The heavy minerals in the sandstones of the scottish carboniferous rocks

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  • PROC. GEOL. Assoc., VOL. XXIV. P LAT E 10.

    t' pper Keupe r Sand..tone..--- - - - -'-- - - --.,

    Carboniferons S

  • 57

    THE HEAVY MINERALS IN THE SAND.STONES OF THE SCOTTISH

    CARBONIFEROUS ROCKS.By T. O. BOSWORTH, B.A., B.Sc., F.G.S.

    (Read December 6th, 1912.)

    INTRODUCTION.

    A FEW years ago the writer commenced an investigation ofthese sands, hoping by the study of their distribution, thethickening and thinning, the current-bedding, and the mineralscontained, to learn something as to the source of the sedimentsand the manner of their accumulation. A mere beginning wasmade, but there being now no opportunity to complete the work,such facts as were determined are here put on record.

    About forty sandstones were examined, the rocks selectedvarying from coarse grit to fine sand, some of them being well-known building stones. The specimens were from various parts ofthe Midland Valley and from horizons in all parts of the Carbon -ferous Series, so that both lateral and vertical distribution couldbe studied.

    GENERAL CHARACTER OF THE HEAVY MINERAL CONTENTS.

    The most abundant heavy minerals separated from thesand by heavy liquids were: Garnet, zircon, magnetite, tourma-line, rutile, staurolite, anatase and barytes. An examination ofthese minerals revealed wide differences between the varioussands of the formation, an excessive amount of garnet in manyof them, and marked angularity of the grains.

    Drawings of grains of most of the minerals found are givenin Plate 10, and a series of grains of similar minerals fromthe Upper Keuper Sandstones of Leicestershire is shown on thesame Plate for comparison."

    In general, amongst heavy-mineral grains, degrees of angu-larity or smoothness are much more perceptible than with quartzgrains; thus, for instance, on comparing the Carboniferous sandswith sands from deserts, a far more conspicuous difference isobserved in these grains than in the grains of quartz. Attentionis particularly called to the characters of the garnets describedbelow.

    DESCRIPTIONS OF THE GRAINS.

    Garnet.-In some of the sands no garnets, or only anoccasional one, could be found; but more often garnet forms thegreater part of the heavy-mineral content, and in certain cases is

    .;: See" The Keuper Marls around Charnwood," by T. O. Bosworth. Leicester, 1912.Also Qllart. [ous n, Gecl, Soc., vel, Ixviii (1912), P. 28,.Pxoc. Gsor.. Assoc., VOL. XXIV, PART 2, 1913.]

  • 58 T. O. BOSWORTH ON THE HEAVY MINERALS IN THE

    present in such quantity that a heavy-mineral separation containsbut little else.

    These grains are peculiar in their angularity-presumably theeffect of transportation in water-for the dodecahedral cleavagehas been developed out to such a degree that the grains haveelaborate zigzag shapes, with innumerable sharp edges, anglesand re-entrant angles, repeated all over them.

    Plate II shows micro-photographs of some of these garnets, butthey do not fully exhibit the angularity of the grains, because onlya small portion of the surface of the grain can be in focus at once.Angles, such as those here focussed on the periphery, are foundall over the grain, and come into sight and disappear successivelyas the focus of the microscope is gradually changed from thebottom to the top of the grain.

    Zircon (see Plate 10) is always present, generally plentiful, andcommonly forms a large proportion of the heavy-mineral content.It occurs mainly as whole crystals or elongated grains formedfrom complete crystals. Many of the crystals are almost perfect,and are hardly worn at all, with edges sharp and pyramids atboth ends unbroken, but there are some grains which may bedescribed as worn prisms. There is a wide range in size and shape,even in one sand, some crystals being long and slender, andsome (generally less numerous) short and stout. Thoughmost are colourless, some have a pale pink colour, and .occa-sionally greenish yellow and faint mauve-coloured grains occur.

    ;Magnetite (see Plate 10) is always present, and sometimesabundant, mainly as irregular shaped grains which are frag-mental, but occasional grains show the octahedral form. It isthe most readily rounded of all the grains here described.

    Rutile (see Plate IO).-The deep amber-coloured rutile isgenerally abundant, and the golden yellow variety also is present,though in much smaller amount. Many crystals are almostentire and unworn, the shape varying from stout and stumpyto the less numerous slender prisms. Other grains are angularand subangular, being derived from broken crystals. There areoccasional geniculate twins.

    Sometimes there is much deep coloured rutile in the form ofprismatic stick-like pieces, which have irregular sides and nowell-developed faces, nor do they bear the pyramids at their ends.Evidently these are fragments broken off from the large crystalsor crystalline masses.

    Tourmaline (see Plate 10).-The ordinary brown tourmalineis generally abundant, and blue and greenish grains also arefairly numerous. The majority of the grains are angular, andusually are almost complete prisms, or pieces derived from com-plete prisms by cross fracture'. On a fair proportion of piecesthe pyramid is included. Much-worn grains, however, are by nomeans rare.

  • PROC. GEOL. Assoc., VOL. XXIV.

    Crush d Ga rne t X 30.

    PLATE I I

    Garnets fro m S cor rish Ca rbon iterous Sandstone .x -t3 )(. JO~_ _

    Gar ne ts (rom Up per Keu per Sandstone . Lei cest er.x ::;0.

    - - - - - --------- - - - --

    F]{AGMENTS AND GRAINS OF GARNET.-T. O. Bosuorth, Microphoto.

    To face page 58.

  • SANDSTO:ilES OF THE SCOTTISH CARBONIFEROCS ROCKS. 59

    Staurolite (see Plate IO) is not plentiful in the sands examined.The grains observed are extremely angular, and have their edgesragged or "toothed." Staurolite in the state of sedimentarygrains apparently gives way readily along the cleavage, so thateven the much-worn grains show a frayed margin. In thesesands the outline of the grains is irregular and the" teeth" arelarge.

    Anatase (see Plate 1:2 ) is only occasionally present, but it isnoteworthy that when present it is usually rather plentiful. Itoccurs in well-formed plates of steel-blue colour.

    Barytes (see Plate 12 ).-This, when present, is in suchquantity as to mask the occurrence of the other heavy minerals.It is almost certainly a cement, although some perfect tabularcrystals are obtained in addition to the cleavage fragments.

    I have found this mineral present in several good building-stones of various ages. It is often first detected in a sandstoneby the labour expended in crushing.

    SOURCES OF SEDI~IENTS.

    The heavy-mineral contents of the different sands were foundto differ widely, indicating considerable variety of sources forthe sediment. Thus, for instance, in some sands rutile is con-tained in small quantity, but in others it forms an importantfraction of the total amount of heavy grains. In somesands the rutile occurs as grains, or more or less worn crystals,but in others it is present as rod-like fragments (e.g., in the sand-stone of Blochairn Quarry, Glasgow).

    Similarly tourmaline, though generally in rather small amount,is occasionally one of the main constituents (e.g., in the sandstoneat Levenseat).

    Zircon is usually either first or second in order of abundance,but in some sands the amount is comparatively quite small.But the most noticeable difference between the sands is in respectof the garnets, for whereas in some sands garnets arc scarce orabsent, in others they arc so abundant that all other heavy grainsseem few and far between.

    Thus it seems probable that sediments were being pouredinto the basin from a number of different sources.

    Attention was directed chiefly to the garnets, for by thismeans the sands are easily divisible into at least two kinds,which must have been derived from areas of entirely differenttype.

    (I) Sands containing much garnet.(2) Sands containing very little, or no garnet.

    From the abundance and unworn character of the garnetgrains we may infer that the garnet-sands were derived from the

  • 60 T. O. BOSWORTH ON THE HEAVY MINERALS IN THE

    neighbouring Highland schists to the north and north-west of thebasin, whilst the sands almost devoid of garnets were probablyintroduced from the north-east, east, and south.

    In the following list the sands examined are placed in strati-graphical order. The garnetiferous sands are indicated by thesign" 0," and those which are non-garnctiferous by the sign "x."

    RED MEAse RES.o Rutherglen.

    COAL MEASURES.o Cambuslang .o Chapelhall, Shettsburno Chapelhall, Shettsburno Chapelhall, Shettsburno Fauldhouse Quarryx Chapelhall, Shettsburn

    MILLSTONE GRIT.x Bilston Burn, near Edinburghx Glasgow, Blochairn Quarry .x Muirhouse Bore, Lanarkshire x Muirhouse Bore, Lanarkshire .x Levenseat . x Balfour Bore, Fife, 129 fms,

    deep .

    o Bilston Burn

    Above Humph Coal.24 ft. above Lower Drumgray Coal.Just above Lower Drumgray Coal.Just below" " "Above Crofthead 4 ft. Coal.Below the Coals, faulted, and near

    Millstone Grit.

    Roslin Grit.Above Fireclays.Depth 30 ft.Depth [74 ft.Above Curdley Ironstone.

    12 ft. above Levenseat Limestonehorizon.

    6 ft. above Castle Carey Limestonehorizon.

    "

    CARBONIFEROUS LIMESTONE SERIES.

    o Balfour Bore, Fife, 264 fms.deep .

    o Bilston Burn. . .o Balfour Bore, 347 fms. deepx Giffnocko Balfour Bore, 382 fms. deepo Bilston Burn .o Bishopsbriggs, Hunters' Hillx Kirkintilloch o Linlithgow x Bilston Burn .x Balfour Bore, 540 fms. deepo Balfour Bore, 614 fms. deepx Balfour Bore, 636 fms. deepo Bilston Burn .x Bilston Burn

    CALCIFEROUS SANDSTO:

  • P ROC. GEOL. Assoc., VOL . XX IV. PLATE 1 2

    [T. O. B osw orth, M icropl!Oto.

    A-A NAT ASE (x 40) OUT OF A SANDST O" E FROM T H E B ALFOU RBORE, FIFE.

    X 40

    X 50

    [T. O. Bo sworth, M icroph oto .

    :B .- B A RYT ES OUT OF A SAN DSTON E FROM THE B AL FO U R BORE, F IFE.

    T o face pa ge 60.

  • SANDSTONES OF THE SCOTT ISH CARBONI FER OUS ROCKS. 6 1

    LATERAL AND VERTICAL D ISTRIBUTION .

    It seems likely that garnetiferous sands were deposited in theone part of the basin contemporaneously with the deposition ofnon-gametiferou s sands in other par ts. There is not, however,yet sufficient evidence collected to show to what extent thi s isth e case, but the observations on the sands of th e Carb oniferousLimeston e Series show that at any rate both kinds of sand existon near horizon s in that series.

    With regard to vertical distribution also the present amountof evidence is too small to be anything more than suggestive.

    In the R ed M easures and Coat Measu res.- Of the sevenspecimens examined all are highly garne tiferous save one, andin that one case field evidence was ind ecisive as to wheth er thesand belonged to the Millstone Grit Series or to the Lowest CoalMeasures.

    In the Millstone Grit.-Of the seven specimens all save oneare non -garnetiferous, and that one is within 6 ft. of the horizonselected as the base .

    In the Carboniferous L imestone Series.-Of the fifteen speci-mens nine are garnetiferou s and six are non-garnetiferous ,

    I n the Calciferous Sandstone Series.-Of the four specim ensall are non-garn etiferous.

    C ONCL USION S AND SUGGESTIONS .

    The sands cont aining such an extraordinary quantity ofang ular garnet have presumably heen derived from the H ighla ndschis ts to the north and north-west of the basin, whilst the sandsdevoid of garnet are likely to have come from the north-east, east ,or south .

    It may be possible, by a study of the heavy mineral grains andof current-bedding, and the thickening and thinning of the beds,to subdivide the whole of the Carboniferous accumulation into anumber of great lens-shaped or wedge-shaped bodies of sedimentwhich have been introduced from various direction s, and areinterdigitated in a complex manner. These great lenticles mightbe expressible on maps, and might be helpful in explaining thelateral changes and th e distr ibut ion of the coals.

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