The Dalradian Rocks of the Sperrin Mountains and Adjacent Areas in Northern IrelandAuthor(s): J. J. HartleySource: Proceedings of the Royal Irish Academy. Section B: Biological, Geological, andChemical Science, Vol. 44 (1937/1938), pp. 141-171Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20490442 .Accessed: 09/05/2011 09:23

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141

VIII.

THE DALRADIAN ROCKS OF THE SPERRIN MOUNTAINS AND ADJACENT AREAS IN NORTHERN IRELAND.

BY J. J. HARTLEY, M.Sc., M.ENG., ASSOC.M.INST.C.E., F.G.S.

(PLATES VI-VIII.)

[Read 31 MAY, 1937. Published 8 MARCH, 1938.]

CONTNTS.

PAGE PAGE

1. INTRODUCTION ... ... 141 10. THE BLACK SCHISTS OF BROUGH

2. GENERAL SUCCESSION AND STRATI- DERG ... ... ... 156

GRAPH.Y ... ... 143 11. SUGGESTED CORRELATION WITH

3. NEWTOWNSTEWART QUARTZITIC OTHER AREAS ... 156

GROUP ... ... ... 144 12. INTRUSIVE TYPES OF EPIDIORITES 158

4. DUNGIVEN LIMESTONE GROUP ... 146 13. RELATION BETWEEN EXTRUSIVE AND

5. EPIDIORITES (EXTRUSIVE TYPES) 147 INTRUSIVE EPIDIORITES ... 160

6. THE SCHISTOSE GRIT GROUP OF 14. FAULTS ... ... ... 161

DART ... ... ... 149 15. STRUCTURE ... ... ... 162

7. TOURMALINE SCHSTS OF GLENELLY 151 16. RELATION BETWEEN GEOLOGICAL

8. ALBITE-BIOTITE-EPIDOTE SCHISTS STRUCTURE AND PHYSICAL FORM 167

(GREEN BEDS) ... ... 152 17. TE METAMORPHISM ... 168

9. THE MULLAGHCARN GROUP ... 154 18. SUMMARY AND CONCLUSION ... 169

(a) Albite Schists ... 154 19. LIST OF WORKS TO WHCH REFER

(b) Bedded Types ... 155 ENCE IS MADE ... ... 170

l.-INTRODUCTION.

THE Sperrin Mountains with their associated foothills lie along the borders of the counties of Derry and Tyrone in Northern Ireland, and consist essentially of two ranges as shown in Fig. 1. The highest and most northerly of these, forming the Sperrins proper, trends almost east and west and includes the five summits of Mullaclogha, Dart, Sawel, Meenard,

and Mullaghaneany, all of which exceed two thousand feet in height. The

foothills to the south, which include the Munterloney Hills and the Mullaghearn-Carnanelly group, form a range broader, less continuous and

more detached in character and broken in outline than that of the main

Sperrins. This latter range is slightly lower than the Sperrin mountains.

It culminates at its two ends in Mullaghearn and Carnanelly, both about

1,800 feet high, whilst the central part breached by the two north

PROC. R.I.A., VOL. XLIV, SECT. B. [PI

142 Proceedings of the Royal Irish Academy.

westward flowing rivers, the Owenreagh and the Owenkillew, is cut up and discontinuous.

-Dot~~~~~~~~~~~~

Snic

Newtownsswuf

FIG. 1 Topographical Map of the Sperrin area. Districts lying between 700 and 800 feet elevation are shaded in black, and those below 300 feet are dotted.

The Mullaghearn-Carnanelly range is directed along north-east south west lines and coalesces with the northern range towards the north-east, so that the whole district here described forms a triangle with the northern

edge running east-west and twenty miles in length and the western edge north-south and measuring about twelve miles, whilst on the east the depth tapers away to less than four miles. The three corners of the triangle

are occupied respectively by the towns of Dungiven, Strabane, and Omagh. The western side defined by the Mourne-Strule depression is low-lying

throughout its length. It is from this direction that the country has been

cut away by the complex river system, the headwaters of the Foyle, which

includes amongst other rivers the Glenelly, Glenlark, Owenkillew, Owen reagh, and the Strule-Mourne,

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 143

The northern and south-eastern portions of the district were geologically examined by General J. E. Portlock in 1843, and subsequently the whole

area was'treated in somewhat fuller detail by Messrs. G. H. Kinahan,

J. Nolan, P. W. Egan, S. B. Wilkinson, and other members of the Survey.

Their results are shown oni the one-inch sheets of Geological Survey of

Ireland, numbers 17, 18, 25, and 26, which together with the descriptive

memoirs were published between the years 1884 and 1889.

Since the area was one of glacial deposition,' the drift deposits reach

locally a considerable thickness, but owing to the high relief and fairly

heavy rainfall post-glacial erosion has been sufficiently powerful for much of the solid floor to have become exposed. Practically all the streams

flowing down the sides of the ranges have cut down to solid rock over their

whole length, whilst the deep, rocky, and steep-sided character of their

gorges makes the larger of them in some cases almost impassable. The

main rivers, though their valleys are of a different type and flat floored

with alluvial deposits, usually expose the solid rock at intervals. In addition, good continuous exposures occur on some of the summit ridges

such 'as Mullaghasturrakeen, Dart ald Sawel, and many of the glacial

spillway channels which cut across the ranges provide excellent sections.

With the above exceptions, however, there is rarely anything to be

seen in situ, and it was probably owing to the paucity of the exposures as

well as to the apparent monotony of the lithological types that much of

the area was only mapped geologically to the scale of one inch to the mile.

The lack of subdivisions or datum lines in the wide expanse of schists

has in consequence rendered the structure difficult to interpret from the published maps, and it is only the more detailed knowledge of schist

texture and composition attained during the last fifty years which has

enabled the problem to be reattacked with more chance of success, and

made it worth while to remap the district on the six-inch scale throughout.

2.-GENERAL SUCCESSION AND STRATIGRAPHY.

The schists of the Sperrins represent the south-eastern termination of

the main outcrop of the Dalradians of Ireland. . In the Sperrin area they

are bounded to the south-east by an overthrust and rest upon Ordovician

rocks as previously described (19), whilst they are cut into from the

north-east, west, and south-west by tongues of Carboniferous shales, sand

stones, and conglomerates which form the lower ground and run radially

into the area along the lines now followed by the rivers Roe, Strule, and

Owenreagh.

1 See J. K. Charlesworth, Glacial Geology of the North-West of Ireland. Proc.

Roy. Ir. Acad., vol. xxxvi, Sect. B, pp. 174-314.

IF' 2j

144 Proceedings of the Royal Irish Academy.

The present paper deals merely with the schists, for which the proposed succession is given in the following table. The order is that found when the beds are traced across the strike fromn north-west to south-east, the north-westerly one being placed at the top:

Quartzitic Group of Newtownstewart with associated Schists. Dungiven Limestone Group. Amphibolites or epidiorites of extrusive origin. Schistose Grit Group of Dart. Tourmaline Schists of Glenelly (north side). Albite Biotite Epidote Schists ("Green Beds"). Tourmaline Schists of Glenelly (south side). Mullaghcarn Schists; chiefly albite schists with siliceous bands. Black Schists of Broughderg. [Thrust Fault.]

Palteozoic Series of Tyrone.

The table would appear, from a preliminary examination of the map with its predominating north-westerly dips, to represent the Dalradian beds in their stratigraphically correct order, but it seems probable, though not certain, that the order is inverted, and that the Quartzitic Group of Newtownstewart is the lowest and not the highest member of the series. The rock groups will therefore be described, beginning with those lying towards the north-west.

The various subdivisions are treated as groups rather than as individual bands in order to avoid the uncertainties and complications which would otherwise be introduced through the frequent difficulty and occasional impossibility of deciding whether, for example, two bands of limestone are stratigraphically equivalent or lie at different levels. That there is a gradual and general change when the beds are traced across the strike is

certain, but local isoclinal folding introduces complexities in detail difficult to completely unravel.

3.-NEWTOWNSTEWART QUARTZITic GROUP.

Bounding the area on the north and north-west is a group of beds

in which quartzites predominate. The quartzites, associated with quartz sericite and at times somewhat phyllitic schists, or with bands of siliceous

grits indistinguishable from much of the material found further to the south-east, gradually diminish in amount in this direction, till they are

only represented by thin isolated lenticular patches. Their abundance to the north-east would appear to be a definite stratigraphical feature.

Although this group lies for the most part outside the area mapped

and has not been examined in detail except in the west and north-west, it

appears to extend more or less continuously along the northern edge of

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 145

th.e Sperrins till it is overlapped by the'Carboniferous a few miles west of Dungiven.

The quartzites are yellowish or pinkish in colour and very finer in graln. Colour banding, probably indicating original bedding, was referred to by J. R. Kilroe (6, p. 9). Both felspars and light and dark micas are present

to a small extent in all the specimens examined, and the quartzites usually

pass by indistinguishable gradations into schistose grits, which they closely resemble, though the latter contain tiny elongated or rounded pebbles of both clear quartz and felspar, and are more definitely foliated with

pronounced iron staining along the foliation planes. In many cases it is these felspathic grits which are in the earlier literature referred to as gneiss or gneissic schist.

The mica schists associated with the quaitzites call for little remark. They are somewhat poor in mica, very siliceous and frequently felspathic. Though a few thin bands of a lead-grey or blue-grey coloured and more micaceous and pelitic type occur on Bessy Bell as well as amongst the

quartzites to the south of Dunnamanagh, they are quite subordinate to the more psammitic varieties.

Albite is rare and tourmaline practically absent from the schists, schistose grits, and quartzites of this group. In such an environment, however, albite might be easily overlooked.

The thin bands of micaceous and black schists which occur amongst

the quartzites are well seen in the roadside quarry south of Dunnamanagh

illustrated in P1. VI, fig. 1. Taking a general view of the quartzitic group as compared with those

discussed below, it would seem to have been less altered. Secondary biotite is rare, and when present forms very small crystals. The larger

portion of the group represents a series of sediments of an arkose type

only slightly affected by regional metamorphism. The outcrop of the siliceous series appears to be steadily broadening

westwards. Towards the east, although the numerous quarries recently

opened at this horizon for road metal have shown that the outcrop of

quartzite is much more extensive than the existing maps would lead one

to suppose, yet the breadth of the whole series is diminishing both in

general width and in the persistence and breadth of the siliceous bands.

The northern edge of the series has not yet been determined with

sufficient closeness to enable it to be shown on Plate VIII, but it seems to

the author that the whole series disappears entirely to the north or north

east of Dungiven.

As to whether this disappearance is due to overlap, overthrust, or to

the effect of pitch on an anticlinal or synclinal fold is a question which

requires more extensive mapping before it can be satisfactorily answered;

but in the author's opinion the last hypothesis is the most probable one

of the three.

146 Proceedings of the Royal Irish Academy.

4.-TmE DuNGivEN LIMESTONE GROUP.

To the south-east of the schists and quartzites of Newtownstewart the

amount of siliceous material diminishes, and the quartzite is replaced first by lenticles and then more or less continuous bands of limestones, the

essential constituent of the Dungiven Limestone Group. The width of outcrop of the Limestone Group varies considerably. It is

widest to the east of Dungiven, where, however, its continuity as shown on the published maps appears to the author to have been somewhat over

estimated. The thickness of the calcareous beds also shows a great divergence. In

one or two instances 100 to 250 feet of limestone is developed without

pelitic layers as noted by J. Nolan (5, p. 16); in other cases these banids

are only a few inches thick. Lack of sufficient exposures and difficulty in

correlating the seams make it impossible to say definitely whether this is

due to rapid lateral variation, or to the splitting up of a particular bed

such as has been recorded from Argyll (13,, p. 25). The actual south

eastern boundary of this division is difficult to fix, as the limestones again

diminish in this direction more or less gradually, the limestone outcrop changing from bands to discontinuous lenticles before it disappears entirely.

Even when the fullest allowance is made for close folding and repetition

the thickness of the whole group would appear to be considerable and

greater, for example, than that found in north-east Antrim.

The general texture and colour of the limestones are very >ariable.

Usually they are dark-blue and somewhat schistose, but around Newtown

stewart they are white and saccharoidal, while they are yellowish near

the Butterlope Pass, and occasionally black and presumably graphitic,

especially near their contact with the epidiorites. It may be pointed out

that the black massive varieties with large crystals of dark calcite up to

ten millimetres in length, identical with those which occur at Torr Head,

Co. Antrim (cf. 1, p. 169), are only found near to the thickest of the

epidiorite intrusions such as the one illustrated in PI. VII, fig. 2, from

Dungiven and those in the Altnaheglish valley. At only one locality

(Tamnagh Bridge) do the limestones show traces of dolomite, a feature

which seems more typical of those lying further to the north (cf. 6, p. 10).

Rounded grains of quartz are almost invariably present, the proportion

being unusually high around Craig and Dungiven. Dark mica is very common. It occurs as films parallel to the foliatior

and in six-inch bands (presumably bedding planes) at the Butterlope cand

elsewhere. Its abundance in the limestones has been commented on by

Portlock (25, p. 181), Kinahan (4, p. 20), Wilkinson (6, p. 10), and others.

Iron pyrites is another common constituent, and the crystals some

times reach 10 mm. in diameter around Newtownstewart.

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 14'

The occasional presence of muscovite and absence of any recognisable orthoclase or microcline felspar would indicate, according to Prof. C. E. Tilley's note2 on the Loch Tay limestones in Argyllshire and Perthshire, that the metamorphic grade is not here higher than that of the Biotite

Zone. The schists associated with and intercalated amongst the limestones

are rarely felspathic or gritty, but more usually dark-grey or black, soft and very rich in biotite.

To the south-east of the limestone group in the Glenelly valley and to the east of Gortin two exposures of limestone are marked on the one-inch

maps. These outcrops, not now exposed, are referred to in sections 8 and 9. Since both these limestones, which were very thin and of such inferior quality that their working has been long since abandoned, are not associated with epidiorite, it is considered improbable that they represent the Dungiven series repeated by folding, but rather that they mark a

separate and distinct calcareous horizon.

5.-EPIDIORITES. (EXTRUSIVE TYPES.)

North of Plumb Bridge the limestones are succeeded by two bands of dark-green schists. The northerly band is narrow, but the southerly one is broader with a maximum width of about half a mile, and is well exposed

on the western side of the Butterlope Pass. To judge from the dips and

the general structure the northerly band is a repetition of the one to the

south brought up by folding as an inlier. Both outcrops become narrower

when traced in either direction along the strike, and their maximum

lateral extent does not amount to more than about seven miles, while the

greatest thickness would not be more than three or four hundred feet.

Many of these sehists, as, for example, the wide outcrop at the head of

the Butterlope, were not previously differentiated from the mica schists

and schistose grits by which they are surrounded, but as shown on P1. VIII

they include a few of the exposures such as those of Stranagalwilly and

Craig which were shown on the Survey maps as epidiorites and were

referred to as such in the memoirs (cf. 5, p. 17).

As seen in the field they are all massive dark-green schists with rusty

weathering. Though too uniform and fine in grain for any of the

constituent minerals to be identified in the field, they usually show

effervescence with hydrochloric acid. Microscopic examination shows them to be as a rule well foliated and

to consist essentially of a felted mass of very pale green, sometimes almost

colourless actinolitic needles, with extinction angles ranging from 18 to

2 ' ' Vesuvianite and Grossular as Products of Regional Metamorphism.

' ' Geol. Mag.,

vol. lxiv (1927), p. 374.

148 Proceedings of the Royal Irish Academy.

20 degrees. Epidote and zoisite3 are usually present as scattered grains or granular aggregates, and they show in addition irregular patches of calcite and occasionally quartz. Small grains of albite have been noted, but this mineral rarely forms definite phenocrysts or porphyroblasts.

In one or two cases, such as specimens from Stranagalwilly, the amphibole builds larger and more platy crystals. It is darker in colour and associated with crystals of ilmenite and leucoxene. The epidote and zoisite instead of occurring in scattered grains form granular aggregates about 2 mm. in length, suggestive of pseudomorphs after felspar. A similar type also occurs to the west of Dungiven.

These particular types seem to approach in character the intrusive epidiorites which are discussed below in section 12.

Other specimens, from the Dungiven area, represent varieties in which irregular phenocrysts of chlorite and felspar are present, while there is

much white mica in the hornblendic matrix. With such variation in texture and degree of foliation the types range

from hornblende schists to amphibolites, and since there was little in the way of igneous features to be seen under the microscope the use of the term epidiorite seemed hardly justifiable till field evidence showed that the rocks were, at least in part, not only of igneous but of effusive origin.

The evidence for this statement consists of the pillow-structure, a feature which is so far as known confined to rocks of volcanic origin.

The epidiorites of Craig, a village lying about six miles niorth-west of

Plumb Bridge, supply one of the best of the few exposures showing definite pillow-structure. This section is illustrated in P1. VI, fig. 2. Here the pillow-like forms are rather small, averaging about eighteen inches across. They do not show the usual signs of chilled margins, or of concentric

amygdules, but these are perhaps hardly to be expected when the generally

fine grain of the material as well as the metamorphism to which they have

undoubtedly been subjected are taken into account. Microscopically they differ little from the amphibolites of the Butterlope

described above. They are distinctly schistose, and small needles of tctinolite felted together make up the bulk of the rock. Grains of zoisite

are fairly plentiful, and a few small oval-shaped and quartz-filled lenticles may represent amygdules.

The material between the pillows is deeply weathered in. It is grey

green in colour and spongy in texture as shown on P1. VI and consists

almost entirely of granular zoisite and quartz, with a little pale actinolite, muscovite, clinozoisite, calcite, and hematite. Though it may represent an altered sediment, such sediment must have been of approximately the

' Zoisite was observed amongst epidiorites to the north-west by both J. E. Portlock

(25, p. 209) and J. S. Hyland (4, p. 3-6). The latter writer gives excellent descriptions

of some of these rocks, though outside the area here described.

HAxRTuY-Dalradian Rocks of the Sperrin Mountains. 149

same composition as the lavas themselves, and its general type is somewhat

unusual. It is quite distinct from the cherts found amongst the pillow

lavas of the Tyrone Igneous Series (19, pp. 228 and 236), from the "green

beds," limestones and slates of Strabane (23, p. 17), and from the quartz

hornblende, or quartz-mica oligoclase assemblages found by Tilley4 to compose the interstitial material of the pillow lavas of Cornwall.

Further to the north-west Dr. McCallien has described (23, pp. 13-22) pillow lavas from two horizons at Strabane and Altigarvan.

Although the pillow lavas of Craig do not resemble those of Strabane

they show a closer similarity to some of the non-porphyritic types of Altigarvan, except for a scarcity of albite.

It is not possible to say with certainty that the Altigarvan and

Butterlope-Craig rocks lie on the same horizon. The two exposures are not directly continuous, but are separated by some six miles of a calcareo

siliceous assemblage not yet examined in detail, while, in addition, there is a certain amount of faulting between them. There is, for example, almost certainly a fault, running north - east south - west through Craignagapole, which cuts off the limestone on the east.

Nevertheless the present author thinks it highly probable that the two sets of pillow-lavas lie at the same horizon.

As previously mentioned the outcrop of the quartzites narrows towards the east and shows signs of thinning out altogether in that direction, while it is succeeded to the north-west by a band of limestone and a series of

pebbly grits which are separated by lavas similar to those at the Butterlope and Craig. The threefold succession found on the two sides of the quartzite is so similar as to suggest repetition. In both cases we find an association

with epidiorites, and although the two bands cannot actually be traced into each other owing to the overlap of the Carboniferous they are running eastwards in a manner which would if continued lead apparently to ultimate convergence. Though such evidence is perhaps not strong enough to be conclusive, it is at least of considerable presumptive value.

It may be noted incidentally that while the Strabane section as described

by McCallien (23, p. 16) reverses structurally the general Sperrin succession, that of Altigarvan repeats it in the same order (23, p. 14).

6.-THE SCHISTOSE GRIT GRouP OF DART.

Succeeding the epidiorite lavas towards the south-west is a rather thick

succession of schists of somewhat varied type, but distinguished in the mnain by their gritty and siliceous character.

*" Metasomatism of Greenstone Hornfelses, Cornwall.

" Min. Mag., vol. xxiv (1935),

p. 185.

150 Proceedings of the Royal Irish Academy.

At their north-westem edge there occurs a pebbly band with pebbles up to about three quarters of an inch in diameter composed of quartzite

and vein quartz. This band is seen north of Plumb Bridge, east of the

Butterlope Pass, on the slopes of Oughtnager and south of Craig. No compound pebbles have been detected in any of the exposures.

The succeeding beds consist mainly of grits and highly siliceous granulites or sometimes quartzites, which form massive and rugged features (ef. 11, p. 11) along much of the summit ridge of the main Sperrin range.

They are especially well seen near the summits of Dart and Sawel. Another feature is the quartz phaeoids which though of sporadic

occurrence in some of the other groups are most extensively developed amongst the Schistose Grit Group of Dart. They occur in various forns,

as knots, strings or lenticular seams usually conforming to the convolutions of the foliation but occasionally transgressing these in a sill-like manner.5

In some instances, especially in the neighbourhood of known fault planes, they show kataclastic modifications with an alteration of texture

from crystalline to a more granoblastic type. When much broken down

these varieties sometimes take on a breeciated appearance of pseudo

sedimentaxy type. The quartz appears to be the product of the breaking down of the

higher silicate compounds originally present in the sedimentary material from which the schists are produced, and not derived from external so. ces.

As pointed out by P. W. Clark,6 quartz is a bye-prduct of the sericitization of felspar either orthoclase or plagioclase, whilst the production of albite, together with epidote, zoisite, or actinolite from plagioclase, as well as tourmaline from more basic silicates would in every case lead to an excess of silica.8

There is of course a strong tend,ency for the silica so released to segregate in lenticles or inconstant bands, the process of segregation being one of continued local solution, diffusion and recrystallisation, and the transference being effected along lines perpendicular to the maximum pressure,9 that is to say along the planes of schistosity, its extent varying

with the intensity of the metamorphism.10

The true schistose grits are somewhat variable and impersistent in

their distribution. They are, where mapped, indicated with circles on P1. VIII. Their outcrops usually form lenticular areas which thini and thicken rapidly when traced along the strike. Petrologically they are often very coarse in grain as illustrated in P1. VII, fig. 1.

B Cf. E. Greenly, Geology of Anglesey, Mem. Geol. Surv., p. 48.

Data of Geochemistry, U.S. Geol. Sur. Bull., 695 (1920), p. 597. 7

loe. cit., p. 598. 8A. Harker, "

Metamorphism, > >

London (1932), p. 121. 8

loa cit., pp. 203 and 207. 10 loc. cit, p. 67.

HIARTLEY-Dalradian Rocks of the Sperrin Mowntains. 151

The quartz is usually white or water clear, but rounded grains of a

blue variety occur south of Tamnagh Bridge and in a few other localities.

Occasionally the grits are felspathic and contain rather large, up to

10 mm., rounded crystals of felspar. The localities of the more felspathic

types are indicated by the heavy black dots on P1. VIII. The feilspars

are too cloudy and decomposed for exact determination, but the optical

properties would appear to indicate albite or albite-oligoclase. The general gritty character is the special feature of this gtoup, and

as an indication of its lithological similarity to those beds with which

it is proposed to correlate it, one may note that the coarse gritty and

conglomerate character of the Glen Sluan or Pitlochry Schists of Kintyre,

and of North East Antrim, is a feature to which attention has frequently

been drawn (cf. 3, p. 418, and 1, p. 170).

Further south-west and at structurally higher levels a more pelitic

type of schist is developed associated first with albite and then with albite

and tourmaline.

7.-TouRvALiNE SCOLISTS Or GLENELLY.

The Schistose Grit group of Dart is succeeded to the south by a series

of quartz-muscovite-biotite schists with much tourmaline. They form a belt nearly four miles wide in the Glenelly valley. This belt is broken

through the centre by a narrow band of dark green schists (also tourmaline

bearing) which are discussed below in section 8. Although the tounmaline bearin,g schists on either side of the green

schists are indistinguishable in the field the difference in the groups by which they are succeeded to the north-west and south-west respectively

makes it probable that they are not repetitions of the same band but succeed

one another stratigraphically. On Plate VIII they are thus distinguished as the Tourmaline Schists of Glenelly (north) and Glenelly (south), but

since they are identical petrologically they will be here treated together. Though tourmaline does occur in one or two exposures outside the main

t&urmaline bearing areas as, for example, west of Dungiven and Eden

Bridgfe, south-east of Dreen and east of Altnaheglish, such occurrences are small, rare and isolated. It is not possible to say whether they mark

inliers of the main band or a repetition of the same sedimentary conditions

at another horizon. These tourmaline schists of Glenelly differ from the Dart group in

their higher percentage of mica and generally more pelitic character. Derived felspar is rare, though lentieles and bands of authigenie

albite, which reach their maximum in the succeeding division, begin to show sporadic development.

The tourmaline is most obvious in certain soft grey coloured varieties which are very rich in mica. It occurs in the form of acicular needles

up to one centimetre in length.

152 Proceedings of the Royal Irish Academy.

As noted many years ago by General Portlock (25, pp. 180 and 196),

the needles run both along and across the foliation. This writer also

r efers to the frequently curved character of the crystals, a feature especially

common on the Glenedra Pass where they frequently curve round through

an angle of ninety degrees.

Since the tourmaline oceurs in Tyrone as inclusions in the albite in a

similar manner to that recorded from Cowal (9, p. 78) it may, if Tilley's

views as to the albite marking by its abundance a particular type of

sediment be accepted, have been also inherent in the composition of the

original mud from which these schists were derived and hence be of

iefinite stratigraphicaJ value. Its relative abundance certainly shows no signs of bearing any relation

ship to the intrusive epidiorites or to pegmatite veins.

The only change which may be of metamorphic origin is some increase

in size shown by the tourmaline in the gamnetiferous areas. This is only

to be expected, since the facility of tourmaline for growth under suitable

conditions and its rapid recovery from kataelastic accidents is a well

lknown feature (ef. 22, p. 17) and such recovery would be assisted by

entrance into the garnet isograd. Tourmaline, as described from the Dalradian rocks of other localities,

seems to be most typically developed in the Beinn Bheula or Ben Ledi

Group or its equivalents, as well as the Green Beds and Glen Sluan or

Pitlochry Schists. It has been noted at the first horizon in Argyll (13, p. 16), Cowal (9, p. 40),

Kintyre (3, p. 414) and North-East Antrim (18, p. 131); in the Green Beds

(,f Argyll (13, p,. 19), Dalmally (11, p. 13), Cowal (9, p. 35), and Kintyre

(3, p. 416); and in the Pitlochry Schists of the Ben Lwers area (24,

p. 307), etc.

It is these three particular groups which are considered to be represented

by the tourmaline schists of Glenelly and their intercalated band of " Green

Beds." In the Scottish memoirs reference is frequently made to the association

of tourmaline and albite (cf. 9, pp. 45, 78, and 299, 3, p. 416, 13, p. 10

18, p. 131, etc.). In Tyrone, though this correspondence is a very common

one, it is not by any means exact. The albite types overlap the tourmaline

area both to the north-west and south-east, whilst many of those schists

richest in tourmaline are albite-free.

8.-ALB1TE-BioTITE-EPIDoTE SCHISTS OR " GREEN BEDS.

For purposes of correlation with those areas lying towards the north

east perhaps the most important stratigraphical horizon is that of the

albite-biotite-epidote schists which are equivalent to the Green Beds of

the Dalradians of Scotland as well as to those of North-East Ant-rim

described by Dr. E. B. Bailey and Dr. W. J. MeCallien (1, p 167).

llARTLEY-Dalradian Rocks of the Sperrin Mountains. 153

UIp to the present these Green Beds have not been separated from their

surroundings in Derry and Tyrone. On the published maps they are

usually given the same colour as the "mica schists" though occasionally

marked as epidiorites, but their importance was not of course realised in

the seventies of the nineteenth century when Scottish Dalradian geology was but little advanced.

These particular beds occur in a thin but apparently fairly conitinuous

band which can be traced across the area from Omagh to Draperstown.

This main band is always narrow and not more than about sixty yards

across. This is about the same as its outcrop in Antrim (1, p. 173), but

considerably less than the half to three-quarters of a mile recorded (12, p. 8, antd 3, p. 415) for Kintyre. On Plate VIII the breadth of the outerop

has for the sake of clearness been somewhat exaggerated.

In addition to the main band several isolated exposures of the same

type of sediment are found to the north-west and south-east. The longest

and most continuous of these is interpreted in the section on P1. VIII as

being an inlier of the main outcrop, since such a view fits in most readily

with the folding of the surrounding beds. The other exposures, however,

are more likely to indicate either an earlier incoming or later persistence

of the same special type of sedimentation conditions.

In Scotland "Green Beds" have been recorded from various horizons

ranging from the Dunoon Phyllites (9, p. 89), through the Beinn Bheula

or Ben Ledi (2, p. 98, 12, p. 24) to the Ben Lui Schists (2, p. 98), and it

is stated that some of these horizons cannot be referred to a single group

reduplicated by folding (2, p. 100). In one or two localities, such as the Oughtboy Burn north of Cranagh

and at Garvagh Bridge in the Glenelly valley, the Green Beds are

associated with very caleareous lenticles or even thin impure limestones

which though of very poor quality have been at one time worked. This

calcareous nature of the Green Bed horizon is comparable with the

characters described by McCallien (8, p. 161) as found in South Knapdale.

The Green Beds of Glenelly are in the main albite-biotite-epidote schists.

Albite is an invariable constituent. It forms good porphyroblasts, ranging from 0 5 mm. to 2 mm., crowded with inclusions of epidote, zoisite,

etc. Simple twinning is fairly common, but the lamellar type has not

been noted. Dark mica occurs interstitially between the albite porphyroblasts in

considerable quantity, and by its abundance enables these beds to be

distinguished from the epidiorites (cf. 1, p. 173). It is as a rule greenish

and chloritised, but the colour becomes darker and more definitely brown

in the garnetiferous areas. The foxy red type found amongst the more sericitic schists of the succeeding Mullaghearn Group has not been observed

amongst the Green Beds.

154 Proceedings of the Royal Irish Academy.

The quantity of epidote is very variable; though sometimes plentiful, it may, in other cases, be almost completely lacking.

Quartz as microscopic lenticles or discontinuous bands of granulitic texture, suggestive of sheared pebbles, occasionally occurs, but the Green

Beds of Tyrone are not characteristically gritty. White mica is occasionally present in small amounts, but it is never

so plentiful as in the beds to north and south.

The garnets, which are discussed more fully when dealing with the question of metamorphism in section 17, are always small, never more than 0 5 to 10 mm. in diameter.

In spite of the occasional development of garnet, hornblende has not

been observed in any of the slides of Green Beds examined from this area.

The absence may be due to the small amount of the, presumed potash

bearing, white mica which has an inhibiting effect on hornblende develop

ment as pointed out by F. C. Phillips (10, p. 253).

9.-THE MULLAGHCARN GROUP.

The Green Beds are succeeded to the south-east by another variable

group of schists which have a width of outcrop of about four miles.

They are on the whole very similar to those of the Dart Group, though

more micaceous ;1 but felspar, excluding albite, is distinctly subordinate in amount, and there is a gradual increase in the percentage of mica when

traced towards the south-east till, at the boundary fault, we have a

thoroughly pelitic rock.

Speaking generally, they are characteristically rather silvery quartz

muscovite or quartz-muscovite-biotite schists and chiefly consist of a

granulitic admixture of quartz and sometimes albite lying between foliae

of sericitic mica and biotite. In addition to the above there are many intermediate varieties which

can be distinguished in the field.

There are first a few thin bands of very siliceous quartz-mica granulites

similar to those found to the north-west, though here less extensively

developed. Albite schists are exceedingly common, while in addition certain

horizons at which alternation of sediment type enables the original bedding

to be distinctly seen and a few impure limestones deserve special mention.

9a.-THE ALBITE SCHISTS.

Albite is as common a constituent of the Mullaghearn Group as it is

amongst the Beinn Bheula or Ben Ledi Schists of Antrim (1, p. 175),

Kintyre (3, p. 414), Cowal (9, p. 39), or Arygll (13, pp. 13 and 15). It

u In Kintyre the Glen Sluan and Beinn Bheula Schists are at times lithologically

indistinguishable from each other, cf. 12, p. 26.

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 155

also extends into the Green Beds to the north-east and into a few scattered

areas along the southern portion of the Dart Group. The albites possess the usual characters, being clear and colourless,

fresh and undeformed, while they are usually crowded with inclusions. Their freshness renders them at times difficult to distinguish from the quartz without the use of convergent light. The size varies from about 2 to 10 mm. The largest crystals occur at the head of the Finglen River,

where what appears to be a segregation vein of quartz and albite, with a

width of about four inches, runs parallel to the foliation. The nature of the felspar and the absence of any definite intrusions from this portion of the district would seem to militate against any suggestion of igneous action at this particular spot, while segregation has been suggested for similar veins in Antrim (1, p. 166), Knapdale (12, p. 18), Oban (11, p. 11), and

elsewhere. The albite of the Green Beds is frequently confined to definite bands

of a width of about 1 cm. which run parallel to the true dip, and the

intervening layers equally well foliated are albite free. Such development which is similar to that recorded by Clough from Mid-Argyll (13, p. 15) would certainly appear to support the view of Professor C. E. Tilley (17,

p. 108) that the presence of the albite depends on the nature of the original

sediment. There are, however, other areas such as the Glenelly valley east of Cranagh, where the edge of the albite development follows a sharply

marked irregular line having (apparently) no relation to strike or dip. Lines of incipient overthrusting, and the generally higher degree of shearing stress which developed towards the sout4i evidently favoured segregation, and therefore an increase in the average size of the albite

crystals.'2 Those found in the Mullaghearn Group of Schists are usually larger than those which occur in the less highly sheared Dart Group.

9b.-BEDDED TYPES.

There are many localities, especially near the northern margin of the

Mullaghearn schists and the southern edge of the Dart Group, where the

original stratification and aqueous character of the beds are revealed by the

presence of distinct bands varying in texture and composition. Fine or

coarse grained grits alternate with softer micaceous layers representing altered sediments of more pelitic type.

The localities of these particular horizons, which enable the directions

and amounts of the true dips to be determined, need not be described in

detail, since many of them are shown on the map (P1. VIII) by the positions

of those arrows which lie within the schists.

In the bedded types the thickness of the bands may be as much as six

inches, though usually somewhat less, and although it is as a rule difficult

12 A similar relation apparently occurs in Cowal (9, p. 43) and Argyll (13, p. 15).

156 Proceedings of the Royal Irish Academy.

to follow individual sections for more than a few yards at a time, eases

occur where they can be traced for a quarter of a mile, and when this is

possible the dip continues practically unaltered with no signs of sharp

changes or intense isoclinal folding, except on a very minute scale.

A similar alternation of schistose grits or quartzites and micaceous

bands has been noted among the Ben Ledi (or Beinn Bheula) schists of

Kintyre (3, p. 414), Argyll (13, p. 15), and Oban (11, p. 11), as well as

in the Pitlochry and Ben Lui Schists of central Perthshire (24, p. 307).

The thin band of impure limestone brought up to the east of Gortin

by a small anticlinal flexure may be compared with the lenticles of

calcareous schist which occur in the Beinn Bheula schists of Argyll (13,

p. 12) and elsewhere.

10.-THE BLACK SCHISTS OF BROUGHDERG.

The petrology and general distribution of these beds have been discussed in a previous communication to this Academy (19, pp. 225-226) and need

not be recapitulated. They may be briefly described as very fine grained

pelitic schists of a dull black or lead-grey colour composed of quartz,

graphitic dust, and in some cases a little finely divided iron ore.

In the paper above referred to the writer was inclined to group the

Black Schists with the Tyrone Igneous Series (19, p. 226), but the present

mapping has shown that they occur in smaller exposures well within the

limits of the schists of the Sperrins, apparently as infolded outliers or

inliers.

These Black Schists also share in the higher degree of metamorphism

found amongst the schists of the Sperrins, and are even more intensely

puckered and crumpled. It seems most probable, therefore, that they should be included with

the Sperrin series. It may be noted in this connection that Black Schists

or phyllites, lithologically similar to the above, have been described by

C. T. Clough from among the Dunoon Phyllites of the Cowal area (9, p. 9),

and it is possible that the Black Schists of Tyrone may represenit this

horizon.

11.-SUGGESTED CORRELATION WVITH OTHER AREAS.

The lithological variations of the different groups in Tyrone has been

briefly discussed together with the close parallels offered by the Dalradians

to the north-east in order to examine the possibility of correlation with

Antrim and S.W. Scotland. When the relative geographical positions

with respect to the direction of strike, and the order in which the various

types sucteed each other along the dip are also taken into account, it is

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 157

thought that the following correlation may be put forward with some confidenee as to its correctness:

Sperrins. South-west Scotland and Antrim.

Newtownstewart Quartzitic Group. Stonefield or Ben Lui Schists. Dungiven Limestone Group. Loch Tay Limestone. Epidiorites and Amphibolites

(extrusive types). Schistose Grit Group of Dart and Glen Sluan or Pitlochry Schists.

Tourmaline Schists of Glenelly (north).

Albite-Biotite-Epidote Schists of Green Beds. Glenelly.

Tourmaline Schists of Glenelly Beinn Bheula or Ben Ledi Schists. (south) and Mullaghcarn Schists.

Black Schists of Broughderg. Dunoon Phyllites (in part).

As to the correctness of these suggested correlations there would appear to be, with the exception of the first and last, very little room for doubt.

The evidence derived from the numerous lithological similarities shown

by the rocks in Tyrone to those beds to the north-east with which they

are correlated is considerably strengthened by the discovery of the " Green Bed" type of sediment in its expected position. In addition, not only

individual groups but the whole general succession agrees very closely with

that described from Antrim, Kintyre, and Cowal. The question of the position of the Black Schists has been discussed

already in section 10, but a few remarks may be made as to the suggested

correlation of the Newtownstewart Group. That the greater part of it

represents the Stonefield or Ben Lui schists appears to be certain, but this

correlation may not apply to the whole, even of the mapped portion, of

this group, and some of it may possibly though not probably represent the Erins Quartzite.

Although these two groups were considered'3 to be separated in S.W.

Scotland by an important tectonic break (2, p. 96, and 3, p. 424), they

are evidently very similar lithologically. Thus J. B. Hill, referring to

their occurrence in North Kintyre, says (12, p. 29) that there is no sharp

division between them, and that "the boundary drawn upon the map is

in places so indefinite that it loses much of its stratigraphical value (12,

p. 33). Dr. E. B. Bailey also refers to the difficulty of separation (2, p. 96),

as do Dr. G. L. Elles and Prof. C. E. Tilley (16, pp. 628 and 637).

"Recent evidence, however, suggests this view may require some modification. Cf.

Abs. Proc. Geol. Soc. (1936), No. 1304, p. 37.

PROC. R.I.A., VOL. XLIV, SECT. B. l1v]

158 Proceedings of the Royal Irish Academy.

The bands of lead-grey schist, referred to on page 145, which have

been observed amongst the quartzites of this horizon on Bessy Bell and

between the Butterlope Pass and Dunnamanagh seem also to be a feature

common to both groups in Scotland (cf. 12, p. 31, and 2, p. 96), so are

evidently of little.correlative value. At the same time locally important

bands of quartzite and quartz schist are known to occur amongst the

Stonefield Schists (12, p. 27, and 3, p. 420).

The question of the presence or absence of the Erins Quartzite must therefore be left open till the mapping has been carried further to the

north-west.

12.-INTRUSIVE TYPES OF EPIDIORITES.

The extrusive types have been discussed above, but there are, in addition, numerous exposures of hornblende schist, some of which are definitely intrusive and differ in texture from the rocks of similar composition previously described.

They are extensively developed amongst the schistose grits of the Dart area, along the main watershed of the Sperrins where they form bold

crags, and in the south-eastern portion of the limestone series. Most of the main exposures are indicated on the published maps of

the Geological Survey, but additional areas have been discovered on the eastern side of the Glenshane Pass and in other localities. In at least oine

case, however, at a point about two miles north-east of Newtownstewart,

the rocks mapped as epidiorites, here definitely intrusive into the lime stone, are quite unfoliated and unaltered dolerites, probably of tertiary age.

The joining up of isolated exposures where the outcrops are narrow and exposures infrequent is of necessity somewhat conjectural, but the general trend directions and the variety of type have been carefully borne

in mind in the reconstructions. The junctions with the schists as seen on the western ridge of Dart, in

the Stranagalwilly Burn, and east of the Glenshane Pass, etc., are sharp

without gradual passage. As in Scotlarnd, the epidiorites are on the whole more in the nature of

sills than dykes. This is not only shown by their general conformity to the

strike and dip of the associated schists, a feature to which attention was

frequently drawn by the earlier surveyors (4, p. 25, 5, p. 12, 6, p. 12),

but it is also clearly seen, for example, on the western portion of the Dart

summit ridge where small inliers of epidiorite can be discerned below the gently inclined schistose grits, and in the section at the old Priory Church,

Dungiven, illustrated in P1. VII, fig. 2. On the other hand they are occasionally transgressive. Evidence of

intrusion has also been pointed out by Egan and Nolan (5, p. 12) and is

clearly shown in P1. VII fig. 2.

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 159

One point in the occurrence of these rocks is somewhat difficult to explain, that is their apparent local disappearance, or at any rate diminution, in the area to the south of Dungiven. That this is a real absence and not due to the lack of exposures seems to be indicated by the

almost continuous sections supplied by the Finglen, Glenedra, and other rivers. The most probable explanation is that the epidiorites actually represent a comparatively limited number of sills, and that they appear

more numerous to the east and west owing to the close folding, with the ground surface here running not only approximately parallel to, but also closely adjacent to, their surface.

Microscopic Petrology.-The great amount of foliation and alteration to which these rocks have been subjected has left few, if any, traces of ophitic or other igneous texture. If it were not for the field relations they would in many cases be described rather as amphibolites or even hornblende schists than epidiorites.

The minerals present include horublende, felspars, ilmenite, and leucoxene, also epidote, chlorite, quartz, biotite, and garnets.

The hornblende occurs in plates with ragged edges about 0 5 mm. in length and breadth. Both the colour and pleochroism are distinct and strong. Occasionally, however, the amphibolite element is represented by pale-green actinolitic needles similar to those found in the basic lavas.

The felspar is chiefly albite in small granules scattered amongst the quartz. The crystals are untwinned and not distinctly porphyritic, except in one locality on the Finglen River where they reach 0 6 mm. in diameter.

At this particular exposure oligoclase showing lamellar twinning is also present.

Ilmenite is usually plentiful either as scattered grains or as pseudo morphic replacements after pyroxene. In many, though not in all, cases the ilmenite is partially or entirely replaced by sphene and leucoxene.

Epidote and chlorite are never-plentiful and sometimes entirely absent. The occasional lack of the former mineral amongst the Scottish epidiorites is a feature noticed by Wiseman (15, pp. 359 and 387).

The chlorite when present frequently shows anomalous interference colours, browns and purples, comparable to those recorded by Phillips (10, p. 244) and Wiseman (15, pp. 360, 361, and 365).

Quartz occurs as a granular mosaic which builds up small lenticles

amongst the amphibole. Mica is represented by brown biotite. The green variety has not been

observed. Small red garnets occur within that portion of the epidiorites which

lies within the line marked by crosses on Plate VIII.

The two chief features of interest in connection with these epidiorites

are, firstly the nature of the original material from which they were derived, and secondly the grade of metamorphism to which they have attained.

[Q 2]

160 Proceedings of the Royal Irish Academy.

Wiseman in his valuable discussion of the epidiorites of the Highlands suggests (15, p. 371) that the amphibole and chlorite were formed

simultaneously from pyroxene and plagioclase felspar with the extraction of alumina from the anorthite molecule and liberation of albite, silica, and

lime. No doubt the same process has been operative here, as in any case

the origin of the albite in these intrusions must be essentially different from its mode of formation in the detrital schists. The quartz and sphene

would account for the superfluous silica and the sphene and epidote for

the liberated lime. The comparative scarcity of these lime-bearing minerals and the

occsional presence of unaltered ilmenite would, however, suggest even in default of analyses that the intrusive epidiorites were originally much richer in soda than in lime.

As regards the metamorphic grade the evidence would seem to point to a horizon either just above or just below the garnet isograd. The

optical characters of the hornblende, the scarcity of chlorite (cf. 15, pp. 361 and 380), the nature of the mica (cf. 15, p. 387), and the occasional

presence of small garnets all point in this direction. Although the presence of garnet does not necessarily imply a high

grade of metamorphism but may depend on the original composition of the

rock,'4 yet, if Wiseman's criterion (15, p. 407) for the Highland epidiorites be accepted, and "the presence of garnet combined with -an albite felspar

is indicative of an epidiorite situated at the commencement of the garnet

zone as defined in normal pelitic sediments," some of the epidiorites of

Tyrone would lie within this zone. Such a conclusion would also be in

agreement with the character of the mica schists which bound the epidiorites

on either side.

13.-RELATION BEWEEN THE EXTRUSIVE AND ITRUSIVE EPIDIORITES.

There is little doubt that, although it has not been found possible to

trace the two types of amphibolites or epidiorites directly into each other,

they are closely associated and probably contemporaneous.

Their close proximity in the field and similar degree of metamorphism

would support this conclusion. They are both on the whole " epidiorites"

to use in its original sense the field term introduced by C. W. Von Giimbel5

to signify a metamorphosed igneous rock (extrusive or intrusive) containing

hornblende. Although extrusive and intrusive types were not separated on the Irish

survey maps either from each other or, in many cases, from the surrounding

14Cf. C. E. Tilley, ''Met. Rocks of Start Area," Quart. Jour. Geol. Soc, vol. Ixxix

(1923), p. 198; also references to a paper by J. Suzuki by Wiseman (15, p. 378).

^Die pal?olithischen Eruptivgesteine des Fichtelgebirges, Munich (1874), p. 9.

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 161

schists, the surveyors recognised that both types might be present, and J. R. Kilroe suggested (6, p. 12) that, though generally intrusive, some of them might be lavas on account of their general conformity to the

schists. This is certainly the case in Scotland, where B. N. Peach showed

the presence of pillow structure in the epidiorites of Argyll (14, p. 43) and Kintyre (12, p. 83), and now holds also for the epidiorites of Northern

Ireland.

14.-FAULTS.

Excluding the boundary and overthrust faults to the south-east which have been already treated in a previous communication (19, pp. 263-264),

there are four important faults in the Sperrin area, which are arranged

in the form of a somewhat sprawling W, and each fault downthrows to

the inside of the two V's which build up this letter. Of these four faults

those running east-west through Gortin, and north-south towards Magherafelt have the most important effect on the structure. They down

throw in each case towards the inside of the area and bring in the two

tongues of Carboniferous which run respectively up the basin of the Owenkillew and the higher part of the Roe valley, while beyond them the

schists are again brought up as inlies or semi-inliers.

These two faults, along one of whic movement has certainly and along

the other most probably been either i itiated or renewed in Tertiary or

more recent times, control the topogrnphy oI the distriet and also to a

certain degree the curved character of the rock outcrops shown on Plate

VI II. The third fault is shown on the map running somewhat doubtfully

between Dungiven and Omagh. Only the end portions, where it is bounded

on one side by the Carboniferous, were marked as faults on the original

maps, but from the effect on the outcrops of the Green Beds anld the

epidiorites it appears to the author that it is most probably continuous

between these points. This suggestion is to a certain extent confirmed

by the important gaps by which this suggested line crosses the three main

ridges of the Sperrin Group. Along this line the shattering of the rocks

has apparently given rise to an important transverse feature of erosion.

The pass'6 at Cloghornagh east of Sawel where one of the only two

passable roads crosses the main Sperrin range, Barnes Gap, and Gortin

Gap all lie along its suggested course. Though all three of these served

as glacial overflows and were in consequence deepened and widened, they

must have initially marked depressions in the ridges and therefore -have

been presumably lines of weakness.

16 This depression is continued to the north-east through a prominent gap between

Barnes Top and Meenard.

162 Proceedings of the Royal Irish Academy.

To judge from its effect on the Carboniferous, which adjoins its two

ends on the north-west, this fault downthrows in that direction, but its

effect on the outcrops of some of the Dalradian horizons, as well as the

character of the slickensiding, suggests that a lateral element may have

preceded or been associated with this vertical movement and that it may

have been initially a wrench or torsional-tear fault, similar to the one

which occurs in the Loch Tay area, and of the type described by

Cunningham Craig (in Tilley, 17, p. 112), Elles and Tilley (16, pp. 637

and 645), Clough (9, p. 90), and Barrow (20, p. 133).

The fourth fault running south-east through Dungiven downthrows to the north-east over at any rate most of its length. Its main importance lies

in the light which it throws on the character of the folding as discussed

in the following section.

15.-STRUCTURE.

Taking a broad genreral view of the whole of the Sperrin area the

structure would seem to be comparatively simple. The directions of dip marked on Plate VIII show a general inclination towards the north-west, the

angles being as a rule comparatively low (about 20 or 30 degrees), though

they become vertical south-east of the limestone outcrop. It must be noted

that all the dips marked on Plate VIII are true dips and not foliation dips.

They have been obtained from those sections which show close interbanding

of beds of divergent texture and composition, from the thin bands of mica

schist between the limestone, from the alternating psammitic and pelitic

types previously referred to on p. 155, or from the narrow siliceous bands

in the MNullaghearn Group.

In a few cases, such as Barnes Gap, the dip can be seen on a large

scale even from a distance, in others such as the Butterlope Pass where

the contact of two different beds can be mapped accurately across a valley

it is deduced from the relation between outcrop and contour lines.

In this connection it may be noted that, so far as the author 's

observations go, the strike of foliation and, in consequence, the direction

of foliation dip agrees with the true dip, but the two may vary considerably

as regards the angle of inclination.

Since the dips are to the north-west it might be assumed (without

entering into the question of relative age of the beds) that the Newtown

stewart Group is at the top of the succession and the Black Schists of

Broughderg at the base. The whole succession could then represent either

the north-western half of a simple anticline or the south-western portion

of a syncline.

Although there appeared nothing definitely to contradict such a view

when the mapping was commenced in the south-east of the area, yet, as

the work progressed north-westwards and the rock types berame more

varied and the outcrops more continuous and better exposed along the

HARTLEY-Datradian Rocks of the Sperrin Mountains. 163

higher ridges and the intersecting glacial overflow channels, it became obvious that such a simple view would require some modification.

The first difficulty was the Black Schist which was found typically developed high up on the southern flanks of Carnanelly. Though this apparently represented an infold of the band to the south there was no evidence as to whether the structure was anticlinal or synclinal.

The same difficulty as to a simple succession arose in the Butterlope area to the north of Plumb Bridge where a series of limestone bands, parted by bands of mica schist, is brought in between two belts of extrusive epidiorites, the northerly one being lenticular and dying out to east and west; and where, in addition, the Schistose Grits of the Dart Group appear to overlie the main epidiorite band which in its turn rests on limestone.

Since the Butterlope Pass, formerly a spillway from the glacial lake in Glenelly, represents one of the two glacial channels which traverse the main Sperrin range, it cuts a deep rocky gorge along the dip with the

road across the pass reaching a maximum elevation of less than 800 feet, while the hills on either side rise to 1300 feet or more. In consequence of this the mapping could be carried out in a more detailed form than

was usually the case. This fact, together with the numerous dips, varied

rock types and good exposures, suggested the possibility of constructing a detailed section across this line, but, when this was done, it was not

found possible to meet all the facts without the type of folding illustrated

in Fig. 2 in which the limestone occurs as a syncline between the epidiorites.

A further section to the east through the pillow lavas of Craig based

on the relationships shown in Fig. 2 seemed also to fit the facts better than the earlier and simpler hypothesis first adopted.

IV_ Al.

.... ~ ~ ~ ~ ~ ~~ N

~- -

FIG. 2.-Section across the Butterlope Pass looking east-north-east. The pebbly conglpmerates referred to on page 166 occur at the point marked X and about three-quarters of a mile from the road.

164 Proceedings of the Royal Irish Academy.

From a study of these and other sections it became evident that there

was a considerable amount of fairly large scale isoclinal folding present causing local inversions of the apparently simple succession.

Such a conclusion is in accordance not only with the smaller structures

seen in some of the hand specimens but with the general opinions generally

accepted for similar rocks in Cowal and elsewhere and illustrated for

example on Plate XVIII, prepared by Dr. B. N. Peach, for "Chapters on

the Geology of Scotland." In view of the somewhat limited height of the sections available the

possibility was entertained that the isoclinal folding might be of such an

extent that the actual sheet dips of the various groups were not actually

north-west but rather towards the south-east and that the limestones, for

example, overlay the quartzites. In order to test this hypothesis the most

hopeful line of attack appeared to be to investigate the effects of pitch

and dip faulting on the character of the outcrops and direction of the strikes. Both pitch and faults might be expected to bring within reach of observation horizons otherwise inaccessible either owing to thickness of cover or the removal of beds by denudation.

The Effect of Pitch.

In the Sperrin area the beds as shown by the dips pitch definitely to the east or north-east in the east part of the map, and slightly south-west in the west, the culmination of the fold occurring in the centre. The north-easterly pitch'7 is the more strongly developed of the two, the south westerly one is only apparent in the extreme south-west corner near Omagh where it brings in an arc of Black Schists above the Tyrone Series, and it does not seem to persist so strongly towards the north. The dominant strike structure would thus perhaps be best described as consisting of a

north-easterly pitch interrupted by local south-westerly undulations. This direction of pitch agrees with that found in North Antrim (cf.

1, Plate II), while it is opposed to the south-westerly pitch of Cowal (9,

p. 87) and Kintyre (2, p. 103, 3, p. 428). To judge from the map given

by Dr. E. B. Bailey (2, PI4 1), the pitch is again north-easterly around

Loch Tay. The outward pitch in both directions in the Sperrins with the axial

culmination at the centre shows a doming which repeats the similar

culmination shown by the central inlier of schists and gneisses in the

Tyrone Igneous Series to the south-east, and it is in consequence tempting

to ascribe it to the early Devonian overthrusting.

It is however certain that the post-carboniferous radial faults which

bound the Carboniferous tongues of the Dungiven and Gortin areas on the

17 This pitch was noticed by Portlock (25, p. 171) in 1843.

H1ARTIEY-Dalradian Rocks of the Sperrin Mountains. 165

north-east and south-west respectively, and indicate in each case a considerable drop, must also have been instrumental in producing or at least modifying this axial pitch. The feature is thus at least in part post-carboniferous, and, so far as the north-easterly one which affects the basalts is concerned, late or post-tertiary.

Again it will be noted that the summits of the main Sperrin range increase in altitude from each end and rise from 1800 feet towards Sawel at the centre with a height of 2240 feet.

This suggests that both the pitch elements are fairly recent. Similar processes may have determined the form of the inlier when the axial tilting would then in each case be most probably late Tertiary and associated with the downward movements which produced the depressions of the Foyle valley and Lough Neagh.

Whatever its date, however, the pitch is distinct and one may now return to investigate the light, if any, which it throws on the question of the extent of the local overfolding.

This evidence appears decisive in favour of the view that the general dip is towards the north-west. To the east of Dungiven there is strongly

marked bending of the limestone group towards the south and the outcrops

of the beds begin to advance steadily in this direction as they come within

the influence of the Dungiven depression. A similar effect though not so pronounced is shown in the south, where

what is probably a continuation of the same depression occurs in the

neighbourhood of Draperstown. To the west the curvature of the strike though appreciable is not

perhaps quite so definite as it is at the eastern end of the range. This may

be due to the fall in the ground level compensating the axial depression.

Nevertheless, so far as it is evident at all, the strike curvature is south

wards and this adds support to the above conclusions.

The only important faults which bring schist against schist, and of

which the direction of downthrow is at the same time known, is the one

which runs north-west south-east through Dungiven and the somewhat

doubtful one between Dungiven and Omagh. At its north-western end

the former definitely downthrows to the north-east, and its effect on the

Dalradians is immediately apparent in the southward transgression of the

limestones for a distance of some two miles. This again confirms the

evidence as to the dominance of north-westerly dip supplied by the pitch.

Though the displacements of outcrops produced by the Dungiven-Omagh

fault would also seem to point in the same direction, their support is

weakened through ignorance as to the exact amount of lateral movement

which may have taken place along this line.

The section shown on Plate VIII is intended to give a general idea of

the suggested structure, but it must of necessity be somewhat diagrammatic.

The main lines of the minor folds shown have been obtained chiefly from

166 Proceedings of the Royal Irish Academy.

the (suggested synclinal) area of black schists on Carnanelly, from the

epidiorite sill near to the summit of the main Sperrin range and north

of Dart and Sawel, and from the vertical and then overturned fold exposed

on the Butterlope. The general turning towards the west or north-west at higher elevations

is more conjectural, as the only evidence available for this construction is the assumption that the pillow-lavas of Strabane and Altigarvan lie at

the same horizon as those of Craig. If, however, this hypothesis is correct, the fold connecting them must be as shown, that is anticliwal and not

synclinal, since it narrows to the north-east wlth a north-easterly pitch. The extension of the beds below ground-level towards the south-east

is also conjectural. Such a construction would, however, place the beds in their position believed to be stratigraphically correct and at the same

time fall in with the type of fold (the Ben Lui fold) which has been

observed at this level in Scotland.

The further question as to the relative ages of the various groups has

not yet been touched upon, and it is perhaps not possible at present to

give a convincing answer to the question as to which are the younger

and which the older beds of the series.

Nevertheless the opinion of the author is that the order given in the

table on Plate VIII, with the Quartzitic Group of Newtownstewart repre

senting the older rocks, is the correct one. This opinion is based in part

on the thin bands of pebbly conglomerate which occur in the schistose grits

of the Dart Group only a few feet above their junction with the under

lying lavas on the north-western slope of Oughtnager. These bands of

conglomerate have a well-marked base line, while the upper edge is less

well defined and there is a gradual passage into the overlying grits.

In the Knapdale area conglomerates occur in the Loch Awe Series at

Kilmory Bay. Of these conglomerates J. F. G. Wilson wrote (12, p. 64)

as follows:- The individual bands of conglomerate have, in every

instance, a well defined base line and the pebbles come in thickly at the

very start, but they decrease in number quite gradually, so that in this

direction the conglomeratic bands have no definife boundary-the tops

and bottoms of the conglomeratic bands are thus strongly contrasted, and

the contrast is of type one meets with not uncommonly in unfolded

conglomerates. In fact it is exceedingly difficult to escape the inference

that these conglomeratic seams are right way up."

This evidence was, as stated by Dr. Bailey (2, p. 97), suggestive rather

than conclusive, but nevertheless gave valuable support to the testimony obtained by Peach (12, p. 68) from the difference between the tops anid

bottoms of the pillow lavas.'8

18 It may be noted, however, that even this line of evidence has been recently queried.

Cf. Abs. Proc. Geol. Soc. (1936), No. 1304, p. 38. For the value of the conglomerate evidence cf. W. H. Twenhofel, "Treatise on Sedimentation/' London (1932), p. 210.

HARTLEY-DalTradwn Rocks of the Sperrin Moutntains. 167

In the opinion of the author the features seen in the conglomerates

of Oughtnager correspond very closely to those described from Kilmory Bay, and are therefore perhaps worth recording in default of more definite

evidence. It must be admitted, however, that Dr. Bailey attaches little

weight to this particular exposure. Such a conclusion, if correct, would place the Black Schists of

Broughderg as the youngest, the Newtownstewart Quartzites as the oldest members of the Sperrin Series, and would mean that the succession

deduced from the general order of superposition is an inverted one. Only

locally are the beds in their correct stratigraphical position. This conclusion as to the actual succession would be in agreement with

the views of Dr. E. B. Bailey (2, p. 101) as to the structure of the equivalent

beds in Scottish areas. It is opposed, however, to the order held to be

correct by A. Geikie, G. Barrow, J. W. Gregory, and other workers.

16.-THE RELATION BETWEEN GEOLOGICAL STRUCTURE AND PHYSICAL FORM.

From what has been already mentioned it will be realised that the

Sperrin area consists, broadly eaking, of a series of beds differing in

composition, and therefore in ldness, which outcrop in a series of

concentric arcs which swing round rom a south-easterly trend on the

east to a south-westerly one oni the wet, and that the beds have a general

inclination towards the north or north- est. Two important radial depressions hay opened out the lines of attack

to the forces of river denudation and along these lines the two post

cretaceous dip rivers the Owenreagh and the Roe have cut deep salients,

and paying, as such rivers do, but little attention to the characters of the

rocks which they traverse have somewhat obscured the main structural lines.

In the development of their strike tributaries, especially the rivers of

Glenelly, Glenlark and Coney Glen, however, we find a more marked

relationship between structure and form. These rivers have picked out

the softer and more micaceous horizons amongst the schists, and their

courses follow very closely the line of strike of these particular beds. The

hard schistose grits and massive and tough hornblendic epidiorites are

carefully avoided and remain to build up the prominent ridge of tlle main

Sperrin range.

The relationship is not complete, but it is evidently progressing rapidly.

In traversing the streams it is frequent to find them following the strike

of a specially favourable horizon for considerable distances and then,

when they lose it, taking a sudden jump across to return to their original

foundation.

168 Proceedings of the Royal Irish Academg.

The feature of dip slopes and escarpments also shows signs of development. The watersheds of both the Sperrin Mountains and the

Munterloney Hills lie closer to their southern than their northern boundaries. The southern slopes are in each case the steeper, less drift covered, and more sharply incised.

At the northern boundary of the area mapped the development of strike features is again reflected in the topography. The grits and epidiorites are s4cceeded by the soft and easily denuded limestones, the course of which is represented by the low-lying ground occupied over

much of its length by the Inver River. Further north again the lower

Quartzitic Group gives rise to a ridge of higher ground, which includes

the thousand-foot rounded summits of Crockdooish, Slieveboy, and Straid.

17.-THE METAMORPHISM.

The general metamorphic position of the intrusive epidiorites having been already discussed on page 160, it is only necessary to refer to the

position of those schists which were originally of detrital origin. The greater part of the area occupied by the detrital schists lies within

the biotite zone of metamorphism, but garnets are developed in two distinct areas, the boundaries of which are marked by small crosses on Plate VIII.

In the first place garnets occur along a belt about a mile and a half

wide, the centre of which coincides roughly over the greater part of its

length with the main line of outcrop of the Green Beds. At its south

western and north-eastern ends, however, where the effects of pitch become apparent in diverting the outcrops of the stratigraphical divisions towards the south, the metamorphic isograds continue their original directions almost without change.

Along the centre of the belt is an oval shaped area, the rocks of which

lie within the biotite zone. Further south another garnetiferous locality occurs forming a triangular

area about five miles in length between Mountfield and Greencastle. This

area is bounded-to the south by the main boundary fault.

The garnets of the northern belt are always small pinkish garnets.

They never reach a diameter of more than 0 5 mm. and are as a rule

about half that size.

Since they are so small as to always require microscopic examination

for their determination, and since, in addition, the exposures are comparatively few, the line marking the garnet isograd must of necessity

be here somewhat tentative, but sufficient slides have been examined to

ensure that the main outline is correct.

In the Mountfield area the garnets are much larger, sometimes 20 to

30 mm. in diameter, frequently margined with a broad chlorite rim, strong

evidence of considerable retrograde metamorphism.

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 169

The question has been considered as to how far the above described distribution of the garnets supports the views put forward by Elles and Tilley (16 and 17) as to the structure of the varying grades of regional

metamorphism. These authors have suggested that in North Kintyre the garnet isograd forms, like the beds, a recumbent fold closing towards the south and containing towards the north a core of higher metamorphic grade. (Cf. 16, Fig. 2, p. 629.) In the Cowal area (16, Fig. 1, p. 627)

only the lower edge of the garnet zone comes sufficiently low to touch the surface, so that this particular belt is represented by a thin veneer of garnetiferous material resting on less metamorphosed sediments. The zones like the beds (?) are claimed to be inverted.

The conditions shown in Cowal would appear to be possible in the case of the Sperrins. Taking a general view of the most northerly of the garnetiferous belts it is seen to correspond roughly with the southerni slope of the main Sperrin range. It occupies high ground, but the garnets are always small. They show no signs of increasing size with increase in the depth of the sections, but on the contrary tend to disappear, and to be

replaced by biotite. In the centre of the belt also the garnet band is thin and discontinuous

and broken up by lenticles of biotite grade. It is only towards the ends

where the axial depression (equivalent to a topographical elevation of the ground) comes into play that the garnetiferous area is broader and more continuous.

The southern garnetiferous area around Mountfield is, however, in a different category. It is almost everywhere confined to low ground and its outcrop forms a broad V running down the valley of the Owenreagh.

When the beds are traced along the strike in either a north-west or south

east direction we pass into garnet free rocks lying at a higher topographical

level. Here the garnet zone cannot represent a simple veneer, but it

must underlie the biotite schists unless it has been faulted or folded down

to its present position. This is a possibility, for the area it occupies is in

intimate association with a Carboniferous outlier lying to the south-east

dropped down amongst older rocks. It might also represent an overfolded

"outlier" of this particular zone, or a narrow wedge lying along the

boundary fault which has been overthrust from the north-west by schists

of a lower grade.

Nevertheless the simplest view of the character of the metamorphism and the one that most readily meets the facts observed in the field is that

it increases, not decreases, in intensity with structural depth.

18.-SUMMARY AND CONCLUSION.

The present communication establishes a stratigraphical succession for the, Dalradian rocks of the Sperrin district in Northern Ireland. Although the boundaries of the various divisions are in some cases not easy to fix

170 Proceedings of the Royal Irish Academy.

exactly owing to complication by minor folding with the development of

impersistent lenticles, or to local lack of exposures, it is believed that a

sufficiently definite succession has been established to enable the main structure of the area to be followed from an examination of the map, and at the same time to permit f detailed correlation with other districts on

the same stratigraphical horizo

A detailed correlation with e Dalradian rocks of Antrim and the

south-western portion of Seotlan is suggested and the general structure

briefly described. The thanks of the author are due to Dr. E. B. Bailey and Dr. W. J.

MeCallien for useful discussion, and to the former for his helpful criticism

in the field, also to Professor J. K. Charlesworth for reading through the

mss. and for help in other ways. To Mr. P. Clarke, of Gortin, the late

M. A. Robinson, and the engineering staff in charge of construiction of

the Londonderry waterworks, to Mr. G. Barnett, of Sixtowns, and to many farmers, landowners, and residents in the district he is indebted

for local aid and information. To Miss W. J. Sayers and Rev. E. M.

Gumley he is obliged for facilitating certain features of the above

investigations when the time available was somewhat limited.

19.?List of Works to which Reference is made.

1. Bailey, E. B., and McCallien, W. J.?"The Metamorphic Rocks of

North East Antrim." Trans. Roy. Soc. Ed., vol. lviii (1934), pp. 163-177.

2. Bailey, E, B.?"On the Structure of the South-West Highlands of

Scotland." Quart. Jour. Geol. Soc, vol. lxxviii (1922), pp. 82-131. * 3. McCallien, W. J.?"The Metamorphic Rocks of Kintyre." Trans.

Roy. Soc. Ed., vol. lvi (1929), pp. 409-436.

4. Kinahan, G. H., and Others.?Mem. Geol. Surv. Ireland. Sheet 17

(1889). 5. Nolan, J., and Egan, F. W.?Mem. Geol. Surv. Ireland. Sheet 18

(1884). 6. Wilkinson, S. B., and Others.?Mem. Geol. Surv. Ireland. Sheet 25

(1887). 7. Nolan, J.?Mem. Geol. Surv. Ireland. Sheet 26 (1884). 8. McCallien, W. J.?"The Green Beds of South Knapdale." Geol.

Mag, vol. lxx (1933), pp. 156-167.

9. Clough, C. T.?"The Geology of Cowal." Mem. Geol. Surv. Scotland

(1897). 10. Phillips, F. C.?"The Green Beds of the Scottish Dalradian." Min.

Mag., vol. xxii (1930), pp. 239-256.

11. Kynaston, H, and Others.?"The Geology of Oban and Dalmally."

Mem. Geol. Surv. Scotland. Sheet 45 (1908).

HARTLEY-Dalradian Rocks of the Sperrin Mountains. 171

12. Peach, B. N., and Others.?"The Geology of Knapdale, Jura, and North Kintyre." Mem. Geol. Surv. Scotland. Sheet 28 (1911).

13. Hill, J. B., and Others.?' ' The Geology of Mid Argyll." Mem. Geol.

Surv. Scotland. Sheet 37 (1905). 14. Peach, B. N., and Others.?"The Geology of the Seaboard of Mid

Argyll." Mem. Geol. Surv. Scotland. Sheet 36 (1909). 15. Wiseman, J. D. H.?"The Central and South-West Highland

Epidiorites" Quart. Jour. Geol. Soc, vol. xc (1934), pp. 354-417.

16. Elles, G. L., and Tilley, C. E.?"Metamorphism in Relation to

structure in the Scottish Highlands." Trans. Roy. Soc. Ed., vol. lvi

(1930), pp. 621-646.

17. Tilley, C. E.?"Metamorphic Zones in the Southern Highlands of

Scotland." Quart. Jour. Geol. Soc, vol. lxxxi (1925), pp. 100-112.

18. McCallien, W. J.?"A Contribution to the Correlation of the

Dalradian Rocks of Scotland and Ireland." Geol. Mag., vol. lxviii

(1931), pp. 126-133.

19. Hartley, J. J.?"The Geology of North-East Tyrone," Proc Roy. Irish Acad., vol. xli (1933), pp. 218-285.

20. Barrow, G., and Others.?"The Geology of Blair Atholl, Pitlochry and Aberfeldy." Mem. Geol. Surv. Scotland. Sheet 55 (1905).

21. Hinxman, L. W., and Others.?"The Geology of Corrour and the

Moor of Rannoch." Mem. Geol. Surv. Scotland. Sheet 54 (1923). 22. Harker, A.?"Metamorphism." London (1932). 23. McCallien, W. J.?"A Note on Dalradian Pillow Lavas." Proc.

Roy. Irish Acad., vol. xliii (1936), pp. 13-22.

24. Elles, G. L.?"The Geological Structure of Ben Lawers and Meall

Corranaich." Quart. Jour. Geol. Soc, vol. lxxxii (1926), pp. 304

331.

25.?Portlock, J. E.?"Report on The Geology of Londonderry and of

parts of Tyrone and Fermanagh." Dublin (1843).

Paoc. R. I. ACAD., VOL. XLIV, SECT. 1B. P'LATE V.

(Photo, J.J.J!.)

FIG. 1.-Quairtzites of the Newtownstewart GIroup withl tlinii ban(ds of micaceous and

black sclists. The hammer is restiing on the Ihigher of the two mica bands

shown. The exposuIre occurs about two miles north of the Butterlope Pass

anid soutlh of Dunnamanagh.

'P6

(Photo, R. J. Welch.)

Fi: 2. Epidiorites of Craig slhowing pillow strueture and the sponigy naturie of tlle

se(liments ( ) between the pillows.

HARTLEY: DALRADIAN Ro(Ks ;OF SPEUIZINS.

PROC. R. I. ACAD., VOL. XLIV, SECT. B. PLATE VII.

(Photo, J.J.II.) FIG. 1.-Coarse grained schistose grits of the Dart Group as exposed near Mullaghcarn.

The sub-angular fragments weathleiring out are white quartz and aver age half an inch in diameter.

(Photo, J.J.H.) FIG. 2.-Junction of transgressive sill of epidioi ite (below) and bedded limestone of

the Dungiven Group (above). Old Piriory Clhurch, Dungiven, Co. Derry.

HARTLEY: DALRADIAN ROCKS OF SPERRINS.

Pioc. R. I. ACAD., VOL. XLIV, SECT. B.

Geological Map of

THE SPERRIN AREA NORTHERN IRELAND

by

J. J. Hartley. M Sc.

Dunnamanagh

Altigarvan

A

Strabane

Dung'iven

Craignagapole1 ~~Slileveboy\\

\ \\ \

-A T

C roc h 8 o o i s ~~~~~~~~~~~~~~~~~~ ~~Mllghnen

raig~~~~~~~~~~ic re

n%,Cr R;%-cr~r

PLATE VIII.

Dung'iven /,vr I,dv ,' / \

Le

Edenbridg6

CU. N.

Geological Map of

THE SPERRIN AREA NORTHERN IRELAND

by

J. J. Hartley. M Sc.

Dunnamanagh

Altigarvan

A

Strabane

PlumblfB

\~~~~~~~\\

Mary\

Newtowustewat~

2 ?essy<,,Bell -

Dung'iven

0

~~~~~~~~~~~~~~~o

<-/' \

- raig -- - __ ~g lwullayhaneany

-~~~ tuager - ~ ~ ~ ~ ~~M% T-VIulaMU1oghan

-- ~~~' ~~' \Carnanell~~~00'awe

\ ~ ~ ~ ~ ~ ~ ~~~~~~' ~ ~

OWez2Lllle Owegerkllaclogrugien

\~~~~~~~~~~~~~~~~~~~A

9~~~~~~~~~~~~ulgat AV ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ i- - %

710 r~~~~~~~~~inagramtcScinaogteln

;.-' r-r'4'

Dungiven /.r

S:

Y.,~~~~~~~~~~~~,

< T > ~~~~Margin of Garnetiferonls Areas marked thus xxxx.

INDEX.

B?laclc Schist of Broughderg.

Alullaghcarn Schists.m

1 oun-naline Schists of Glenelly South, ]X

(;reen Beds.|_

1Tourmaline Schists, of Glen,tlv Nortb,

SChistose G rit Gr-oup of Dart.

I-'illow lavas anid E;pidiorites.(extrusive) |

rDungiven Limestone Group.

Newltownstewvart Quartzitic Group. J,~

, ?,-. I~~~r.triisive Epidorites.

Quiartzite Bands.

I mestones. 12~\

Felspathic Grits.b.*.g.

XNw I' / \\\~~~~~~~~~~~~~lv

,Mary\

Orn2agh

OWe~~~~~~LiJj " '~~~~~~~~~~~" ~~~~~' ~ ~ ~ ~ .- ----

Mu11a~ ~ ~~~~Sal Mle

HARTLEY: DALRADIAN ROCKS 'OF SPERRINS.

INDEX.

Blackc Schist of Broughderg. -

AMullaghcarn Schists.

1Tounrnaline Schists of Glenelly

South,1 Green Beds.

'I 'ourmaline Schist-s of Glettl1v Nnrtbi,1

SChistose Grit Group of Dart.

I-'illow lavas and Epidiorites.(extrusive)

Dungiven Limestone Group. I

Newtownstewart Quartzitic Group. ___

lr.trnisive Epidorites.

I/hIIh| Qiiartzite Bands.

I mestones.

Schistose Grits. I I

Felspathic Grits.

Albite Schists.