RADIOACTIVITY AND THE MEASUREMENTOF GEOLOGICAL TIME.

By ARTHUR HOLMES, A.R.e.S., D.Le., B.Sc., F.G.S.

[Read May 7tft, 19/5.]PAGE

I.-INTRODUCTION 289

I I.-RADIOACTIVITY 290

III.-RADIOTHERMAL ENF:RGY AND THE COOLING OF

THE EARTH • 293IV.-THE ACCUMULATION OF HELIUM 294

V.-PLEOCHROIC HALOES 297

VI.-THE ACCUMULATION OF LEAD 298

VI I.-BIBLIOGRAPHY . 308

I.-INTRODUCTION.

RAD IOACT IVIT Y is a science of fundamental importance,. not only on account of the searching light which it throws

on the constitution of matter, but also because it offers to US, asgeologists, new data and new methods with which to attack someof our most difficult problems. It has gone far in revealing tothe physicist the internal structure of the atom, and just as surelyit helps to disclose the hidden architecture of the interior of theearth. Radioactivity deals intrinsically with the disintegrationof certain unstable elements, and the rate of their disintegrationserves as a clock with which to measure geological time. ] ust asa clock is maintained by the energy of a spring which slowlyunwinds itself, giving out its energy as it runs down, so the radio­active elements are maintained by the internal energy of theiratoms, and, as they decay, that energy is slowly given out in theultimate form of heat. But while a clock records time in ticksthat are heard and pass away, the radioactive elements areengaged, after the manner of an hour-glass, in keeping a morematerial register of the passage of time. The unstable elementssteadily evolve end products such as helium and lead, and sincethese are stable, they gradually accumulate. It is in these tworespects, the constant emission of heat and the slow accumula­tion of the end products, that radioactive phenomena have aspecial significance for geologists.

Professor Joly has shown that the thermal effects of theradioactive elements (which are everywhere distributed throughthe rocks of the earth's crust) playa critical rOle in determiningthe location and upheaval of mountain ranges. A study of thedistribution of the radioactive elements in rocks and meteoritesleads directly to a revision of earlier ideas regarding the heat ofPROC. GEOL. Assoc., VOL. X XVI, PART 5, I915.J 2 [

ARTHUR HOLMES ON"

the earth, and this in turn suggests important conclusions as tothe structure of the interior of the earth, the evolution of the" crust," and the genesis of igneous rocks. In th is paper thesefar-reaching questions cannot bt: even touched on. Attentionwill be restricted to those phases of the subject which bear onthe measurement of geological time. T he methods which havebeen developed with this end in view are four in num ber, and arerespectively based on :-

(a) The effect of radiotherrnal energy on the coolingof the earth;

(b) The accumulation of helium in radioactive minerals ;(c) The depth of colour of pleochroic haloes ; and(d) The accumulation of lead in radioactive minerals.

II.-RADIOACTIVITY.

In r896, Becquerel made the remarkable observation thaturanium salts and minerals give out invisible rays which arecapable of penetrating black paper, and revealing themselvesby their effect on a photographic plate wrapped within it. Mme.Curie followed up this discovery by systematically examiningother elements, and she was soon able to announce that thoriumalone. among the then known element s, possessed rad ioactiveprop erties similar to those of uranium. Pitchb lende, however,was found to be far more powerful in its emission of rays thancould be explained by its uran ium conte nt, and this curiousdiscrepan cy led to the discovery of radium. To-day we knowof the existence of over thirty radioactive elements, all of whichcan be genetically grouped int o two families,* the respectiveparents of which are uranium and thorium.

Th e spontaneous rad iations of these elements, known inhono ur of their discoverer as the Becquerel Rays, were foundby Sir E. Ru therford to be of three types, which he named n-,13-, and y-rays.

a-rays are positively charged atoms of helium, explosivelyemitted with velocities approaching one-tenth that of light .

,B-rays are nega tively charged electrons, having a mass ofabout r/[,250 that of a hydrogen atom, and expelled even moreswiftly than the a-parti cles.

y-rays are not material particles in rapid motion, but appearto be of the same nature as X- or Rontgen-rays.

The disintegration of uranium is illustrated in the adjoin ingdiagram, in which the daughter elements are placed in suc­cessive columns of the Periodic classification accordin g to theirche mical properties. It will be noticed that the expul sion of an

* Actinium is the pare nt of a sub-family which branches from the uranium family ,and rep resents about 8% of the whole.

RA

DIO

AC

TIV

ITY

AN

DM

EA

SU

RE

ME

NT

OF

GE

OL

OG

ICA

LT

IME

,2

91

::0 ;.. t:l <3

Go

ld1

97

'2r:

Me

rcu

ry2

00

'6-

-\r:

1®

-T

ha

lliu

m20

4-- :-

'\.

~~

e~

-L

ea

d2

07

'1<

1l;)

~

""'-

-1

I

~~

::J:t

lB

ism

uth

20

8:"

III"

'<'<

""'-

-(1

1<I

I

@~

33

08

<""

,.P

olo

niu

m""

,.:-

<ll

<II

r--C

l.

~

[Io

din

e]

< r:

®R

a,E

ma

na

tio

n9

[Co

osi

um

]r:

CDR

adiu

m2

26

'2- r: -

[La

nth

an

um

]- r:

0CD

Th

ori

um

23

2'4

- ~\ .le

. 0[T

an

talu

m]

:"-c

....\

~~e

CDU

ran

ium

23

8'2

<0)=

r:'

c~3

[Iod

ine

]< - :-

ARTHUR HOLMES ON

a-particle, or helium atom, causes the resulting element to moveits position in the table, by two places to the left. When a {3-raychange takes place, the resulting element moves one place inthe opposite direction.

After three helium atoms have been discharged in the mainline of descent from uranium, radium is reached. At first, radiumand its companions are generated more rapidly than they decay.They therefore increase in quantity relative to a given mass ofthe parent uranium. The rate of disintegration is, 'however,directly proportional to the quantity present, and thereforeaccumulation only proceeds until the amount which decaysexactly equals the amount generated. Once this equilibrium isestablished there is a constant ratio between the amount ofuranium and the amount of anyone 0 the unstable daughterelements. The ratio of uranium to radium, for example, is3,000,000 to one. Radium in turn spontaneously disintegratesinto radium emanation, liberating another helium atom in theprocess. The emanation is a gaseous product which gives riseto a further succession of changes until finally a stable endproduct is reached in the main line of descent. This endproduct naturally falls into the space already occupied by lead.

When the whole family has fallen into equilibrium, eachatom of uranium which disintegrates implies the disintegrationof an atom of each of the daughter elements, and the generationof an atom of the stable end product, lead. Laterally, stableatoms of helium are given off at eight stages. *

An atomic equation representing the ultimate change wouldbe as follows :-

Ra. + 3 He.226'2 + 12------r­

238'2

Pb. + 8 He.206'2 + 32-----....,....---­

238'2

Thorium disintegrates through a long series of elements in amanner closely analogous to that of uranium, and gives offhelium atoms at six stages. In this case, however, the identity ofthe end product in the main line of descent is still unrecognised.

In 1903 a startling announcement came from the Curies'laboratory in Paris. It had been discovered that radium wasconstantly giving out heat, and since then it has been ascertainedthat many of the other radio-elements share the same property,though in varying degrees. The source of this heat is to befound mainly in the expulsion of helium atoms with an averagespeed of 12,000 miles per second. A helium atom with such avelocity may be said to have a temperature of 6S,ooo,000,ooooC.

• Radium C breaks down into radium D in two ways, part giving an a-ray and thena ~-ray, part giving a ~'ray and then an c-ray. Whichever road is taken, each atom of Cbecomes transformed into D j with the accompanying liberation of one helium atom.

RADIOACTI VITY AN D MEA SU R E ME NT OF GEOLOGICAL TIME. 293

The total generation of heat from one gram of uranium when inequilibrium with all its pro ducts amounts in one year to 6'5calories, In itself this is a trifling quantity. Its real meaningcan only be grasped when the total amount of uranium in theearth is taken into account, and when it is realised that out of7 ,000 million atoms of that uran ium, only one disintezrat esdu ring a single year. e

What is the effect of this slow but never failing supply of heaton the earth ? The earth is known to be losing heat, for itstemperature gradually increases in depth when the crust is pene­trated by mines and boreholes. Kn owing (a) the average ratea t which the temperature increases, de/dx ; (b) the average rate atwhich rocks conduct heat , k j and (c) the area of the ear th 'ssurface, A; it is easy to calculate the total quanti ty of heat,Q, which is lost from the earth's interior, for Q = k-A» dejdx.The question then arises: How much uranium would be neces­sary to provide a heat income sufficient to maintain th is expenditure? It will be found on calculation that a distribution 01

uranium throughout the earth amounting to five parts in 100

million parts of rock would just suffice to maintain a balancebetween heat gained and heat lost.

III.-RADIOTHERMAL ENERGY AND THE COOLINGOF THE EARTH.

This suggest ion was made by Rutherford in 1905 , and thefollowing year it was directly tested by Stru tt, who had alread ydevised an accura te method for determin ing the small quantitiesof uranium held by common minerals and rocks. He found thatthe rocks contained much more uranium than was necessary.Hi s work has been con tinued by j oly and others, and as a resulto f the examination of rocks of all kinds and ages taken fromvarious representative par ts of the world, the distribution of theradioactive elements is now well kno wn. The average amount ofuranium in igneous rocks is found to be 600 parts in 100 millionparts of rock, i.e., there is 120 times more than is required. Butthe discrepancy is worse than thi s. The thorium series isresponsible for even a greater emission of heat than th e uraniumseries, and finally, takin g all the radi o-elements together, we arriveat the conclusion that the rocks appear to be 300 times richer inthese elements than thermal equ ilibrium demands.

It is impossible to believe that heat is generated within theearth 300 times more rapidly than it is lost! The difficulty canbe avoided only by assuming that the earth's interior is not rad io­active, i.e., either (a) the radio-elements must be actually con­centrated in the earth's crust, leaving the interior free from them ;or (6) if uranium and thorium are present at great depths, the

ARTHUR HOLMES ON

high temperatures and pressures of those regions must inhibittheir disintegration and evolution of heat.

The second alternative can at once be ruled out. Tempera­tures up to 2,50ooC, and pressures up to 160 ton s to the squareinch-conditions which cannot be generally reached within theearth at depths less than 50 miles-leave the radioactive pheno­mena quit e unaffected. It is therefore safe to assume that withinthe outer 50 miles heat is generated as readily as it is in the labora­tory. Rut even th is outer zone cannot be as rich in the rad io­elements as th e surface rocks, for if it were, there would still befive times more heat evolved than is wanted. It follows con­clusively that the rad io-elements are spec ially concentra ted in thesurface rocks, and that they gradually become less abundant, andfinally die away, in depth.

Fo r our pres ent purp ose it is more important to notice thebearing of radioactivity on the method of estimating the age ofthe earth originally developed by Kelvin. Kelvin's later treat­ment of the problem indicates that if the earth had cooled downfrom a molten state, it would have arrived at its present con­dition within about 20 million years. Now, since heat is con­stantly being generated by the radio -elements, it becom es clear,eithe r that the earth is not cooling down at all, or that it iscooling down very slowly. Let us see what the effect act ually is,on the assumption of cooling from a surface initi ally molten :

P roportion 0./heat lostattributed tv radioactiuity,

°1/+ or 25%1/2 or 50%~ /3 or 67%3/4 or 75%4!S or So91

Pe riod ofcoolillg Ylquired toreach present conditions,

co million years4°

120SSo

1,6007,200 "

The abo ve figures show how rapidly the " age" increases asthe influence of radioactivity in mainta ining the earth 's heat isgiven greater weight. It is very difficult to believe that less thanthree-quarters of the ear th's heat is supplied by radio thermalenergy, and we are therefore prepared for very much higherfi gures than have previously been accepted. If the assump­tion of an initially molten surface is made, then the earth's agemay be 1,600 million years or more. If, for other reasons,an age of that order be accep ted, then radioactivity does not,as was originally supposed, imply that the earth can never havebeen molten.

IV.-THE ACCUMULATION OF HELIUM.The proof is complete tha t helium is a stable product of the

decay of both uranium and th orium. It is always present inrad ioactive minerals. In 1903 Ramsay and Soddy demon-

RADIOACTIVITY AND MEASUREMENT OF GEOLOGICAL TIME. 295

strated its genesis directly from radium emanation. In 1910

Strutt went further, and measured its rate of formation inpitchblende and thorianite. His results show that one gramof uranium * generates helium at the rate of I cc. in 10 millionyears, and that one gram of thorium - generates helium at therate of I cc. in 30 million years. These figures have beenverified quite independently by directly counting the a-particles­helium atoms-emitted by various members of the radioactivefamilies. Strutt found that the thorianite which he used in hisexperiments originally contained 280 million times more heliumthan the amount which the same mineral could generate in ayear. The inference is clear. That large volume of heliummust have taken 280 million years to accumulate.

Before going a step further and asserting that this is also thetu;« of the mineral, two questions must be asked and answered:

(a) Was there any helium present in the mineral at the timeof its crystallisation, or has it all been generated since?

(b) Can we be sure that no helium has escaped from themineral during the period which has elapsed since its crystal­lisation?

In the first case it is only necessary to notice that ordinaryrocks and minerals contain only the slightest traces of helium,and that what little there is can be fully explained by the smallquantities of the radio-elements which are always present. Ifany strongly radioactive mineral in an igneous rock did containa little helium as an original impurity, its amount would soonbecome negligible in proportion to the large quantities sub­sequently generated.

The second question is not so easily disposed of. It hasbeen experimentally demonstrated that as soon as a mineral isexposed to the air it begins to lose its helium. When it ispowdered for analysis, still more leaks away. Consequently, thehelium now found in a mineral can only be a part, rarely as muchas one half, of the total amount which has been generated withinit during its life-time. This must be carefully remembered ininterpreting the helium contents of minerals.

Returning to our thorianite, it will now be clear that itwould be erroneous to suppose that its age is only 280 millionyears. Its age must be much greater than that. Thorianite occursin Ceylon in sands and gravels, where it has been exposed to theaction of the weather for thousands of years-ever since it wasbroken away from its original nome in the pegmatite dykes ofthat island. During all that time its store of helium has beenleaking away, and present measurements give only a minimumestimate of its age.

In the course of his work Strutt investigated phosphaticnodules and iron ores from sedimentary rocks, and zircons and

* When in equilibrium with all its daughter elements.

ARTHUR HOLMES ON

sphenes from igneous rocks, the two latter being among the mostradioactive of the commoner rock - forming minerals. Theresults for iron ores and zircons are the most valuable andinstructive, for they represent widely different periods, and theyshow that in spite of the unavoidable leakage, the older mineralscontain far more helium than the younger ones.

In Table A, some of the highest of the ages based on Strutt'sresults are given for each of the geological periods represented.I t will be seen that with few exceptions they stand in closerelation to the geological ages of the minerals. For comparison,the corresponding ages based on the accumulation of lead aregiven in brackets in the cases where these have been measured.The figures clearly bring out the limitations of the heliummethod. All that it can tell us is that the age of a mineral isgreater than a certain minimum value.

TABLE A.

Geological Mineral. I Locality.I

Helium

IMillions0

Period. Ratio.· Years.I

Recent. Zircon. Mt. Somma. < o-or 0'[Pleistocene.

"Mayen, Eifel. o-oq 1'0

Pliocene."

Campbell Is" N.Z. 0'223 2'5Miocene.

"Expailly, Auvergne 0'57 6'3

Oligocene. Siderite. I Niederpleis,f 0'7 6 8'4I Rhine Provinces.

Post-Eocene. Heematite. Co, Antrim. 2'8 30'8Permian 1 Zircon. N,E. Tasmania. 3'88 42'7

Upper ILimonite. Forest of Dean. 13'3 146 (3 20) tCarboniferousCarboniferous

} Zircon. 1Green River, N. J

147 (330)to Cambrian, Carolina. i 13'4Devonian.

"Brevig, Norway. 4'94 54 (380)

"Hsematite, Caen, 13'2 145

Upper } Zircon. { Cheyenne Canon, } 12'8 141Pre-Cambrian. Colorado,

" "Miask, Urals. rq-o 20g

" " Ceylon. 26'0 286

Middle(1) Thorianite.

" 27'9 3°7

re-Carnbrian. } Sphene. Arendal, Norway. 36'8 405 (1,200)

1Twedestrand, I 40'8 449 (r,200)" " Norway, {

Lower I Zircon. 1Renfrew Co" On- l 56'6 623re-Cambrian, ( tario, Canada. )

" Sphene."

65'0 715 (1,500)

P

P

~ The ratio of helium in ccs. to an amount of uranium (U) equivalent in its rate ofhehum generation to that of the uranium and thorium present in the mineral.Age= He/V x II million years.

t The figures in brackets represent the agein millionsof years based on lead ratios ferthe correspondingperiods.

RADIOACTIVITY AND MEASUREMENT OF GEOLOGICAL TIME. 297

V.-PLEOCHROIC HALOES.

The presence of uranium and thorium in rocks is sometimesrevealed in a most beautiful way. When mica and tourmalineand a few other minerals are examined in thin sections under themicroscope, small circular spots known as pleochroic haloes aresometimes seen. At the centre a minute crystal of zircon or ofsome other radioactive mineral can usually be detected. In1907 Joly demonstrated beyond doubt that these intenselypleochroic spots were due to the radioactivity of the tinyinclusions at their centres.

The a-rays, or helium atoms, discharged from the differentradio-elements have not all the same velocity. Those fromuranium can penetrate about an inch of air, and they give upmost of their electrical charge just before coming to rest. Sincethey must be discharged equally in all directions they come tooccupy a spherical surface around the central uranium. Thehelium atoms from radium travel more rapidly and thus penetratefarther, forming another spherical surface. Those from radiumC have the maximum range and form a sphere which encloses allthe others. The ranges of the helium atoms from the thoriumfamily are of the same order, but slightly greater than those fromthe uranium family, and consequently the resultant spheres are ofsomewhat greater diameter.

The distance to which a helium atom can penetrate dependson the density of the matter through which it is passing. Theranges in air have heen carefully measured, and the correspondingvalues for biotite can easily be calculated. The largest spherefrom the uranium family should have a diameter of 1/30 mm., andthat from the thorium family a diameter of 1/25 mm. Now themaximum diameters of haloes have been carefully measured, andsurely enough there are two types of haloes with exactly thediameters theoretically to be expected. Clinching the argumentby practical demonstration, Rutherford has made artificial haloesin glass and in flakes of biotite.

The colour of a halo depends on two factors, (a) the radio­activity of the central inclusion, and (b) the age of the mineralin which it occurs. As the helium atoms accumulate in theirspherical shells the colour gradually deepens. It is interestingand significant to notice that haloes are found only in fairly oldrocks. Granites of Tertiary age are practically free from per­ceptible haloes, whereas in biotite granites of Permian andDevonian age they are frequently well developed.

Recently Joly and Rutherford have attempted to estimatethe age of the biotite haloes in the Leinster Granite of Co.Carlow (Lower Devonian) by means of their colour. Artificialhaloes were made in the same biotite under controlled conditions

298 ARTHUR HOLl\IES 0 1'\

which made it po ssible to measure accurately bo th th e radio­activity and the tim e for which it acte d. Ar ti ficial haloes aremade with a h igh degree of radioacti vity acting for a short tim e.Natural haloes are made with a low degree of radioactivit y act ingfor a long time. Suppose that a na tural halo is found withexactly the same depth of co lour as an artifi cial one. If no wth e radioac tivity of th e centra l inclusion can be est imated, the nthe tim e for which it mu st have act ed can easily be calculat ed.The period of tim e so foun d would, of course, be a measur e ofth e age of th e hal o, and of th e min era l in which it had developed .O nce the natural hal o has been matched in colour with a nartificial halo, th e only pro blem that remains is th e estimatio nof the radi oacti vity of the zircon at the centre of the naturalhalo.

The volume of th e inclusion can be determined by workingwith a high-power mi croscope. Un fortunately it is impossible toseparate the zircons and measur e th eir uranium contents direc tly.T he difficulty is surmounted by an appeal to probable limits.No zircons are known to contain more than 10%of uranium.If thi s figure is employed th rou ghout for all the haloes examined ,some of them, th e paler ones, will give ages that ar e ob viouslytoo low ; while oth ers, th e darker ones , will give ages thatapproa ch the correct figu re, an d may perha ps even exceed it.The values actually arrived a t by J oly and Rutherford variedbetwee n 50 a nd 470 mill ion years, th e la rger figure beingcerta inly nearer the truth than the smaller one . Whether it istoo high or to o low ca nnot as yet be determined by the methodof attack pursued in this inv est igat ion.

VI. - T H E ACCUMULATION OF L E AD.

The suggestion that lead is the fina l pr oduct of the uraniumfamily was mad e by Holtwood in 190 5. All th e evide nce whichhas beco me avai la ble since that dat e co nvincingly uphold s thecorrectnes s of his view, which from the first was mu ch morethan a mere guess or assu mption. U p to th e present th egeneration of lead from a radioactive preparation has not beend irectly demonstrat ed, but the reason is not far to seek . Such apreparation, originally free from spectroscopic tra ces of lead,would require many years to gene rate within itself a perc epti bleaccumulation of lead. Tw o or three experiments were begun afew years ago with this end in view, but more time mu st elapsebefore there can be any hope of dete cting the final produ ct . Thefollowing lines of deduct ive evidence, however, leav e no room fordoubt that the final product really is che mically ident ical withlead.

Rl, DI OACTI VITY AN D MEASUR EMENT OF GEOLOG ICA L TIM E. 299

(a) When th e radio-elements are arranged in their properposi tions in ~he Periodic classification, the final product ofuranium naturally falls into th e division already occupied bylead .

(b) The atomic weight of ordinary lead is 207' ! (Inter­national, 1915) , or 2°7 '19 accord ing to Baxter, T horvaldsenand Gro ver (!915). However, according to the equation onp, 292 the atomic weight of the final product should be206 '2, or 206 according to H onigschrnid 's measurement ofthe atomic weigh t of radi um. This was for some years asource of difficul ty, but it has now led to the discovery thatlead which has accumulated in radioact ive minerals, andwhich is recognised as lead by its chemical and spectro­scopic behaviour, has actu ally a lower atomic weight thanthat of ordinary lead. Richards and Lembert, in America,Maurice Curie, in France, and H onigschmid, in Austria,have investigated lead prepared from pit chblende and oth eruranium minerals, and the values of the atomic weight foundby th em range from 206 to 206'5'* The higher values areall from pi tchbIen des of secondary origin, which are liable tocon tamination with ordinary lead owing to their associationwith galena. The lowest values are from pure uraninite(German East Africa), and from a similar mineral, Broggerite(Norway), both of which were fresh primary minerals quitefree from trac es of galena. Atomic weight evid ence, there­fore, strongly supports the view that the final pro du ct ofuran ium is an isot opic var iety of lead .

(c) It is found that in fresh, primary, uranium-bearin gminerals of the same geological age, the amount of lead isclosely proport ional to th at of ura nium. That is to say, th eratio Pb/U (referred to as the lead ratio) is practically CO ll ­

stant, For each gram of uran ium in a mineral, the amountof lead generated and accumulated is th e same as in anyother mineral of equal antiquity, always prov ided that nolead has been lost or gained from external sources duringthe period concerne d.

(d) When series of minerals of d ifferent ages are com­pared, it is found that th e lead ratio s vary in sympathy withthose ages. The older the mineral the high er is the leadratio.

T he rate at which lead is genera ted from uranium can easilybe calculated from the equation on p. 292 , T he rate of produc­tion of helium is accurately known, and the mass of lead set freein the same tim e is roughly 6 '5 tim es that of th e helium liberated.In a year one gram of urani um generates 1'25 x 10.10 grams oflead, an d at th is rat e, one gram of lead would be produced in

* For references, and a table of results , see : Holmes and Law son, Phi l. Mag.,vol. xxi x, p. 68z, I915.

30 0 ARTHUR HOLMES ON

8,000 million years. If a mineral contains a percentage ofaccumulated lead of radioactive origin represented by Pb, and apercentage of uranium represented by U, then the age of themineral is given approximately c by the expression :-

PbU x 8,000 million years.

Before applying this method to the actual measurement ofgeological time, it is necessary to examine closely a number ofassumptions which are implied. It is clear that if any leadshould have been originally present in a radioactive mineral atthe time of its genesis, a serious difficulty will have arisen. Inall the ordinary minerals of igneous rocks, lead is a negligiblequantity. The difficulty may often be overridden by analysingonly these minerals which are much richer in uranium than themain body of the rock. Within them, lead will steadily accumulate,and any original lead will, as time goes on, become of less andless importance in proportion to the whole. The difficulty is not,however, wholly dispelled in this simple way. If original leadwere to be present in troublesome quantities the amount is likelyto vary from mineral to mineral, and the lead ratios will lack thatconstancy which is the criterion of their value. Moreover, thelead from such minerals is of two kinds-" ordinary" lead and" uranium" lead -distinguishable by their atomic weights, thoughnot by chemical methods of analysis. If the lead is whollyoriginal its atomic weight should be about 207"1; if generatedfrom uranium the atomic weight should be about 206'2. Valuesbetween these figures imply a mixture of the two types. Thecriteria pointing to the absence of original lead in perceptiblequantities are (a) constancy of the lead ratios in a series of fresh,primary minerals of the same geological age, and (b) an atomicweight value of the order 206'2.

In cases where the lead ratios are not constant, or where theatomic weight is too high, the presence of original lead is to besuspected, and the ratios become worthless as an index of age.The thorium minerals from Ceylon afford an instructiveinstance.

U + 0'575 Pb x 8,000 million years.

* Since the amount of uranium present is slowly decreasing as the helium and leadaccumulate, it is clear that the amount of uranium U0 originally in the mineral musthave been greater than the amount U now present. For periods of time less than 2,000million years, the average amount of uranium present in a mineral throughout itshistory-the lime average-is almost exactly (Uo + U)!z. Now U o is given byU + Pb + He, or by U + !"IS Pb. Consequently (U o + U)!z = U + 0'575 Pb, and amore accurate expression of the age of a mineral than that given above is

Pb

RADIOACTIVITY AND MEASUREMENT OF GEOLOGICAL TIME. 30r

TABLE B.

THORlUM MINERALS FROM CEYLON (AGE UNKNOWN),

Analyst. Mineral. Lead. Uranium. PbjUr.

johnston" Thorite 1'z8 4'57 oz8Blakej Thorianite z'66 rO'4 2 0'26

Boltwood'[" 2'70 II'20 O'Z4

Johnston* Thorite 0'78 3'5 0 O'Z2Soddy and Hyman§

Tho;;anite0'36 1'62 0'22

Blaket Z'10 9'50 O'2Z] ones] II Z'36 11'40 0'21

Buchner'- " 2'30 II'IO 0'21Blaker II 2'42 r aBo 0'19

Ramsay" "I 1'87 13'10 0'14

Iones] " 2,78 z3'o O'IZ

" " z'7° Z3'75 01 I

" " 2'16 24'8 0'°9

" " :'38 27'8 0'°9:

The ratios vary from 0'09 to 0'28, and the atomic weightof lead from thorianite has been found by Richards and Lembertto be 206'83. Ratios of the order 0'2 are therefore far too high.It was at first thought that these minerals were of two differentages. This is unlikely. The Archrean platform of Ceylon is madeup of ancient gneisses, with crystalline limestones, etc., whichhave been invaded (a) by a charnockite series, (b) by pegmatites,some of which contain thorite and thorianite, and (c) by pyro­xenites and allied rocks, The pegmatites are probably post-Middlepre-Cambrian and pre-Gondwana in age. The former view thatthe so-called "older" radioactive minerals of Ceylon occur inthe oldest known rocks of the earth's crust must be abandoned.It is refuted by the ratios themselves, by atomic weight evidence,and by the actual sequence of igneous intrusion and sedimenta­tion in the island.

The thorium minerals of the Devonian igneous rocks of theChristiania district also exhibit wide discrepancies, and cannottherefore be accepted as a criterion of the age of the rocks inwhich they occur (Table C),

* Col, Rep, Misc., 87 Ceylon, Cd, 7'75. p, 6, '9'4.+Proc. Roy, Soc" vol , 76 A, p. 253, 'g05,tAm. J. Sci., xxiii, p. 80, 1907.§ Trans. Chem, Soc., vol, 105, p. 1404, ]914.11 Proc, Roy, SOL, A vol., p. 546, ,g06., Nature, vol. 75, p, ,65, '906.**Nature, vel, 69, p, 5591 1904.

An analysis of pitchblende from Ceylon is given in Col. Rep. Mise., No. 37 Ceylon.Cd. 3'go, p. 38, 1906. U = 7>'88; Pb = 4'65; PbjU '" 0'064,

3° 2 ART H UR H OLM ES ON

TABLE C.

THORIU M M INERA LS FROM B RE VIG, NO R\\ ' AY. ( DEVON IA:<)

Analyst , Mineral. L ead . Uranium. Pb/U.

Damour ' O rangite 0 '082 1'0 2 O'SIRammelsberg, • Thorite 0 '076 1'4-8 0 '5 2

Boltwood j" <:0 ' 1 O'{O <: 0 ' 25

Holrnesf" 0 '080 0'7 2 0 '112

" ,. 0 '°76 0'70 0 '1 ° 9

" Orar'J'gi te0 '020 0 '{07 0 '°48

" 0'057 1'244- 0 '046

" " 0'°54 1 '1 8 3 0 '0 4 6

"Urano-thorite. 0'428 10'1°4 0 ' ° 4 2

Schilling* II 0'335 8'703 0'03Q

",. 0'297 8'930 0 .033

There seems now to be little doubt that primary lead isfrequentl y present in minerals rich in thorium. It has beensuggested by Soddy that lead having an atom ic weight of 208 '4may be the final produ ct of thorium. Since, however, the PbJUratio is often quite independent of the amoun t of thoriumpresent in a mineral, it seems impossibl e that this " thorium lead II

can accumulate. It may possibly be formed , but if so, it must beunstable. Neverth eless, minerals rich in thorium are thoroughlyunsui tabl e for the measurement of time. and ratios obtainedfrom them, unless independent ly verified, are of no use for ourpurpose.

:\ second assumption must now be examined. Can we assertthat since their initial crystallisation the minerals we investigateha ve suffered no chemical changes whereby concentration orelimination of lead may have resulted ? In many cases, un­fortunately, we cannot. The famous radioactive minerals fromLlano Co" Texas, furnish a warning example of the effect ofalteration. (Table D.) All these minerals occur in pegmatiteswhich are of late Pr e-Cambrian age, They are all, however,riddled with secondary produ cts, and as the writer has found bynumerous attempts, it is impossible to purchase unalteredminerals from any of the localities concerned. The lead rati oscited in Table D are eloqu ent of the uselessness of this series ofminerals as an index of geological time.

* C. Hi ntz, N ineralogie, vel. r , p. I67S.t Proc, RDJ'. Soc.,A , vol. 85, p. 2 ~4 ( I~ II ) ; P hi l . Ma g . . x xvhi , p. 8]2 (I 914).~Am . J, SCi.• vel . xxiii , p . 88 (' 907).

RADIOACTIVITY AND MEASUREMENT OF GEOLOGICAL TI~lE. 303

TABLE D.

RADIOACTIVE MINERALS FROM LLANO CO" TEXAS, U.S.A,*

Analyst, Mineral. Lead, Uranium. Pb(Ur.

Mackintosh Yttrialite 08 0'7 1 1"13

" "0'8 0'75 1"°7

"Fergusonite 1'3 1'28 ro r

Hillebrand Yttrialite 0'76 1'5 0'5 IMackintosh Fergusonite 1'8 6'q 0'29Hillebrand Mackin toshite 3'65 19'6 0'186Mackintosh Cleveite IO'25 55'2 0'186Hillebrand Mackintoshite 3'47 197 0'176

"Uraninite 9"4 55'0 0'170

Mackintosh Nivinite 9'45 56'1 0'168

"Broggerite 7'82 67"2 0'u6

"Thorogummite 2'01 197 0'102

Generally it is possible to determine by field and microscopicevidence whether a mineral is reasonably fresh or not. To bevaluable for age determinations a mineral must be stable andfresh, and it must be a primary product of the crystallisation ofthe rock in which it occurs. If these conditions are fulfilled thelead ratios of minerals of the same age ought to be constant.If alteration has taken place the ratios will not be constant, andthey therefore hold within themselves a test of their reliability,Here, again, the criteria of a satisfactory series of minerals are thatthe ratios should be constant, and that (if it can be determined)the atomic weight of lead prepared from those minerals should beof the right order.

We may now proceed to consider a number of analyses ofminerals which may fairly be used for the measurement ofgeological time. In Table E. many of the analyses compiled byBoltwood in 19°7 are quoted, together with several new oneswhich have since become available.

TABLE E,

47I " " 9Geological age: Carboniferous,, Average age t from Lead Ratios: 320,000,000 years.

Minerals. Locality. Lead. jUranium. , Pb/U. Analyst.

IGlasronbury,Juraninite Conn" U,S,A, } 2'9 70 0'°41 Hillebrand,

" " 3° 7° 0'°43 "" "

2'8 7° 0'04°"

" "3'0 72 0'°42

"Z' Z .0'0 °

* Am. ] Durn. Sci., vol, xxviil, p, 431, and p. 493, 1884; V0!. xxxviii, p. 480, 1889 ;vol, xlvi, p. 98, ,893. Pb

+Calculated in each case from the formula U T o~5-i5ph X 8,000,000,000 years.

3°4 ARTHUR HOLMES O N

TA BLE E.-(Continurd).

:ll inerals, Locality, Lea d , Uran ium. PbjU, Analyst ,

"

H illebrand,'

Bolt; ood,'H illebrand,Holmes.j

0'0 5 10'05 50'° 490 '0460 '0 470 ' 0 4 2

""

Spruce Pine • 3' 9 77North Carolina 4 ' 2 77

3"3 6 7Marie;h, S,C , 3"3 71North Car olina 0 "0036 0 '076

" 0 "0055 0 "130

Geological age : Cambrian to Carbo niferous,Average ag e from Lead Ratios : 37°,000,000 yea rs.

" (co rrected for primary lead): 3 30 ,000,000 years ,

Uranini te

I I.

III,

Zircon Brevig , Ch rist iania 0 '037 0 '9 3 I 0'040Homelite dist rict, N orway 0 '012 0'244 0 '°50Zircon , oooq 0 '194 0 '046Pyrochlore . ,, 0 '0 1 2 o'19J 0 ' 0 6 2

" 0 '009 0'186 0 '04 8Biotite 0'007 0'160 0'044Zircon 0'006 0'146 0 '04 1Tritomite ,, 0 '0 0 3 0 '063 0'048F reyalite , 0 '003 0'053 0 '° 56Mosandrite ,, 0'002 0 '043 0 '047iEgirine " " 0 '00 2 5 0 '015 0 '060Eucolite, " '" 0 '001 0 '0 17 0 '059

Geological age : Devonian ( probably Lower or MidAverage age from Lead Rat ios: 380,000,000 years.

H olmes,

"

Ie),

Uran inite Annerbd , Norway 8 '4 66 0 ' 1 3 Hillebrand,

" " " 7"8 6 8 0' 1 2 Blomst ran d,Annerodite ,

" "2 ' 2 IS 0 '15 "Uran inite Elvestad,

" 9'3 66 0 '1 4 H illebrand,., "

8 '0 5 7 0 ' 14

" Skaartorp ,"

8'8 65 0 '1 35

IV,r H ugg enaskilen, f 8'8 68 0'13

" 1 Norway "Tho;ite " "

9 '0 7 6 0'12 Lorenzen,Hi ttero,

"1'2 8 '2 0 '15 Li ndstrom,

Broggerile : Norway 8 "6 1 67"4 0 ' 1 3 Hofmann.j

" " 8 '49 67 0' 13"Geological age : Middle Pre-Cambri an (p re-jatul ian),

Average age from Lead Ratios : 1, 000,000,00 0 years,

Ura ninite Arendal, No rway 9 '8 56 0 ' 18 Hi llebrand ,

"10'2 6 1 0 '17 "

" 9'4 56 0 ' 17 Lindstrdm.

V, Thorite ,." 1 '5 9 0 ' 1 7 Nordcnskiold .

Orangite Landbo,"

1' 2 7'5 0 '1 6 Hidden,Xen otime Narest o, 0'62 2'9 O"2 ! Blomstrand.

Geological age: Middl e Pre -Cambrian (pre-jatulian) .Average age from Lead Ratios: 1 .200,000,000 year s,

• Atomic weig ht of lead (R ichards and L em bert, 1914), 206 4, This suggests tha t201. of the lead may be primar y,

t " T be Age of the Ea r th," p , 160 (1913),: Be y, d; d. Chem Ges" vol, xxxtv, p , 914 (lgCl), Atomic weigh t of lead IHooig.

schmid, IgI4), 2c6'06,

R ADIOACTI VITY AND .\I EASU RE;\n:N T OF GEO LOG I CA L 'J I 1\1 E. 305

TA BLE E.-( Colllinutd) ,

Miner als, I L ocality, Lead. Uran iu m. PbjU. An alyst.

VI. f Fergu~o~ i te IYtt erby, Sweden 0 '18 1 '0 6 0 ' 17 H olmes,Gadolinite , " " 0 ' 3 6 2 ',p 0 ' 15 "

l Geologica l age : Middle P re-Cam brian (Ser-ar ch:ean Granites) .Average age from Lead Ra tios : 1, 100,000,0 0 ::> yeMs,

( " I{Villene uve 'I IUrammte. Q be 0' tari 10'I.J. 60VI I ue c, n ano' 1 Geological ag e: l\Iiddle Pre-Cambrinn ,

l Age from Lead Ra tio : 1,200,000,0 ::>0 yea rs,

IO' 17 IH illebran d.

{

' Uran in iie . j { J\l orog:o ro ,Ge~man l 6'98 174 ' 54 10 '0 94 I Ma rckwald ."VI II " . East Africa , 6'88 17 4' 7 2 0 '0 9 2

. Geological age: U nde termine d, bu t younger than IX and X.Average age from Lead Rati os: 700,000,000 years.

l'' I Nrassi Basin, f I 0'17 IZircon . I M oza mbi que 0 03 2( 0'193 II H ol mes.

j Mon ap o RiVer,} IIX." .) Mozam bique 0 '026 \ 0'171 0 '15 "

Biotite . Ligon ia, Zambesia 0 '01 4 10'0 9 7 0 '14

Geologi cal age : Unde termined, but younger than X.\ Averag e age from Lead Ratios: 1,1 0 0 ,000,0 00 years.

{

Zircon . I :\Iozambiq ne . / 0 '05 4 /0 257 10'21 I H olmes.X. Geological ag e : Un determi ned : oldest gn eissose granites .

Age from Lead Rat io : 1.500 ,000 ,0 0 0 years ,

1.-T he uranin ite of Glastonbury, Conn., U.S.A., occurs inpegmatit e dykes which are associated with a granite probably oflate Carboniferous age. The granite intrudes Lower Carboni­ferous strata and is certainly pre-T riassic.

n.-Further south in the Appalachians of N. and S. Carolina,uraninite is found in coarse pegmatites of which the age, unfor­tu nat ely, is obscure . The strata cut by the pegmatites may beanywhere from Cambrian to Carboniferous. It will be noticedthat the lead ratios vary from 0 '°42 to 0'055, the discrepanciesbeing partly due to alteration. The specimens analysed byHillebrand were not fresh and the results are therefore ofdoubtful value. However, lead has been extracted from theNorth Carolina uraninite, and its atomic weight indicates thatonly 80% of the lead can safely be referred to a radioactiveorigin. When the lead ratios are corrected for the error thusintroduced, they are found to be in close agreement with thoseof the Glastonbury uraninites. It seems probable that all theuraninite pegmatites of the Appalachians are of the same age­late Carboniferous.

• Atomic weig ht of lead (Ho niJ(schm id, 1915), 206'04.P ROC. GEOL. Asso c., V OL. XX VI, PA RT 5 , 191 5.] 22

306 ARTHUR HOLMES ON

IlL-Unfortunately, uraninite does not occur in the Devonianigneous rocks of the Christiania district of Norway. As hasbeen already shown, analyses of thorite are far from reliable astests of age, and the writer has felt reluctantly obliged to abandonthem from that point of view. The remaining analyses giveratios varying from 0'04 to 0'062, the discrepancies being heredue to the difficulty of accurately determining small quantitiesof lead. The average age given in the table may be too high,and must be regarded as indicating no more than an approxima­tion to the correct figure. It is significant that the somewhatolder granites of Co. Carlow give a maximum age of 470 millionyears, as determined by the pleochroic halo method.

IV.-Turning to the Pre-Cambrian rocks of Scandinavia,there are three series of igneous intrusions containing radio­active minerals. All are intrusive into the older schists andquartzites of the Pre-Cambrian, and all were worn down by thedenudation which prepared the platform on which the ]atulianformations were laid down. They all, therefore, belong to themiddle division or Pre-Cambrian time. The position of theserocks in the Pre-Cambrian sequence will be made clear by thefollowing tentative classification:

UPPERPRE-CA.MBRIAN

orEp-ARCHA':A.N

FENNO.SCANDIA.

(RapakilJi Granir«Jotnian

l~

CANADA.

Keweenawan~

Anirnik ie

Ep-Archsean lnterval~~

MIDDLEPRE-CAMBRIAN

OrMES-ARCHA':AN

Sn--A1'C/UEan and postKalevzan GranitesUpr. Kalevian~

Lr. Kalevian~

Post Bottnian GranitesBottnian

A /gtlman and postHuronian GranitesUpr. Huronian

Lr. Huronian~

Granite I ntrusionsSudburyan

Epi-Laurentianlnterval~~

LOWER {' Post Ladogian Granites Laurentian GranitesPRE-CAMBRIAN 1Grenville Series

Or Ladogian Keewatin "PROT-ARCHA':AN Coutchiching i,

The first group of minerals-from the Moss district of S.Norway-belong to pegmatites associated with granites of post­Kalevian age. The lead ratios only vary from 0'12 to 0'15 andthe atomic weight of lead from Broggerite (lead ratio = 0'13) wasfound by Honigschrnid to be 206.06. Here, then, all the criteriaof a thoroughly satisfactory series of minerals are fulfilled, and theage assigned to the rocks may be accepted with confidence.

RADIOACTIVITY AND MEASUREMENT OF GEOLOGICAL TIME 307

V.-A similar suite of minerals is found in the pegmatites ofthe Arendal district of S. Norway. In this case the associatedgranites may be post-Kalevian or post-Bottnian. The agreementof the lead ratios is good except tor the abnormal value given byXenotirne.

Vr.-Although many analyses have been made of mineralsfrom the famous pegmatites of Ytterby (Ser-archsean granites)no determinations of lead have hitherto been published. Twoanalyses by the writer indicate that the minerals are of the sameorder of age as those of the two preceding groups. Geologicallythe Ser-archsean and post-Kalevian granites cannot be distin­guished in time, and as far as correlation has been attemptedthey have been grouped together.

VII.-A valuable analysis of a Canadian Middle Pre-Cambrianmineral is that of the uraninite from Villeneuve, in Ontario. Thewriter is indebted to the Canadian Geological Survey for the in­formation (due to the work of Mr. M. E. Wilson) that thepegmatite in which the uraninite occurs is associated with a granitewhich (a) is intrusive into the Grenville series and into thepyroxene granites which penetrate the Grenville series, and (b)is intruded by diabase dykes of Keweenawan age. Thesedetails make it clear that the pegmatite belongs to one of theperiods of granite intrusions contained within the Middle Pre­Cambrian of the above classification. This conclusion is com­pletely in harmony with the age deduced from the single analysisavailable.

VIII-X.-The remaining results are given, partly to illustratehow the method may be employed to determine the approximateage of igneous rocks where other evidence is lacking, and partlyto show that where a succession of igneous intrusions can bemade out, the lead ratios vary in accordance with that succession.

In German East Africa and Mozambique at least threeimportant periods of granite intrusions can be recognised. Theoldest rocks are, as usuaL schists and crystalline limestonesassociated with banded gneisses and gneissose granites. Intothese penetrated granites having a granulitic texture, and stilllater the complex was invaded by intrusions of massive granitesand pegmatites. Zircon from the gneissose granites of Mozam­bique gives an age of 1,5°° million years, suggesting a correlationwith the Lower Pre-Cambrian of other areas. Zircon and biotitefrom the granulitic granites of Mozambique give an average ageof 1,100 million years, suggesting a correlation with the lateMiddle Pre-Cambrian. The massive granites and pegmatites ofMorogoro in the south of German East Africa have fortunatelyprovided a pure uraninite which has not only been analysed, butfrom which lead has been prepared. The atomic weight of thelatter guarantees its radioactive origin, and the age deduced fromthe lead ratio-e-joc million years-is therefore thoroughly reliable.

ARTHUR HOUIES ON

CONCLUSIONS OF SECTION VI.

The method of determining geological time by the leadratios of radioactive minerals gives results consistent amongthemselves and in harmony with geological evidence, whereverthis is clear. Rejecting minerals in which alteration, or thepresence of primary lead, has vitiated the results in advance,the evidence is conclusive that the ratio Pb/U is nearly constantfor minerals of the same age, and that the value of the ratioincreases as the geological age of the respective minerals increases.

2. The results are in keeping with those deduced from otherradioactive methods.

3. The' age of the Carboniferous and Devonian intrusions is ofthe order 300-400 million years; the age of the granite intrusionsof the Middle Pre-Cambrian is of the order 1,000'[,200 millionyears.

4. Where geological evidence is obscure, as, for example, inthe Appalachians of the Carolinas, the method is capable (whensuitable minerals are present) of being applied to the determina­tion of the age of granitic and similar intrusive rocks.

5. The method may be used comparatively for the correlationof igneous intrusions in various parts of the world, and inparticular for the correlation of the Pre-Cambrian rocks.

VI I.-BIBLIOGRAPHY.

BAXTER, G. P., THORVALDSEN, T., and GROVER, F.L.-" A Revision of theAtomic Weight of Lead." yourn. Am. Chtm. Soc., vol. xxvii, NO.5.1915·

BECKE[{, G. F.-" Relations of Radioactivity to Cosmogony and Geology."Bull. Ceol. Soc. A m., vol. xix, P: I I 3, Ig08.

------.-" The Age of the Earth." Smith Inst. Misc. Col., vol. lvi,No.6, IgIO.

BOLTWOOD, B. B.-'· On the Ultimate Disintegration Products of the Radio­active Elements." Phil. }Wag., vol, ix, p. 6I3, IC)05 ; Am. J. Sci., vol. xx,P: 253, 1905 ; Am. J. Sci., vol. xxiii, p. 77, Ig07·

CHAMBERLIN. T. C.-" The Bearing of Radioactivity on Geology." YOU,.".

Geol., vol. xix, p. 674, 19I I.GRAY, J. A.-" Liberation of Helium from Radioactive Minerals by Grind­

ing." Proc. Roy. Soc. A., vol. Ixxxii, P: 30I, rqcq.HAMBERG, A.-" Die Radioaktiven Substanzen und die Geologische

Forschung." Geol, Form, Stockholm, Forhand, Bd, 36, p. 31, 19I4.HARKER, A.-" Some Remarks on Geology in Relation to the Exact

Sciences, with an Excursus on Geological Time." Proc, Yorks. Geo],Soc., vol. xix (I), p. I, IgI4.

HOLMES, A.-" The Association of Lead with Uranium in Rock Minerals,and its application to the Measurement of Geological Time." Proc. Roy.Soc., A., vol. 85, p. 248, IgII.

-----.-" The Age of the Earth." London and New York, IgI3.-----.-" The Terrestrial Distribution of Radium." Science Progress,

No. 33, p. I2, July, IgI4.-----.-" Lead and the Final Product of Thorium." Nature, p. no,

April 2, IgI4.-----.-,. Radioactivity and the Earth's Thermal History." (Part I)

Gea', Mrzg., p, 60, IgI5; (Part II) Geol. Mag., p. I02, I9I5.

Me th ods of Dete rmin ing G eol ogi calA bs. Proc. Gtol. Soc., No. 9i1 . p. 63,

RADIOACT/ Vln' AND "IEASL' R E~IEN"T OF G EO LOG IC AL TIi\IE. 309

HOLME S. A.-Contribution to the Discussion on Radioa ctive Eviden ce of theAge of the Eanh.-Brit. Ass oc., Sec. C, Manchester, 19 [5

HOLM ES, A ., & LAWSON, R. W -" Lead an d the End Produ ct of Thori um .,(P~rt I) Plti /. Mag.,vol. xxviii , p. 8 ~3, [914 ; (Part 1/) Pitt!. M a,( ., vol .XXIX, p. 673, 19 1S.

JOLY, J.-" Rad ioa ct ivity and G eolog y. " L on don, 1909---.-" The Radioac ti vity of Terre str ia l Surface M a terial s." Phil. Jfa£(,

vol. xxiv, p. 6114 , 191z.- - -.- " The Ag e of the Earth:' PllIl. Mag., vol. xxii, p. 357, 191 I.- - - .-" The Bi rt h Time of th e W orld ," Scienet Progress, No. 33, p. 37,

1914.- - - .- " P leochroic Haloes." FlIT'!. •II-I'g.. \'01. xix p 327 and P: 630, 19[0.---.-" Pleochroic Hal oes:' b"'~"ock, N O.4, P.'4.i3 , [913.JOLY, J., & R UTHEl{fORD. E.-'· 1 he Ag e of Pleochroic Haloes." PM/.

Mag.• vol. xxv, p. 6.14, 191 3 .KELVI N _ u On the Secula r Cooling- of t he Earth." Thom son a nd T a it :

A a/ural PltildJOp lty , Appe nd ix D:KOENlGSB ERGER, J.- " Berech nun gen des Er da lter s a uf physika lische

Grundlage." G' ol. Run,/rcltau, vol. I , p . 241, 19 [0.LAWSON. R \Y.-" T he Time Av erage of U ra ni um." Proc. DurltaT1l P hil.

Soc.• \ '01. v. (I) . p . ~6. 19[3 .MfrGGE. 0 .- " Rad ioaktivirar uud Pleochroi t ische HOle ." em/Of..r. Min.,

pp. 65. 1[3, and 142, [gogPOOLE . J. H. J. -" The Average Thorium Co nt en t of th e Earth's Crust."

Plt il. Ma~., vol. xxix, p. +83, 1915.R UTHERFORD, E .-" Rad ioa ct ive Subst an ces and their Radiations." Ca m­

brid ge , 1913.SHELTON, H. S.- "The R ad ioac tive

T ime (Abstract an d DiSCUSSIon)."March 3rd, 19 15

SODDY, F.-" The C hemistry of the Rad io- E lements." London, 19[4 a nd19[5.

STRIJTT, R . J.-"On the Distribution of R adium in the Earth's Crust."Pr oc, N~y. Soc.. A" vol. Ixxvi i, p . 472, Ig0 6; vol. lxxv iii, p. 150 , 1907.

------.-" H elium a nd Radioactiv ity in Rare a n d CommonMinera ls." Proc. R oy. Soc., A., vo l. lxxx , p. 572, 1908.

- - - - - - .- " The Acc um u lation of Helium in Geological Time."( I) Proc. Ro.y, Soc., A., vol. lxxxi, p. 272. Ig08 ;

( I I) Proc. Roy. Soc., A., vol. lxxxiil, p. g5, IgI0 ;(1'1) Proc. Rov. Soc., A., vol , lxxxii i, p. 2911, 1910 ;(IV) Proc, R oy . Sue., A, vol. lxxxi v, p. 194, 19[0.

-----.-" T he Leakage of Helium from R adioactive M ine rals. "Pro c, Roy. Soc, A ., vol. lxxxii, p. 166, Ig09.

-----.-" Mea surements of the Ra te a t which Helium is P roducedin Thorian ite a nd Pit chblende." Proc. R oy . Soc., A" vol. lxxxiv, p. 379,1910 .

N ote ad ded N ovem ber 8th. 1915.-ln an add ress to the Geological Society of America(8 11 11.26, p, ' 7', 1915)G. F. Becker bas attempted to correlate recent dev elopments inradio-geo} oty and is osta sy. He assume s tbilt H ayford ' s level of isost atic compensation,at a depth of 12 1 km., ;s tbe depth of easiest fusion. i ,e, 'b e depth at wblch the coolingc urve most nearl y appro acbe s th e c urve of fusion. On this assumption he finds tbatrad ioactiv ity maintains onll vue -seventh of th e pre sent gradien t, a nd that the age of the­cooling earth is 68 million years. Witlt the data he uses it is calcuiated ehat If two-thirdsof the gradient is matncatned by radtoact ivity, then the age becomes 1,3'4 ml1llon years,and the depth of easiest fusion becomes 300 km. In a series of papers running through.hejo"",al oJ Geology , '914 aud '9'5. J. Barrell bas shown thatthe zone of easie st fusionmust be below the lev el of compensati on, and from considerations based 00 the strengthof the ~2rf.b·s crust. and OD tidal phen omena, he places the source of igneous acti vity atabout tbe middle of ,b. hschenosph<r. , i .e. , at a depth of 350-500 km, Combin ing chisco nclu sion wt rb Beck crs anal ysis, il ls easily se en that tbe age of the cooling earth is not68 million years, but considerably ~re ater than 1,3'4 mlllion years . A. H.