LIBRARY 
 
 OF THK 
 
 UNIVERSITY OF CALIFORNIA. 
 
 Class 
 
RADIOACTIVITY AND GEOLOGY. 
 
1 1 
 
RADIOACTIVITY AND GEOLOGY 
 
 AN ACCOUNT 
 
 OF 
 
 THE INFLUENCE OF RADIOACTIVE ENERGY 
 ON TERRESTRIAL HISTORY. 
 
 BY 
 
 J. JOLY, M.A., Sc.D., F.E.S., 
 
 PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF DUBLIN. 
 
 OF THE 
 
 UNIVERSITY 
 
 OF 
 
 LONDON : 
 ARCHIBALD CONSTABLE & CO., LTD. 
 
 1909. 
 

 DUBLIN: 
 
 PRINTED AT THE UNIVERSITY PRESS, 
 BY PONSONBY AND GIBBS. 
 
PREFACE. 
 
 SHORTLY after the meeting of the British Association 
 in Dublin last year, it was suggested to me by several 
 friends that my Presidential Address to Section C 
 might with advantage be re-written in a more 
 extended and less technical form. The same sugges- 
 tion reached me from a few well-known publishers, 
 foremost among whom was the firm whose name 
 appears on the title-page. The idea was already 
 in my mind, for I had been compelled, by the limits 
 necessarily imposed on a Presidential Address, to leave 
 unsaid much that I thought should be said, not only 
 for purposes of elucidation, but by way of addition. 
 Thus it was that the present volume originated. 
 
 But when the task of re-writing the Address was 
 begun, it soon became apparent that mere expansion 
 of the matter already in print was insufficient to meet 
 the requirements of those who, for various reasons, 
 may be unable to abstract from a highly specialized 
 and voluminous literature the principles under- 
 lying this new branch of geological research. It 
 seemed also advisable that, in addition to a simply 
 
 197682 
 
VI PEEFACE. 
 
 worded account of the principles involved, some 
 practical details should be given which might aid in 
 the prosecution of further research. The two intro- 
 ductory chapters and the final chapter were written 
 with these considerations in view. 
 
 The leading topics dealt with in my Address 
 are, of course, specially dwelt on ; and, it is hoped, 
 to their better understanding. As it is impossible 
 to write upon these many-sided questions without new 
 aspects of them being presented, I have added a con- 
 siderable amount of fresh matter. The chapter on 
 the part which radioactivity may have played in the 
 architecture of mountains refers to views incidentally 
 touched upon in a discussion held at the same 
 Dublin Meeting of the British Association. The 
 chapter on the age of the earth requires, I think, 
 no apology, in view of the interest excited in the 
 application of radioactive methods to the measure- 
 ment of geological time. The short discussion of 
 the radioactivity of the atmosphere is introduced not 
 only because of its direct application to denudative 
 geology, but on account of its bearing on the question 
 of the distribution of thorium in the surface rocks. 
 It is needless to add that if the evidence afforded by 
 the frequently observed presence of thorium emanation 
 in the atmosphere is found to be substantiated by 
 
PKEFACE. Vll 
 
 further work upon the rocks, every conclusion reached 
 in this book as to the geological importance of radio- 
 activity will receive additional emphasis, even if some 
 revision of the numerical estimates be entailed. 
 
 There are, of course, many, even among scientific 
 readers, who are still unacquainted with the consider- 
 able body of fact which enters into the subject of 
 radioactivity as an influence on terrestrial history. 
 Among these the objection is likely to arise that the 
 time is not properly ripe for the appearance of a book 
 like the present. I ask such critics to reconsider their 
 attitude in view of what is herein placed before them. 
 A large number of experiments on the surface-materials 
 of the earth are concordant in revealing the presence of 
 radioactive elements as widely distributed constituents' 
 of the crust. There is nothing a priori improbable in 
 the wide-spread occurrence of the parent substance: 
 uranium. Chemical investigations of the type of 
 Sandberger's have already shown the unexpectedly 
 general distribution of the less abundant elements. 
 Spectroscopic analysis extends this conclusion. And 
 here, in the application of radioactive methods, we 
 work for the first time with a means of detection many 
 thousands of times more delicate than the spectroscope. 
 The results of the new analysis are, in short, but an 
 extension of the old. 
 
Vlll PREFACE. 
 
 To these laboriously detailed observations upon tbe 
 rocks the more generalized ones upon the radioactivity 
 of the ocean have to be added. In its waters and 
 its sediments we are presented with the age-long 
 accumulation of materials arising from the denudation 
 of the primary rocks. Thus the oceanic radioactivity 
 testifies to the world-wide presence of uranium in 
 those surface materials which, throughout the whole 
 of geological time, contributed substances to the seas. 
 When, finally, we compare the results on impoverished 
 detrital sediments with those on the igneous rocks, we 
 find their joint evidence in harmony with that yielded 
 by the ocean. 
 
 Although this evidence of the prevalence of radio- 
 active substances might fairly be deemed conclusive, 
 other supporting facts may be mentioned. The presence 
 of helium in rocks and minerals of every description 
 has lately been established. This substance is known 
 to be a product of radioactive change. That the 
 helium in the rocks came into existence as the 
 " a particles " evolved by transforming atoms, hardly 
 admits of doubt. Now, the greater part of the energy 
 liberated upon radioactive transformation is associated 
 with the high initial velocity of these atoms. The 
 presence of helium everywhere in the rocks, notwith- 
 standing its continual escape into the atmosphere, is, 
 
PREFACE. IX 
 
 therefore, independent proof, not only of the wide- 
 spread distribution of the radioactive elements, but of 
 their ceaseless evolution of energy. 
 
 But if, on testimony so strong, the ubiquity of 
 uranium be conceded, then the influence of radioactive 
 energy on the surface dynamics of the globe becomes 
 a matter of first importance. The efficacy of uranium 
 as an almost eternal source of thermal energy seems 
 to be unquestionable. At the present moment this 
 fact appears as well founded as any which fallible 
 human judgment has unravelled from the complex 
 phenomena of the universe. We need submit the 
 uranium ore to neither chemical nor physical processes. 
 It is sufficient to take it from the rocks, and place it in 
 the calorimeter, when the constant outflow of heat will 
 be apparent (p. 11). In a word, uranium is evidently 
 in the rocks ; and its heat-producing property is beyond 
 question. With these fundamental facts before us, 
 why should we regard as premature their presentation 
 as a vera causa in geological dynamics ? 
 
 There is, indeed, much in this book which is 
 admittedly speculative. Theories regarding the interior 
 condition of the earth are, of course, unprovable. Here 
 we can only choose, among many possible issues, those 
 which are most in consonance with the little real 
 knowledge we possess. Again, in dealing with denu- 
 
X PREFACE. 
 
 dative data we find ourselves often so restricted for 
 want of accurate information, that deductions founded 
 upon existing figures must necessarily be open to 
 modification as knowledge advances. As an instance 
 I might refer to a suggested method of investigating, on 
 the basis of denudative data, the supposed existence of 
 former great continents now submerged in the oceans. 
 The matter is referred to in Chapter VI. Now, the 
 value of the conclusions we draw entirely depends 
 upon the reliability of data at present only imperfectly 
 known. Although we are thus left in uncertainty, the 
 inclusion of such considerations is, I think, justified. 
 The development of geology as an exact science can 
 only be along the lines of perfecting geological 
 mensuration ; and any prospective applications of such 
 work which have the effect of emphasizing its import- 
 ance must be tolerated. 
 
 I have received kind assistance from so many friends 
 in obtaining materials for investigation that full 
 acknowledgment is here impossible. More especially 
 Sir John Murray, Professor Agassiz, and Sir Henry 
 Holland have contributed materials which without 
 their aid would have been unattainable by me. To the 
 Trustees of the British Museum I am under similar obli- 
 gations. It is needless to say that the samples of sea- 
 water from all parts of the ocean were only obtained 
 
PREFACE. XI 
 
 by the kindness of friends willing to undertake the 
 not inconsiderable labour attending their transmission. 
 Kock specimens, too, of much value have come to me 
 from many parts of the world. 
 
 A careful revision of the proof-sheets of this book has 
 been made by Mr. Horace H. Poole. Mr. J. R. Cotter 
 has from time to time given me much help of the 
 most valuable kind. The proofs have also been read 
 by Professor H. H. Dixon. To these friends, as well 
 as to the many to whom individual reference is not 
 made, I have pleasure in tendering my thanks. 
 
 J. J. 
 
 TRINITY COLLEGE, DUBLIN, 
 
 February, 1909. 
 
CONTENTS. 
 
 Chapter Page 
 
 I. THE FOUNDATIONS OP RADIOACTIVE SCIENCE, . . 1 
 
 II. PRINCIPLES UNDERLYING THE MEASUREMENT OF RADIUM 
 
 IN TERRESTRIAL MATERIALS, 12 
 
 III. THE RADIUM IN THE EARTH'S SURFACE-MATERIALS, . 35 
 
 IV. UNDERGROUND TEMPERATURE AND RADIOACTIVITY, . 70 
 
 V. THE INSTABILITY OF THE EARTH'S CRUST, ... 94 
 
 VI. THE INSTABILITY OF THE OCEAN FLOOR, . . .117 
 
 VII. RADIOACTIVITY AND MOUNTAIN STRUCTURE, . . . 140 
 
 VIII. RADIOACTIVITY AND THE INTERIOR OF THE EARTH, . 154 
 
 IX. RADIOACTIVITY IN THE EXTERIOR PARTS OF THE EARTH, 173 
 
 X. THE RADIOACTIVITY OF THE ATMOSPHERE, . . .210 
 
 XI. URANIUM AND THE AGE OF THE EARTH, . . .211 
 
 XII. RADIUM MEASUREMENTS IN PRACTICE, . . . 252 
 
 APPENDIX A, 271 
 
 APPENDIX B, 274 
 
LIST OF ILLUSTRATIONS. 
 
 Underground Temperature and Radioactivity : Section of the 
 
 St. Gothard Tunnel ; after Stapft. PLATE I., Frontispiece. 
 The Radioactive Elements ; after Rutherford. PLATE II., p. 3. 
 
 Apparatus for Extracting the Emanation of Radium. Fig. 1, 
 
 p. 26 ; and fig. 4, p. 266. 
 
 Electroscope. Fig. 2, p. 27 ; and fig. 3, p. 253. 
 Pleochroic Halo. PLATE III., p. 66.* 
 
 Distribution of Radium in the Rocks of the St. Gothard Tunnel. 
 PLATE IV., p. 87. 
 
 Diagrammatic Section through the Swiss Alps ; after Schmidt. 
 PLATE V., p. 146. 
 
 Formation of Recumbent Folds. PLATE VI., p. 152. 
 
 * Reproduced from the Philosophical Magazine by kind permission of 
 Messrs. Taylor and Francis. 
 
X>** ^^ 
 
 31TY J 
 
 \ S 
 
 \.>. 
 
 EADIOACTIVITY AND GEOLOGY. 
 
 CHAPTER I. 
 
 THE FOUNDATIONS OF RADIOACTIVE SCIENCE. 
 
 THE FAMILIES OF RADIOACTIVE SUBSTANCES GENETIC RELATIONS OF 
 RADIUM WITH URANIUM THE THEORY OF RADIOACTIVE CHANGE 
 EQUILIBRIUM AMOUNTS OF GENETICALLY RELATED ELEMENTS DIS- 
 COVERY OF IONIUM POSITION OF ACTINIUM THERMAL PHENOMENA 
 ATTENDING RADIOACTIVE CHANGE THE HEAT -CONSTANT OF RADIUM. 
 
 NOTHING can better illustrate the eagerness with 
 which new openings to knowledge are pursued in 
 our time than the history of that branch of science 
 which is devoted to radioactivity. First definitely 
 opened up in 1898 by Mme. and M. Curie, when 
 polonium and radium were discovered, to-day we are 
 in possession of established views in contradiction to 
 the cherished tenets of centuries. It is true that the 
 time-honoured view as to the unalterable stability of 
 the atom fails only in a sense, and not owing to any 
 interference on our part ; nevertheless it is now one 
 of the most assured facts of science that the atoms 
 of certain elements change in atomic weight, radiating 
 a portion of their mass and giving up a part of their 
 internal energy in the process. Such, along with a 
 multitude of related facts, fundamentally altering and 
 
 B 
 
2 RADIOACTIVITY AND GEOLOGY. 
 
 enlarging our ideas of matter, has been the harvest 
 reaped from the one discovery, and gathered in one 
 short decade. 
 
 For our present purpose the presentment of the 
 new science in its geological aspect certain facts only 
 in the history of this great advance need be reviewed. 
 The starting-point of the geological importance of 
 radioactivity is the discovery in 1903 by Paul Curie 
 and Laborde 1 that radium steadily maintains its tem- 
 perature above its surroundings. On thermo-dynamical 
 grounds this result might have been reached at an 
 earlier date, and, indeed, in 1901 Kutherford and 
 McClung had calculated the heat equivalent of the 
 ionization effected by uranium, radium, and thorium. 2 
 The subject is of fundamental importance, and will 
 be discussed later on. It is first necessary to take a 
 general review of the extent to which, according to 
 present knowledge, radioactivity prevails among the 
 elements. 
 
 To-day we are acquainted with at least two families 
 of radioactive substances which appear to be separate 
 and distinct : that which is referable to uranium as 
 the parent or originating substance, and that which 
 appears to be derived from thorium. 
 
 The thorium family may ultimately prove of import- 
 ance to geological science. At the present time its 
 general prevalence among the materials of the earth, 
 to any geologically important extent, remains to be 
 proved. The uranium family includes twelve known or 
 
 1 P. Curie and Laborde, C. R., 136, p. 673, 1903. 
 
 2 Rutherford and McClung, Phil. Trans., A, p. 25, 1901. 
 
PLATE I T. 
 
 URANI UM 
 SX I0 9 YR5 
 
 UR.X 
 22 DAYS 
 
 EMANATION 
 3-75 DAYS 
 
 RAD. A. 
 3 MINS. 
 
 RAD- B 
 26 MIN5. 
 
 RAD. C 
 19 MINS. 
 
 ft ACTINIUM 
 
 ION I UM 
 
 RADI-0 
 ACT 
 
 * S N I9'5DAYS> 
 
 RADIUM I \ 
 
 I76O YRsA A-*. a 
 
 ^-^ ACT. X. 
 
 10 DAYS 
 
 ACT A 
 34 MINS. 1 
 
 ACT. 8. 
 3 MINS. 
 
 RAD. & 
 
 4O YRS 
 
 RADIO-LEA 
 
 RAD. E. 
 6 DAYS 
 
 RAD .F. 
 4-5 DAYS 
 
 RAD.G. 
 14 O DAYS 
 
 POLONIUM: 
 
 *f ^\ 
 
 THORIUMf A 
 
 IO'YR5.l L^ 
 
 EMANATION^ A 
 
 3-7SECS.I /--* 
 
 TH A 
 II MRS. 
 
 TM B 
 I MR 
 
 The Radioactive Elements. 
 
 The periods are those 
 of half transformation 
 (after RuCAerford). 
 
 f 
 
THE FOUNDATIONS OF RADIOACTIVE SCIENCE. 
 
 inferred elements in one line of descent from uranium, 
 and probably six other elements derived from uranium 
 on a collateral line. The accompanying diagram, 
 Plate II., after Kutherford, 1 with some modifications 
 to bring it up to recent discoveries, shows the genetic 
 relations of the radioactive elements one to another, 
 as well as the periods of transformation in each case. 
 
 It will be seen that radium itself is a comparatively 
 short-lived substance ; its period of transformation 
 that is, the period required for one-half of any given 
 quantity of the element to transform into the next 
 element in the line of descent is but 1760 years. This 
 result is obtained by a new method of Kutherford's, 
 whereby the process of radioactive change can be more 
 accurately observed than was hitherto possible. 2 The 
 previous determination by Boltwood afforded 2000 
 years for this constant. 3 It is therefore ephemeral 
 compared with such periods of time as geology is 
 concerned with; and obviously there must be a pro- 
 genitor, or source of production, somewhere among the 
 elements. 
 
 The view that radium is genetically connected with 
 uranium may be regarded as established. We must 
 consider more closely the grounds for this statement. 
 So far as the geological applications of radium are con- 
 cerned we might perhaps regard the identification of 
 the originating substance as not of first importance, 
 seeing that some such element must exist. Undoubtedly, 
 
 1 Rutherford, Nature, March 5, 1908, p. 422. 
 
 3 Rutherford and Geiger, Proc. It. S., A, 81, p. 162. 
 
 3 Boltwood, Amer. Journ. Science, June, 1908, p. 493. 
 
 B2 
 
4 RADIOACTIVITY AND GEOLOGY. 
 
 however, our knowledge of the parentage of radium 
 strengthens our hands in making these applications ; 
 and the quantitative results as regards the total duration 
 of radioactive effects thereby brought into geological 
 science are of the highest value. 
 
 The most important evidence for the genetic con- 
 nexion of radium and uranium is that derived from 
 the examination of ancient minerals. 1 In the hands 
 of Boltwood, 2 and later of Strutt 3 and McCoy, 4 this has 
 revealed the existence of a constant relation between 
 the amount of radium and the amount of uranium 
 present in every uranium-bearing mineral. From 
 various parts of the world from plutonic rocks, dikes, 
 or metalliferous veins from whatever source the 
 uranium mineral is derived, the quantitative relation 
 has been found to hold. Now, this remarkable fact is 
 explicable on the theory of the radioactive production 
 of the one from the other, and on this theory only. 
 
 To clearly understand this, let us review the funda- 
 mental position of the radioactive theory. We can 
 do so in no more interesting connexion. 
 
 According to this theory, a radioactive substance is 
 one possessing a certain degree of atomic instability, 
 so that every now and again one of the atoms under- 
 goes partial disruption ; the various* rays, to which 
 we will later refer, being produced in the act. The 
 instability being something brought about by the 
 
 1 Rutherford and Soddy, Phil Mag., 65, 561 (1903). 
 
 2 Boltwood, Nature, May 25, p. 80, 1904. Phil Mag., April, 1905. 
 
 3 Strutt, Nature, March 17, and July 17, 1904. 
 
 4 McCoy, Ber. D. Chem. Ores., No. 11, p. 2641, 1904. 
 
THE FOUNDATIONS OF RADIOACTIVE SCIENCE. 5 
 
 properties of the atom, is inherent and inevitable ; 
 and where many millions of millions of atoms are 
 concerned, on the average a definite fraction of the 
 atoms present at any moment break up per second. 
 Consider now what the result of this must be supposing 
 the newly formed atoms are themselves possessed of 
 an inherent instability, so that in their case also a 
 fractional proportion of the total number present 
 transforms every second. Let us start with the element 
 A. It produces the element B at a rate which depends 
 on how much of A is present, and on the inherent 
 degree of instability possessed by it. B accordingly 
 begins to accumulate. Now, B possesses its own 
 particular degree of instability ; and as B accumulates, 
 the number of B atoms breaking up per second 
 increases. At length B accumulates to such an amount 
 that the number of its atoms transforming each second 
 is equal to the number which A is producing per 
 second. Evidently B cannot exist in greater quantity 
 along with the given amount of A, and evidently, also, 
 as A diminishes in amount in course of time, so that 
 its rate of production of B also diminishes, then, 
 simultaneously, the quantity of B which transforms 
 per second must decrease with the diminishing quantity 
 of B. Thus there will still always be the same pro- 
 portion of B to A. This quantity of B is the amount 
 of B which is in equilibrium with the amount of A 
 present, or the "equilibrium amount." It is a fixed 
 proportion of A, which entirely depends upon the 
 degree of instability possessed by each element an 
 instability manifested by its rate of change, or 
 
6 RADIOACTIVITY AND GEOLOGY. 
 
 " constant of change." In the cases of uranium and 
 radium, the radium is in equilibrium with the amount 
 of uranium present when only a very minute fraction 
 of radium has accumulated. Thus in a gram of 
 uranium the equilibrium amount of radium is only 
 3*4 x 10~ 7 of a gram of radium. No more than this 
 amount of radium can occur as the derivative of the 
 uranium. When ultimately the uranium has diminished 
 to one-half of a gram, then the amount of radium 
 present will be only half the above small quantity, 
 and so on. 
 
 The reason of the smallness of this equilibrium 
 amount is the very, rapid break-up of the radium com- 
 pared with the rate of transformation of the uranium. 
 In fact, while radium is transformed at such a rate 
 that half any given quantity of it would be changed in 
 1760 years, the same would only happen in the case of 
 uranium after the lapse of about 5 x 10 9 years five 
 thousand million years. 
 
 A hydrostatic analogy may conduce to clearness in 
 the apprehension of this fundamental statement of 
 radioactivity. Suppose a vessel Vi empties into a vessel 
 V 2 through a narrow tubulure of a certain bore. The 
 bore of the tubulure stands for the atomic conditions 
 controlling the rate of decay or constant of change, say, 
 of uranium. The outflow from V t will evidently depend 
 on the amount of liquid contained in it ; for the pressure 
 at the orifice of escape is directly as the depth of the 
 fluid. When there is half the fluid, the pressure will 
 be half, and the rate of flow half ; and if the depth is 
 again halved, the outflow will again be halved, or be at 
 
THE FOUNDATIONS OF RADIOACTIVE SCIENCE. 7 
 
 one quarter its original rate. Thus as time advances 
 in arithmetical progression, the rate of flow diminishes 
 in geometric progression. The quantity of liquid 
 represents the quantity of uranium which thus 
 diminishes in geometrical progression with the arith- 
 metical progression of the time, the final disappearance 
 of the uranium taking a very long time indeed. Now, 
 the rate of flow from "V^ at any instant is, of course, 
 the rate of production of the next substance in the 
 line of descent, which has been called uranium X. 
 (See diagram, Plate II.) If the vessel V 2 has also a 
 tubulure of a certain bore (which may be quite 
 different from that of the first tubulure), there is 
 obviously a certain height to which the fluid will 
 stand in V 2 , so that the rate of outflow from it will 
 just equal the rate of inflow from Vi. This height is 
 defined by the rate of inflow from V^ and the bore of 
 the tubulure to V 2 . This bore, again, is the constant of 
 change of uranium X, and the height of the liquid 
 represents the equilibrium amount of uranium X for 
 the particular amount of uranium. If more than this 
 amount of fluid could collect in V 2 , the rate of outflow 
 must increase ; and as the inflow has remained 
 invariable, the level of the fluid must again be brought 
 back to that of the equilibrium amount. A little 
 consideration will show that if the tubulure of V 2 is 
 wide in bore compared with the tubulure of V 1} the 
 liquid in V 2 will only stand at a small height when its 
 rate of discharge becomes equal to the rate of inflow 
 from VL Analogously, the rate of change of uranium X 
 being very rapid compared with that of uranium, the 
 
8 RADIOACTIVITY AND GEOLOGY. 
 
 equilibrium amount of the former substance is very 
 small. 1 
 
 The constant of change is indicated by the letter X, 
 and means the fractional amount of the radioactive 
 substance which transforms each year. 
 
 The weight of the evidence afforded by the naturally 
 existing proportionality of uranium to radium in 
 minerals will now be apparent. Both elements are 
 radioactive, and here is a law which can explain the 
 observed proportionality as a direct result of the radio- 
 activity, and a genetic connexion. When it is re- 
 membered that the law of radioactive change, to 
 which the fixed proportionality can be traced, has 
 been found to rigorously obtain in all cases which 
 have been quantitatively examined, the evidence 
 becomes overwhelming. 
 
 It was thought for a time, before ionium was 
 discovered, that direct observation should show the 
 production of radium from uranium which had been 
 first purified of radium; and when disappointment 
 attended the investigation, the cause had to be sought 
 in the existence of some intermediate substance which, 
 having to take precedence of radium in order of for- 
 mation, and being slow of change, would retard the 
 appearance of radium. If we imagine on the hydro- 
 static analogy that the vessel V 3 , originally supposed to 
 stand for radium, represents an intermediate substance, 
 
 1 Rutherford, Nature, March 5, 1908, p. 422. The author, in an 
 Opening Address to the Dublin University Experimental Science 
 Association, December, 1906, made use of the hydrostatic analogy 
 as above. 
 
THE FOUNDATIONS OP RADIOACTIVE SCIENCE. 
 
 and that this vessel V 3 has a very fine bore, it will be 
 evident that a very long time might elapse before any 
 appreciable amount of radium had been received from 
 the vessel V 3 . At the time of the experiments which 
 failed to produce radium from uranium, no such 
 intervening element was known. It was plain that 
 the rate of change of uranium X disqualifies it from 
 acting as the retarding substance. 
 
 The missing element has recently been found by 
 Boltwood. 1 The new element, ionium, when purified 
 and sealed up, is found to produce radium, and shows 
 a period of transformation comparable to that of 
 radium itself, that is, measured in thousands of years. 2 
 The failure to obtain radium directly from purified 
 uranium is thus fully explained ; and Boltwood con- 
 siders that ionium is undoubtedly a disintegration 
 product of uranium, intermediate between uranium X 
 and radium. 3 This closes the evidence now in our 
 possession for the genetic connexion of radium and 
 uranium. 
 
 Before leaving this part of our subject the position 
 of the actinium group deserves notice. Actinium 
 appears according to Boltwood to be present in uranium 
 minerals in a fixed ratio to the amount of uranium. 
 Kutherford inclines to the view that its connexion 
 with uranium may be explained by supposing that it 
 springs from an indirect line of descent. At some stage 
 
 1 Boltwood, Amer. Journ. Science, xxiv., 370, 1907 ; and xxv., 367, 
 1908. 
 
 3 Boltwood, Amer. Journ. Science, xxv., 378, 1908. 
 3 Loc. cit., p. 381. 
 
10 RADIOACTIVITY AND GEOLOGY. 
 
 of descent certain of the transforming atoms, owing to 
 differing states of equilibrium being possible, give rise 
 to a distinct sequence of elements. Such atoms are 
 in subordinate amount, for the activity of actinium 
 present is not such as would be expected if it was in 
 the line of descent of radium. 
 
 The experimental evidence of radioactive heating is 
 of the most simple and direct character. The radium 
 salt is enclosed in the Bunsen calorimeter, and the 
 quantity of heat generated in a given time measured 
 directly. Curie and Laborde's first determinations 
 indicated that heat was produced at such a rate that 
 a gram of pure radium must yield sufficient heat 
 in an hour to warm 100 grams of water one degree. 1 
 This result has been confirmed as being approximately 
 correct by others. Using a differential air-calorimeter, 
 Kutherford and Barnes determined the rate of emission 
 of heat as 110 gram-calories per hour per gram of 
 elemental radium. They moreover showed that this 
 result is due to the successive changes which con- 
 tinually proceed within the radioactive substances : the 
 primary substance, radium, present in the experiment 
 being responsible only for a share of the energy given 
 out. 2 
 
 It is apparent that such measurements as those we 
 have cited are not inclusive of the energy liberated by 
 uranium, uranium X, and ionium, these radioactive 
 substances having been separated from radium in the 
 
 1 P. Curie and Laborde, C. R. 136, p. 673, 1903. 
 
 2 Rutherford and Barnes, Phil. Mag., February, 1904. 
 
THE FOUNDATIONS OF RADIOACTIVE SCIENCE. 11 
 
 chemical process of obtaining the latter element. 
 Direct and careful experiments of the heat given out 
 by Joachim sthal pitchblende have been made by H. H. 
 Poole. The heat-production here includes that of the 
 substances omitted in previous observations. It must 
 also include that given up by the actinium family of 
 radioactive substances. Poole's experiments are not 
 yet concluded, but it would appear that the final results 
 will come out somewhat higher than would have been 
 anticipated. It is noteworthy that in these experiments 
 we have the direct observation of radioactive heating 
 in situ, before the chemist has separated the con- 
 stituents, and in materials which have been subjected 
 to no physical modification at our hands. 
 
 Pending the completion and corroboration of these 
 experiments, we proceed on safe grounds by accepting a 
 lesser estimate based directly on the radioactive energy 
 of the several members of the uranium family and on 
 the direct thermal experiments. In a letter received 
 by the author early this present year (1908) Professor 
 Rutherford estimates the rate of evolution of heat 
 involved in the presence of one gram of radium and 
 due to the aggregate energy emitted by the uranium 
 family of radioactive elements, as 5'6 x 10~ 2 calories 
 per second. This is probably correct within a few 
 per cent. 
 
CHAPTEK II. 
 
 PRINCIPLES UNDERLYING THE MEASUREMENT OF RADIUM 
 IN TERRESTRIAL MATERIALS. 
 
 THE ESTIMATION OF SMALL QUANTITIES OF RADIOACTIVE MATTER 
 THEORY OF THE MEASUREMENTS THE SEVERAL RAYS IONIZATION 
 OF A GAS BY A RADIOACTIVE BODY ENERGY OF THE RAYS THE 
 ELECTROSCOPE AND THEORY OF ITS DISCHARGE BY A RADIOACTIVE 
 SUBSTANCE NUMBER OF ALPHA RAYS FROM ONE GRAM OF RADIUM 
 AN ORDINARY MEASUREMENT AND THE NUMBER OF IONS CONCERNED 
 
 SENSITIVENESS OF THE METHOD THE USE OF EMANATION IN 
 
 RADIUM MEASUREMENTS CALIBRATION OF THE ELECTROSCOPE 
 PROPERTIES OF THE EMANATION EFFECTS OF EMANATION IN THE 
 ELECTROSCOPE PROCEDURE IN MEASURING RADIUM PRECAUTIONS 
 RADIOACTIVITY NOT AFFECTED BY CONCENTRATION IS IT AFFECTED 
 BY HIGH PRESSURES AND TEMPERATURES ? 
 
 IT occasionally happens that those who have not fol- 
 lowed the development of radioactive science express 
 scepticism as to the measurements upon which the vast 
 deductions of radiothermal geology are founded. It 
 appears to them as if quantities so minute as billionths 
 of a gram must be beyond the limits of sufficiently safe 
 determination. This objection only arises from those 
 unacquainted with the mode of measurement, and who 
 therefore possess no mental substitute for the relatively 
 coarse and insensitive methods of chemical science, where 
 the chemical balance, the use of indicators, or even the 
 spectroscope, must mark the limits of perception of 
 small quantities of matter. If science had indeed 
 been restricted to these means of dividing and evaluat- 
 
THE MEASUREMENT OF RADIUM. 13 
 
 ing matter, it is safe to say that the discovery of radium 
 had never been. But in the application of the electro- 
 scope to chemical science due to the Curies in the 
 earliest days of radioactive investigation a mode of 
 detection, measurement, and separation was introduced, 
 based upon the radioactive properties themselves, 
 thousands or millions of times as sensitive as the older 
 methods. It must be remembered, however, that the 
 new method applies to radioactive matter only, and 
 may even fail to reveal radioactivity under certain not 
 inconceivable forms of the phenomenon. 
 
 A brief account of the new method is necessary here. 
 The method is fundamentally based on the fact that 
 gases become conductors of electricity under the ionizing 
 influence of the rays attending the transformations of 
 radioactive matter. These transformations are accom- 
 panied by the emission of : (a) relatively heavy 
 material particles helium atoms 1 electrified posi- 
 tively and projected with great velocity. These are 
 the a rays. They represent a definite material part of 
 the atom from which they are derived, and probably 
 indicate that helium enters into the constitution of 
 very many of the elements ; (b) electrons, or /3 particles, 
 of very much less mass, but moving at yet higher speeds 
 and electrified negatively; (c) the y radiations, the 
 nature of which is still under discussion, which may be 
 material or ethereal, and which also produce ionization. 
 Of these three forms of radiation the a rays are far the 
 most important, both from the ionization and from the 
 
 1 Kutherford and Geiger, Proc. R. S,, 81 A, p. 172; and Rutherford 
 and Royds, Manchester Memoirs, yol. liii. (1908), No. 1. 
 
14 RADIOACTIVITY AND GEOLOGY. 
 
 heat-production aspects, the greater importance being 
 due to the more considerable mass which, taken along 
 with the high velocity, involves a larger part of the 
 internal atomic energy than is carried by the other 
 rays. The heat effects traceable to radioactivity are 
 therefore mainly due to the stoppage by surrounding 
 matter of these flying helium atoms, their momentum 
 being spent in producing that atomic vis viva which we 
 recognize as heat. 
 
 The intimate nature of ionization cannot be said to 
 be known ; nor will it be, probably, till we acquire a 
 more minute knowledge of the nature of atoms and 
 atomic systems. It appears most probable that ions 
 are formed in a gas by the detachment from the mole- 
 cule of a negative corpuscle or |3 particle, leaving the 
 residual molecule positively electrified. Thus the ions 
 developed in a gas are both plus and minus in electrical 
 sign, two ions originating from each disrupted molecule. 1 
 There is evidence that the detached )3 particle does not 
 wander as a free particle, but immediately attracts a 
 surrounding of neutral molecules, so that in effect its 
 mass is greatly augmented. The possession of a free 
 positive or negative charge is a characteristic and 
 essential property of the ion. 
 
 The changes brought about in a gas undergoing 
 radioactive ionization may, in mechanical language, be 
 described as due to collisions between the projectiles 
 emitted by the radioactive matter and the gaseous mole- 
 cules, the a ray being, as stated, primarily responsible. 
 It can be shown that many thousands of ions are 
 
 1 See, however, Townsend, Proc. R. S., 80 A, p. 207, and 81 A, p. 464. 
 
THE MEASUREMENT OF RADIUM. 15 
 
 created by a single a ray before its energy is sufficiently 
 absorbed to render it incapable of disrupting a 
 molecule. Considerable energy must remain even 
 after this velocity is attained ; but no means of 
 observation now at our disposal, save the slow accumu- 
 lation of helium or, possibly, the development of a 
 sufficient heat effect, would enable us to determine the 
 presence of rays which move with less than the critical 
 velocity required to ionize the particles of a gas or the 
 sensitive molecule of the photographic plate. It is, 
 therefore, possible that certain elements may expel such 
 rays, the fact remaining at present beyond our cogni- 
 zance. The a rays of radium possess a quantity of 
 energy enormously in excess of that which is asso- 
 ciated with the critical velocity ; for on an average 
 about 100,000 molecules of gas are ionized by each 
 ray before the ray becomes ineffective. 1 Nevertheless 
 a small thickness of matter in the liquid or solid 
 state suffices to stop the career of one of these rays. 
 Only the thinnest flake of mica will suffer one 
 to pass through. There is little doubt that the 
 microscopic radius of the pleochroic haloes seen in 
 biotite surrounding radioactive particles marks the 
 limits of the penetrability of these rays or of their 
 ionizing influence in the medium. 2 
 
 The nature of the radiations in the case of each 
 radioactive element is given in the diagram Plate II. 
 
 The properties of a gas undergoing ionization are 
 easily rendered available for the detection and mea- 
 
 1 According to a recalculation on latest data kindly supplied by 
 Professor Rutherford, February, 1909. 
 
 2 Joly, Phil. Mag., March, 1907, p. 381. 
 
16 RADIOACTIVITY AND GEOLOGY. 
 
 surement of radioactivity. We now pursue the subject 
 with the help of an electroscope, which we easily con- 
 struct by placing in an enclosed volume of air a small 
 gold leaf, affixed to a strip of brass, supported by an 
 insulating rod of sulphur or quartz, and so arranged 
 that we can readily electrify the leaf. The leaf shows 
 its state of electrification by its repulsion and diver- 
 gence from the brass strip. Before an ionizing in- 
 fluence is brought to bear upon the surrounding gas there 
 is only a very slow loss of electricity and collapse of 
 the leaf : and, indeed, what there is appears due still to 
 ionization from sources which we cannot control. 
 
 Having observed the rate of collapse of the leaf 
 through a low-power microscope with a divided scale 
 in the eyepiece, let us bring up a radioactive substance 
 to the vessel, or, better still, introduce it into the air 
 surrounding the leaf. Immediately the rate of 
 collapse augments and, in fact, may be instantaneous 
 if the radioactivity of the substance is strong enough. 
 To understand this remarkable and important ex- 
 periment we must consider what goes on in the gas 
 surrounding the electrified leaf. Here the stream of 
 rays coming from the radioactive body is each moment 
 creating ions. True, many of these recombine ; but the 
 supply is continually renewed. The ions are, it will 
 be remembered, both plus and minus in sign. Under 
 the influence of the electric charge on the leaf and 
 of the corresponding induced charge upon the walls of 
 the vessel, the positive ions are urged in one direction, 
 the negative in the other, each moving towards the 
 charge of opposite sign and away from the charge of 
 
THE MEASUREMENT OF EADIUM. 17 
 
 like sign to that which it carries. This constitutes a 
 current, made up of a multitude of minute charges, 
 each ion carrying a fixed and definite amount of 
 electricity. Thus the leaf becomes discharged or electri- 
 cally neutralized. If the charge upon the leaf is at a 
 sufficient potential, the force upon the ions is adequate 
 to keep them from recombining, urging them at once 
 into the streams of ions and utilizing practically the 
 whole of them as carriers, as fast as they are created 
 by the rays from the radioactive substance. In this 
 case the rate of collapse of the leaf would experience 
 no increase by increasing the voltage or electric 
 pressure : that is, in other words, the rate of discharge 
 is the same whether the leaf shows a wide divergence 
 or a small one. The potential at which all the ions 
 present are utilized as carriers is called the " saturation 
 potential," and the current the "saturation current." 
 Of course the saturation potential varies with the 
 intensity of the ionization. In the case of measure- 
 ments such as we will further on be referring to, the 
 potential of the gold-leaf system when divergent a 
 few degrees must attain the saturation value. 
 
 Before the sensitiveness of the method of mea- 
 surement which is founded on the above conditions 
 can be fully appreciated, it must be understood that 
 each transforming atom of the radioactive substance 
 which emits a ray or rays is potent to affect the 
 conductivity of the gas in the electroscope. We will 
 next inquire into the number of rays from a given 
 quantity of radium, confining our attention to the 
 a rays, which are far the most efficient ionizers. 
 
 c 
 
18 RADIOACTIVITY AND GEOLOGY. 
 
 By a recent brilliant research Eutherford and 
 Geiger have determined the number of rays emitted 
 per second by a known amount of radium. On the 
 simplest and most probable assumption that one atom 
 of radium in breaking up emits one a particle, they 
 find that in one gram of radium 34 x 10 10 atoms 
 are transformed each second. But there are three 
 a-ray-producing elements evolved from each of the 
 transformed atoms (exclusive of radium 6r, which 
 takes many years to form); and, according to the 
 experiments of Bragg and Boltwood, these three 
 products emit each an a -ray for one a -ray from the 
 parent radium : so that when the equilibrium amounts 
 of these elements are present, for each a-ray from the 
 radium, there are three rays from its descendants. 
 The total is, therefore, 13'6 x 10 i0 rays. 1 
 
 Ib will be seen later on that in effecting measure- 
 ments of the radioactivity of terrestrial materials, we 
 do not usually deal with radium itself, but with the 
 first product of change the emanation which is a gas 
 at ordinary temperatures. This is what we introduce 
 into the electroscope ; and we take care to observe the 
 rate of collapse of the leaves after such a period of 
 time that we are assured the successively formed 
 substances, radium A, B, and C, are generated in 
 equilibrium amounts. Hence the a-radiations (see 
 diagram, Plate II.) from three elements have to be con- 
 sidered. Each of these produces 34 x 10 10 rays or 
 particles per second, or a total of, say, 10 11 rays per 
 second. 
 
 1 Rutherford and Geiger, Proa. K.S., 81, A, p. 141. 
 
THE MEASUREMENT OF RADIUM. 19 
 
 We can now make the calculation very easily for the 
 case of an actual experiment. We are dealing, let us 
 suppose, with a rock which has only 2 x 10~ 12 grams of 
 radium in each gram. Ten grams of this rock are 
 brought into solution. The solution is closed in a flask 
 until the emanation is in equilibrium with the radium 
 present. The emanation is then extracted by boiling, 
 and introduced into the electroscope. Now the ema- 
 nation from one gram of radium gives rise in round 
 numbers, as we have seen, to 10 11 a-rays per second ; 
 but here we have only 20 x 10~ 12 grams of radium. 
 If n is the number of rays given out by the emanation 
 from so small a quantity of radium, we have : 
 
 n : 10 11 : : 20 x 10' 12 : 1, 
 
 from which n = 2. That is two a-rays are liberated 
 by this quantity per second. This small number of 
 rays gives rise to probably not less than some 150,000 
 ions per second, allowing that only about three-fourths 
 of the effect of the rays results in ionization of the air 
 of the electroscope. In electroscopes of the dimensions 
 described further on, this constant production of ions 
 is attended with a quickening of the normal rate of 
 discharge which is unmistakable. Thus the ordinary 
 unavoidable ionization of air in closed vessels may vary 
 from about 10 to 60 ions produced per cubic centimetre 
 per second, or from 4,000 to 24,000 in an electroscope 
 holding 400 cubic centimetres of air 1 ; here the number 
 due to the emanation is about 150,000 : or the ordinary 
 ionization even at its extreme value is increased more 
 
 1 Kutherford, Radioactivity, 2nd ed., p. 534. 
 C2 
 
20 RADIOACTIVITY AND GEOLOGY. 
 
 than sixfold. The effect of the /3 and y radiation is an 
 additional small increase of ionization. Starting 
 the experiment with careful observations of the 
 normal rate of collapse of the leaves, the increase of 
 this rate even by 50 per cent, is a variation safely 
 beyond errors of observation. It will be presently 
 seen that we possess additional assurance against 
 error in the particular manner of growth of the 
 increased rate of discharge due to the emanation of 
 radium. 
 
 The reasons for the sensitiveness of the new method 
 are obviously contained in the facts that we are here 
 dealing with energy set free in considerable amount 
 even by the disruption of one single atom, and that 
 it is possible by the conditions described to utilize 
 this energy in the creation of an ionic sluice or valve, 
 whereby the relatively enormous pent-up energy in 
 the gold-leaf is suffered to run down. This, it will be 
 seen, is entirely different in theory from any method 
 of measurement previously known to science. The 
 feeble gravitational force on many trillions of atoms 
 is required to incline the chemical balance. Here the 
 kinetic energy of individual atoms derived from the 
 internal stores of transforming atomic systems is called 
 upon to operate the mechanism. 
 
 Since the investigation of the properties of the 
 radium emanation by Eutherford and Soddy, and by 
 the Curies, the estimation of radium has generally been 
 effected by observations in which the emanation alone 
 is introduced into the electroscope. The superiority of 
 this procedure resides in the distinctive and convenient 
 
THE MEASUREMENT OF RADIUM. 21 
 
 properties of this substance. It is the one gaseous 
 element produced by radium in its sequence of trans- 
 formations ; and by suitable treatment of the material 
 under investigation, if there is even the minutest 
 amount of radium present, the equilibrium amount of 
 emanation may be withdrawn from it and successfully 
 introduced into the electroscope. The mere fact that 
 it is the one gaseous body produced enables us to be 
 sure that none of the other products of transformation 
 is withdrawn along with it : these other elements 
 remaining behind in the material under treatment if 
 the experiment is rightly conducted. Knowing that 
 the equilibrium amount of emanation is strictly pro- 
 portional to the quantity of radium giving rise to it, 
 we in turn are safe in assuming that the effects on 
 the electroscope are proportional to the amount of 
 parent radium. 
 
 To Boltwood 1 we owe a simple means of conferring 
 on this method of measurement a strictly comparative 
 character, so that we can calculate directly from the 
 changed rate of collapse of the gold-leaf the amount 
 of radium responsible for it. The emanation from a 
 known amount of radium is introduced into the elec- 
 troscope, and the instrument carefully observed. The 
 rate of collapse read under this condition enables us to 
 compute the radium-value, as we may call it, of the 
 rate of collapse, so that thereafter, when an unknown 
 amount of emanation is ionizing the air in the electro- 
 scope, we infer directly from the attendant [rate of 
 discharge the amount of [radium giving rise to it. The 
 
 1 Boltwood, Am, Journ. Science, 18th Nov., 1904. 
 
22 RADIOACTIVITY AND GEOLOGY. 
 
 instrument is, in fact, standardized or calibrated much 
 as we calibrate a spring-balance by putting known 
 weights upon it, and finding the weight-value of a 
 distension through one centimetre. In such a purely 
 comparative use of the electroscope, we are not re- 
 quired to investigate the electric capacity of the system ; 
 and so long as we preserve all the conditions of the 
 standardizing experiment, subsequent readings are 
 independent of the fixed details of construction. 
 
 From what has been said it will be gathered that 
 the investigator in this line of research is much 
 concerned with the properties of the emanation of 
 radium. Except at temperatures approaching - 160 C., 
 the emanation is a gaseous substance. It is a liquid 
 below this temperature. Accepting 226 as the atomic 
 weight of radium, that of emanation is 222. The volume 
 of the emanation in equilibrium with one gram of radium 
 is 0'585 cubic millimetres. 1 Its rate of transformation 
 is such, that in 3*75 days it is diminished to one-half 
 the original amount: it may therefore be stored for 
 some hours (as when in the electroscope) without 
 serious diminution. The short transformation periods 
 of the three following products of change are note- 
 worthy. Kadium A, B, and C are all perishable in 
 periods measured by minutes : consequently their equi- 
 librium values are rapidly attained. Thus in a few 
 minutes as much of radium A is generated by the 
 emanation as can exist along with it ; radium B 
 rapidly increasing at the expense of A; and while 
 B is growing it is also decaying into radium C. The 
 
 1 Rutherford and Geiger, froc. E. ., 81, A, 172. 
 
THE MEASUEEMENT OF RADIUM. 23 
 
 next product is a rayless substance of great longevity. 
 "We are therefore not concerned with it. The result 
 of all this is that the rate of discharge of the electro- 
 scope, if observed a few minutes after the emanation 
 is introduced, and again after about three hours, will 
 be found to have increased some 30 to 40 per cent., 
 due to the increase in quantity of radium B and 
 radium C. After this lapse of time there is no further 
 increase of the leak, all the successive products of 
 'change being in equilibrium with one another. This 
 behaviour is characteristic of the emanation of radium. 
 The emanations of thorium and actinium, possessing 
 transformation periods measured in seconds, would 
 either not be brought into the electroscope unless 
 special precautions were adopted, but in the process 
 of transfer would assume the solid form, or, if intro- 
 duced, they would not show the characteristic rate 
 of increase of ionization. 
 
 The emanation of radium is a remarkably stable body. 
 It has been subjected to the most extreme chemical 
 and physical conditions by Eutherford, Eamsay, and 
 others. Sparking for hours with oxygen over alkali ; 
 heating to a red heat in contact with various substances ; 
 etc., had no measurable effect on its radioactive pro- 
 perties. Passed through tightly compressed cotton- 
 wool it issued unchanged in amount. It is, in fact, 
 purely gaseous in its properties, and diffuses like any 
 other gas. 
 
 The first products of change as well as the sub- 
 sequent ones are solids. These are deposited from 
 the emanation upon any surface exposed to the gas. 
 
24 RADIOACTIVITY AND GEOLOGY 
 
 The first three derivatives of the emanation give rise 
 to what is called " the excited activity " or " active 
 deposit/' There is evidence that in the mixture of air 
 and emanation in the electroscope, the particles of 
 radium A, B, and C exist suspended for some time 
 before they are deposited on solid objects ; the process 
 of deposition being hastened by the establishment of 
 an electric field. Once deposited, a certain number of 
 the rays emitted in course of further changes must 
 penetrate the solids on which the particles rest, without 
 effecting any ionization of the gas. The behaviour in the 
 gas of the transforming particles appears to be a matter 
 of some obscurity. 1 It is possible to learn from the elec- 
 troscope the result of an experiment by observation of 
 its leakage shortly after introducing the emanation, 
 and without waiting for the full growth of the active 
 deposit ; but in this case we must calibrate for the 
 same stage of growth ; that is, we may read the rate 
 of collapse, say, 15 minutes after introducing the 
 emanation, and, if we have read the leak after the 
 same interval when the standardizing quantity of 
 emanation was introduced, we may by comparison of the 
 readings determine the radium. The sensitiveness of 
 the method is, however, reduced by this procedure, and 
 the assurance derived from watching the phenomena of 
 development of the deposit is to some extent lost. 
 
 The short-lived nature of the active deposit secures 
 the instrument from the accumulation of radioactive 
 materials, which otherwise must soon render it useless. 
 If the air and the minute amount of emanation diffused 
 
 1 Rutherford, Radioactivity, pp. 319, 395. 
 
THE MEASUREMENT OF RADIUM. 25 
 
 therein are removed after an experiment, the excited 
 activity remaining soon dies out. The normal proper- 
 ties of the instrument are thus not sensibly affected 
 by successive experiments. 
 
 The mode of extraction of the emanation corre- 
 sponding to the radium in any substance, and the 
 nature of the procedure in measuring it, will now be 
 described in outline. 
 
 The rock (supposing the substance to be of this 
 nature) is brought into solution. In general it is best 
 to divide the elements of the rock into two solutions 
 an alkaline and an acid solution. They are stored 
 in closed flasks for about three weeks, at the end 
 of which time the emanation has accumulated till it 
 is almost in equilibrium with the amount of radium 
 present. Each flask is then placed in connexion with 
 a glass receiver, which has first been exhausted of 
 air R in the figure, p. 26. A condenser, through which 
 cold water can be circulated, intervenes between the 
 receiver and the flask C in the figure. When the 
 connexion with the receiver is made, by relaxing the 
 pinched rubber tube at B, the pressure falls in the 
 flask and the solution is now boiled, the steam being 
 condensed in the condenser and draining back into the 
 flask. The ebullition is continued for about 30 minutes, 
 during which time the air and emanation are boiled 
 out of the solution, and collect above the liquid in the 
 flask, in the condenser and in the receiver. When the 
 burner is turned off, cold distilled water is run into 
 the flask by the tubulure E, and the whole of the 
 gaseous contents displaced up into the receiver, the 
 
26 
 
 RADIOACTIVITY AND GEOLOGY. 
 
 water being allowed to rise into its lower tubulure. The 
 receiver is now closed by pinching it off at B, and its 
 contents are transferred to the electroscope. To effect 
 this the electroscope is first exhausted of air, and then 
 connected to the receiver, which is so arranged that 
 
 FIG. 1. 
 
 water flows into one of its tubulures while the electro- 
 scope draws the contained gas out by the other. Thus 
 the whole of the boiled-off gas is brought into the 
 electroscope without loss. 
 
THE MEASUREMENT OF RADIUM. 
 
 27 
 
 There are here three important conditions attained : 
 ebullition in vacua; collection of the emanation in a 
 dry vessel ; and complete transfer to the electroscope. 
 
 The electroscope is shown in fig. 2. It is a glass 
 flask with a connexion, a, for leading gas into and 
 out of it through the drying tube, t, which contains 
 phosphoric anhydride. The gold-leaf system is very 
 simple. A short strip of thin brass, about 2^ cms. 
 long, and about 3 mms. wide, is sealed into the end 
 
 FIG. 2. 
 
 of a sulphur tube, s, of fine bore. This sulphur tube 
 has been cast in the glass tube, g (which supports it 
 in the stopper of the flask), and is then pushed forward 
 in the glass tube, into which it is finally cemented 
 air-tight. A narrow gold leaf is attached to the brass 
 strip as shown. The system is electrified by inserting 
 a platinum wire downwards through the bore of the 
 sulphur tube till it makes contact with the end of the 
 
28 KADIOACTIVITY AND GEOLOGY. 
 
 brass slip, the wire being then electrified by contact 
 made above and withdrawn by means of an attached 
 sulphur handle. The leaf should be made divergent by 
 about 30 degrees. It is kept under observation by a 
 reading microscope, having a divided scale in the 
 eyepiece. The interior walls of the electroscope 
 are connected to earth by two strips of tinfoil which 
 enter beside the stopper. 
 
 The normal rate of discharge of the electroscope is 
 first found and entered as so many scale-divisions (of 
 the eyepiece) per hour. This rate remains practically 
 constant if the instrument is pumped out after each 
 experiment, and before making one. When the radio- 
 active air which has been boiled off the solution is 
 introduced, the rate of collapse of the gold leaf at once 
 begins to increase. The new rate is noted. Again, in 
 about three hours, in which time the active deposit is 
 almost in equilibrium with the emanation, the rate is 
 read. The increase of rate will be about 30 or 40 per 
 cent, above the first reading. 
 
 The increased rate of discharge above the normal 
 is converted to its radium value very simply on the 
 knowledge of the constant of the instrument, which has 
 already been determined as described. Suppose the 
 constant tells us that an increased rate of collapse 
 above the normal, of one scale-division per hour, 
 corresponds to 0'7 x 10~ 12 grams of radium, and that 
 the observed increase of rate in the experiment has 
 been 40 scale- divisions per hour ; then the radium in 
 the flask must have been 40 x 0-7 x 10~ 12 = 28 x 10~ 12 
 grams. The alkaline solution is dealt with in the 
 
THE MEASUREMENT OF RADIUM. 29 
 
 same manner, and the radium found added to that 
 determined for the acid flask. The total radium is due 
 to the rock which has been used, and to any radium 
 which might be present in the chemicals used in 
 preparing the solutions. The last allowance has been 
 found by previous experiments on the chemicals, and 
 after it is deducted the remaining radium is calculated 
 as so much per gram of the rock. 
 
 Several conditions have to be attended to if reliable 
 measurements are to be made. Some of these, as 
 involved in the apparatus, have been referred to above. 
 The most important remaining ones are : (1) absolute 
 avoidance of any possibility of contamination by radio- 
 active preparations ; ( 2) the use of the least amount 
 of chemicals possible in preparing the solutions ; and 
 (3) complete absence of any precipitate from the solu- 
 tions; which last condition involves (among other 
 things) the use of hydrochloric acid which has been 
 redistilled over sodium chloride in the laboratory, and 
 contains no trace of sulphuric acid. 
 
 The principles involved in the method of measuring 
 small quantities of radium will now be understood. 
 Perhaps full confidence in its results will only be felt 
 by those who know how repeatedly verified under 
 various conditions, and under various forms, the laws 
 of radioactive transformation have been. On these 
 the whole method rests. And again, from the practical 
 aspect the sensitiveness and constancy of the electro- 
 scope can only be appreciated by practice in its use. 
 In a later chapter the essential practical details are 
 given, mainly in the hope that the work of research 
 
30 RADIOACTIVITY AND GEOLOGY. 
 
 in this branch of science may be extended, but partly 
 in the belief that it is only in the laboratory that 
 science can be rightly studied, and that those who 
 cannot work in laboratories may find clearer views 
 and fuller confidence by more closely following the 
 details of an experiment upon which so much is 
 founded. 
 
 In this chapter a few outstanding questions have yet 
 to be dealt with. 
 
 The idea that the rapid heat-production and con- 
 tinued radiation of radioactive bodies may in some 
 manner be enhanced by the concentrated form in 
 which the radioactive salts are dealt with in the 
 laboratory, and that consequently in nature, where 
 there is enormous dilution with inert matter, similar 
 actions might be much weaker, has, undoubtedly, a 
 certain plausibility. For we would be inclined to 
 expect that a certain amount of auto-excitation of 
 radioactivity might arise from the rays produced. 
 There is, however, a large body of experimental 
 evidence against this view. Kutherford has directly 
 sought for a change in the radioactivity of a radium 
 salt when diluted as much as 1000 times, and could 
 detect no change. If there was any such effect as 
 auto-excitation, it should be apparent here. Eve has 
 investigated some alleged positive results in this 
 direction, and not only failed to confirm them, but 
 found that in every case the activity was directly 
 proportional to the amount of radium present within 
 the limits of experimental error. Again, the radio- 
 active constant or rate of transformation has been 
 
THE MEASUREMENT OF RADIUM. 31 
 
 investigated for many substances over wide ranges of 
 concentration, and never known to vary. In the case 
 of radium emanation, the rate of decay has been 
 observed when the concentration in air was varied 
 from one to a millionfold, but no difference was 
 discoverable. 
 
 But, finally, as regards the geological applications 
 of radioactivity, if we reflect on the nature of our 
 observations when we withdraw emanation from rock 
 materials which have been brought into solution, we 
 perceive that we are not really concerned with the 
 question of concentration. The solution represents 
 even a more dilute radioactive medium than does 
 the original rock. The emanation we withdraw must 
 have been generated in it by the radium present, for in 
 the process of preparing the solution we de-emanate 
 it completely. But if formed in the solution, then 
 must it be formed in the rock; and hence we have 
 proof of the instability of the radium and its progeni- 
 tors. Again, as the emanation does not accumulate 
 indefinitely in the solution, it must be transforming ; 
 and so, practically, the whole series of changes is 
 established, and with it the heat-production which is 
 but the outcome of the absorption of the several rays 
 in the medium. 
 
 Following this same line of reasoning, the verification 
 of the law of the constant proportion of radium to 
 uranium in substances where the concentration was 
 enormously different, is again a proof that concen- 
 tration has nothing to do with the production and 
 continuance of radioactivity. Now, in Boltwood's 
 
32 RADIOACTIVITY AND GEOLOGY. 
 
 experiments this fixed ratio, which, as we have seen, 
 depends entirely on the constants of change of the 
 two substances, has been verified when the quantity 
 of radium present in one gram of the ore must have 
 been about 3 x 10~ 7 grams. And in an experiment by 
 E. "Werner and the author, this constant ratio was found 
 to hold in a rock in which the radium was present 
 only to the amount of 5 x 10~ n grams per gram of rock. 
 The concentration of the radioactive elements in the 
 first experiment is 10,000 times that in the second, 
 yet the change rate has been unaffected. There can 
 be no question about the conclusion : the geological 
 applications of radiothermal actions are not restricted 
 by any limitations arising from concentration. 
 
 But they might be restricted in yet another manner. 
 When we come to apply the facts we have been 
 studying to the large questions of terrestrial physics, 
 it is necessary to consider if the observations of the 
 laboratory carried out at normal air- temperatures and 
 pressures are applicable to the physical conditions 
 which we believe on good grounds must prevail in the 
 interior of the earth. 
 
 From time to time the independence of radioactive 
 phenomena of physical and chemical conditions has 
 been brought out in experiments. Those already cited 
 are in point. More lately it would appear as if radium C 
 is in a small degree an exception, although, as will be 
 seen, there is conflict of testimony. Bronson, 1 using 
 radium and its products, could detect no change in the 
 rates of transformation even after prolonged exposures 
 
 1 Bronson, Proe. E. S., 78 A, 494. 
 
THE MEASUREMENT OF RADIUM. 33 
 
 to temperatures up to 1600 C. and so low as - 180 C. 
 Observations by Makower, 1 while agreeing that there is 
 no change in the emanation, appear to show a small 
 change in radium C, as measured by the )3 and y rays, 
 when exposed to temperatures of about 1100 C. 
 
 Schuster, 2 as well as Eve and Adams, 3 using direct 
 mechanical pressures, found that the highest attainable 
 pressures were without influence on the activity of 
 radium. Eutherford and Petavel* employed the in- 
 tense temperatures and pressures arising from an 
 explosion of cordite about 2500 C. and 1200 atmo- 
 spheres to find if any change in the radiation of the 
 active deposit could be observed ; the emanation of 
 radium being enclosed along with the explosive in a 
 steel explosion chamber, the -y radiation passing through 
 the walls to the electroscope. It was found that while 
 the radiation was not affected during the explosion, a 
 decrease of temporary nature to the extent of about 
 9 per cent, in the y radiation was indicated after the 
 explosion. 
 
 More recently still H. W. Schmidt, 5 in experiments 
 undertaken to decide between the previous contradictory 
 results as to change in radium C. at high temperatures, 
 using a special preparation of this derivative of radium, 
 found that at 1500 C. this substance behaves exactly as 
 at ordinary temperatures. At pressures of 2000 atmo- 
 
 1 Makower, Proe. R. 8., 77 A, 241 and 79 A, 158. 
 
 2 Schuster, Nature, July, 1907, p. 269. 
 
 3 Eve and Adams, ibid. 
 
 4 Rutherford and Petavel, Rep. Brit. Assoc., 1907, p. 456. 
 
 5 Schmidt, Physik. Zeit., Feb. 15, 1908. See also Ion, Nov., 1908, 
 p. 70. 
 
 D 
 
34 BADIOACTIVITY AND GEOLOGY. 
 
 spheres no change in the intensity of the radiations of 
 radium preparations was perceived. 
 
 As the experimental evidence stands, the constancy 
 of the radioactive processes seems proved for radium 
 and its emanation over wide limits of temperature and 
 pressure such as would obtain deep within the surface- 
 materials of the earth. The question of the constancy 
 of the radioactive change of the parent substance, 
 uranium, has not been treated in the laboratory ; but 
 the argument from analogy is all against any departure 
 from the known results. There is, indeed, nothing in 
 the evidence at our disposal to prohibit us from suppos- 
 ing that radioactivity may exist unchanged in its laws 
 even to the centre of the earth. True, the experimental 
 conditions of pressure would hardly apply below 50 
 miles downwards a very small fraction of the earth's 
 radius and the temperature conditions reached in the 
 laboratory might or might not be such as prevailed at 
 the central parts of the globe; but it seems most 
 probable that the laws of radioactive change are 'based 
 on properties of the atom inherent in its intimate 
 structure, and on energy conditions so self-contained 
 that the internal relations of the atom with the ether 
 alone affect its readjustments and its life-history. 
 Modern theory certainly tends to regard radioactive 
 phenomena as due to events progressing within the 
 atom, and, as such, distinct from chemical actions, which 
 involve external effects upon surrounding atoms, and 
 hence the physical conditions under which the atoms 
 are brought together. 1 
 
 1 See Lodge, Nature, June 11, 1903, p. 129; and Thomson, Phil. 
 Mag., Dec., 1903, and March., 1904. 
 
CHAPTER III. 
 
 THE RADIUM IN THE EARTH'S SURFACE-MATERIALS. 
 
 EARLY OBSERVATIONS THE RESEARCHES OP STRUTT THE RADIO- 
 ACTIVE SUBDIVISION OF ROCKS THE RADIOACTIVITY OF IGNEOUS 
 ROCKS RADIOACTIVITY OF THE OCEAN IONIZATION OVER THE SEA 
 
 RADIOACTIVITY OF DEEP-SEA DEPOSITS VARIATION WITH AMOUNT 
 
 OF CALCAREOUS MATTER URANIUM NECESSARILY IN THE OCEAN 
 THE RADIUM IN THE NILE AND IN THE RIO DE LA PLATA URANIUM 
 IN THE RED CLAY ESTIMATE OF THE MASS OF SUB-OCEANIC SEDI- 
 MENTS AND OF THE QUANTITY OF RADIUM THEREIN THE DETRITAL 
 ROCKS SEDIMENTS DERIVED FROM SOLUTION NO SIGN OF DIMIN- 
 ISHING RADIOACTIVITY WITH THE DEPTH THORIUM AND HELIUM 
 
 IN THE ROCKS PLEOCHROIC HALOES. 
 
 THE gaseous nature of the emanation derived from 
 radium was instrumental in first indicating the very 
 widespread existence of radioactive substances in the 
 surface-materials of the earth. In 1902 Sir J. J. 
 Thomson found that air bubbled through the Cambridge 
 tap- water gained in conductivity. In 1903 Elster and 
 Geitel observed that air drawn through the soil con- 
 tained a radioactive emanation; and they separated 
 chemically a radioactive substance from clays and soils. 
 Consistently with these observations it was found that 
 the air in caverns was exceptionally radioactive. The 
 radioactive emanation was also found to be generally 
 diffused through the atmosphere, depositing an active 
 substance on negatively charged surfaces. These and 
 a host of observations by various investigators all over 
 the world pointed to the conclusion that radioactive 
 
 D2 
 
36 RADIOACTIVITY AND GEOLOGY. 
 
 matter was a widely diffused constituent of the surface- 
 materials of the earth. 
 
 Speculation was awakened by such a far-reaching con- 
 clusion : and Kutherford and Soddy 1 and W. E. Wilson 
 independently suggested even a cosmic application the 
 possibility that radiothermal actions may contribute to 
 solar heat. Sir George Darwin 3 pointed out the influence 
 which such a view must have on speculations as to the 
 age of the sun. The present writer 4 drew attention 
 to its probable importance as a source of terrestrial 
 heat, whether only abundant in the crust or deeper 
 down; and the effect of this view on conclusions as to 
 the earth's age which had been founded on the existing 
 thermal state of the crust. Finally, Eutherford cal- 
 culated that a quantity so minute as 4*6 x 10~ u grams 
 per gram of the earth's materials would account for 
 all the heat which, on the evidence of the surface- 
 gradients of temperature in the crust, we know to be 
 escaping at the surface. 
 
 No even approximate estimate of the concentration 
 of radium in the rocks and soils was possible, however, 
 till the appearance of the Hon. K. J. Strutt's researches 
 in the year 1906. 5 For the first time definite quanti- 
 tative measurements on rocks and soils gathered from 
 widely separated localities were here forthcoming. 
 This work also first established the fact that the 
 quantities at the surface of the earth are much in 
 
 1 Rutherford and Soddy, Phil. Mag., May, 1903. 
 
 2 Wilson, Nature, July 9, 1903. 
 
 3 Sir G. Darwin, Nature, September 24, 1903. 
 
 4 Joly, Nature, October 1, 1903. 
 
 6 Strutt, Proc. E. S, t 77 A, 472 and 7 A, 150. 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 37 
 
 excess of Kutherford's estimate quoted above. Con- 
 sequently a purely surface-distribution was assumed by 
 Strutt, extending to a depth of about 45 miles. This 
 estimate would not, however, indicate the true bearing 
 of the experiments according to the views of the in- 
 vestigator; for, owing to an erroneous idea prevailing 
 at the time as to the constant proportion of radium to 
 uranium, which constant enters into the calibration of 
 the electroscope, the results obtained by Strutt require 
 to be reduced by nearly half their value. Hence, on 
 the theory put forward by Strutt, a layer of about 
 twice this depth must be required to account for the 
 earth's heat. 
 
 The measurements made by Strutt, as corrected for 
 the error just referred to by Eve, 1 are as follows : 
 
 IGNEOUS ROCKS. Radium in grams 
 
 per gram. 
 
 Granite, . . . Rhodesia, . . 478 x 10' 12 
 
 ,, . . . Cornwall, . . 4*67 ,, 
 
 Zircon syenite, . . Norway, . . . 4*65 ,, 
 
 Granite, . . . Cornwall, . . 4'21 ,, 
 
 ,, . . . Cape of Good Hope, 3*57 ,, 
 
 ,, ... Cornwall, . . 3'45 ,, 
 
 ... Westmoreland, . 3 '31 ,, 
 
 Syenite, . . . Norway, . . . 2*44 ,, 
 
 Granite, . . . Devon, . . . 1'84 ,, 
 
 Blue ground, . . Kimberley, . . 1'68 ,, 
 
 Leucite basanite, . Vesuvius, . . 1'66 ,, 
 
 Hornblende granite, . Egypt, . . . T22 ,, 
 
 Pitchstone, . . Isle of Eigg, . . 1/03 
 
 Hornblende diorite, . Heidelberg, . . 0'99 ,, 
 
 Augite syenite, . . Norway, . . . 0'93 ,, 
 
 1 Eve, Phil. Mag., August, 1907, p. 231. 
 
38 
 
 RADIOACTIVITY AND GEOLOGY. 
 
 IGNEOUS ROCKS continued. 
 
 
 
 Radium in grams 
 
 
 
 per gram. 
 
 Peridotite, * . 
 
 Isle of Rum, . 
 
 . 0-68 xlO- 12 
 
 Olivine euchrite, . 
 
 ,, 
 
 . 0-64 
 
 Olivine basalt, . . 
 
 Skye, . . 
 
 . 0-66 
 
 Basalt, . * . 
 
 Victoria Falls, 
 
 . 0-63 
 
 Hornblende granite, . 
 
 Leicestershire, 
 
 . 0-62 
 
 Dolerite, . 
 
 Isle of Canna, . 
 
 . 0-62 
 
 Greenstone, ... 
 
 Cornwall, 
 
 . 0-57 
 
 Basalt, ,. . 
 
 Antrim, . . 
 
 . 0-52 
 
 Serpentine, . 
 
 Cornwall, 
 
 . 0-50 ,, 
 
 Granite, .' , 
 
 Isle of Rum, . 
 
 . 0-36 
 
 Olivine rock, 
 
 ,, 
 
 . 0-33 
 
 Dunite, . ' , 
 
 Loch Scaivig, . 
 
 . 0-33 
 
 Basalt, . . . 
 
 Greenland, . . 
 
 . 0-30 
 
 SEDIMENTARY ROCKS. 
 
 Oolite, i -. 
 
 Bath, 
 
 . 2-92 
 
 ' . . ' '.' . 
 
 St. Alban's Head, 
 
 . 2-02 
 
 Marble, . . ,. 
 
 East Lothian, . 
 
 . 1-93 
 
 Kimmeridge clay, 
 
 Ely, V 
 
 . 1-88 
 
 Oil-bearing sandstone, 
 
 Galicia, . . 
 
 1-52 
 
 Roofing slate, . i ' - 
 
 Wales? . 
 
 . 1-28 
 
 Gritty slate, . . 
 
 Cornwall, 
 
 . 1-25 
 
 Gault clay, 
 
 Cambridge, 
 
 . 1-01 
 
 Clay, . . 
 
 Essex, . . . 
 
 . 0-86 
 
 Red sandstone, . . 
 
 East Lothian, . 
 
 . 0-84 
 
 Fine gravel, . . 
 
 Essex, 
 
 . 071 
 
 Red chalk, ';. 
 
 Hunstanton, 
 
 . 0-53 
 
 Flint, . . ' . 
 
 Essex, . < \ 
 
 . 0-53 
 
 White marble, . - V 
 
 India, . 
 
 . 0-27 
 
 Marble, . 
 
 East Lothian, . 
 
 . 0-26 
 
 Chalk, bottom of pit, 
 
 Cambridge, 
 
 . 0'39 
 
 top of same pit, 
 
 n 
 
 . 0-12 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 39 
 
 Eve finds the mean of these results to stand as 
 follows : 
 
 Igneous rocks, . . . . . 1*7 x 1(H 2 
 
 Sedimentary rocks, I'l ,, 
 
 In 1907 Eve 1 made some determinations on Canadian 
 rocks. The results arrived at were : 
 
 Radium in grams 
 per gram. 
 
 Trenton limestone, crystalline, Ordovician, 0'92 x 10~ 12 
 
 ,, ,, weathered, ,, 0'91 
 
 Essexite. Devonian? .... 0'26 
 
 Tinguaite, .... 4'30 
 
 ,, different locality. Devonian? 3'00 
 
 Nepheline syenite. Devonian? . . I'lO 
 
 Boulder clay, Quaternary, . . . 0'80 
 
 Ledaclay, ... 0'88 
 Saxicava sand, ,, . . .0*16 
 
 These rocks are from the neighbourhood of Mon- 
 treal. 
 
 The view that all the radium is collected near the 
 surface in such concentration as the rocks indicate is 
 beset with difficulties to which we must refer in another 
 place. But that there must be a considerable degree 
 of surface-concentration admits of no doubt. This 
 radioactive surface-layer must extend to a depth of 
 some miles. This at first sight seems inexplicable. For 
 the parent substance uranium is the heaviest element 
 known, and its concentration near the centre of the 
 earth is more what we would have expected. The 
 consideration and possible explanation of the observed 
 facts we must postpone. It is sufficient for us at present 
 
 1 Eve, loc. cit. 
 
40 RADIOACTIVITY AND GEOLOGY. 
 
 to recognize that our surface investigations cannot 
 apply to the whole bulk of the earth, and that in view 
 of the importance attached to the downward distribu- 
 tion of radium, measurements effected on igneous 
 materials, probably coming from considerable depths, 
 and on such materials as have been encountered in the 
 deepest borings and tunnels, possess particular interest. 
 The problem, too, of the general surface-richness of the 
 earth is closely associated with the radioactive state of 
 the ocean and its sediments. 
 
 I From the radioactive point of view the surface-rocks 
 may be divided into : 
 
 (a) Igneous ; possessing a store of uranium pre- 
 
 sumably coeval with the association of 
 
 elements which led to the formation of the 
 
 . . 
 originating magma. 
 
 (b) Detrital sedimentary; the more or less disin- 
 
 tegrated and granular remains of rocks of 
 group (a). The uranium in these rocks is 
 mainly residual in the case of the coarser 
 varieties, as sandstones, breccias, &c. ; but in 
 the finer varieties, as slates and fine shales, 
 the uranium was probably derived in part by 
 precipitation from the waters in which the 
 sediment was deposited. 
 
 (c) Precipitated sedimentary ; in which the whole or 
 
 nearly the whole of the materials has been 
 derived from dissolved substances by organic 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 41 
 
 or purely chemical agencies. In these the 
 uranium is in most cases derived entirely 
 from the waters in which the rocks were 
 formed. 
 
 The results obtained by the author on the radio- 
 activity of rocks are in general agreement, and in 
 many cases in close agreement, with those obtained 
 by Strutt and Eve. It is probable that what 
 difference is exhibited may arise from the natural 
 advance in our knowledge of the best conditions of 
 extracting the emanation. Thus it has been experi- 
 mentally found that any trace of precipitation may 
 conceal the emanation, retaining it in the solution even 
 after prolonged boiling. Again, it seems certain that 
 ebullition in vacuo and collection in a dry vessel is the 
 more effective procedure in carrying out the extraction. 
 The concentration of large amounts of dissolved sub- 
 stances in the solutions is also a source of diminished 
 results. It is important, too, to operate on fresh, 
 unweathered specimens of rock. A summation of 
 these influences probably results in the higher general 
 means recorded further on. 
 
 The applications of the surface-radioactivity of the 
 earth to geological problems, which occupy most of the 
 ensuing chapters, are based on values somewhat below 
 the means of all published results. 
 
 The basic rocks of the Deccan, which are of enormous 
 extent, have yielded radium at every point at which 
 they have been examined. A series of tunnel and 
 surface specimens, kindly sent by the Director- General 
 of the Indian Geological Survey Sir Henry Holland 
 
42 RADIOACTIVITY AND GEOLOGY. 
 
 enabled estimates to be made at widely separated 
 points. The specimens examined up to the present are 
 the following : 
 
 Bedded basalt, Ghats near Thuria, . * . 27 
 
 Pavagad Hill, Panch Mahals, . 8'2 
 
 Basalt dyke, near Palasi, Dhar State (2), . . 4'9 
 
 Bedded basalt, tunnel near Palasdari, . . . 3'8 
 
 ,, ,, ,, between Keloli and Vani, . 2 '4 
 
 ,, ,, ,, between Kasara and Igatpuri, 3'6 
 
 Mean, . . .4'3 
 
 The convention of omitting the qualifying expression 
 10~ 12 will be continued, as above, throughout the rest of 
 this book unless occasion arises to specially refer to this 
 factor. It will be, therefore, always understood that 
 the radioactivity expressed by an unqualified number 
 is the quantity of radium in billionths of a gram per 
 gram of material under discussion. A number given in 
 brackets after the designation of a substance refers to 
 the number of tests, generally on independent samples 
 of the substance : the radioactivity in such cases being 
 the mean of the results. 
 
 The basalt of the Giant's Causeway, which is part of 
 the great lava flow of the Inner Hebrides, has been 
 treated in five experiments, all, however, on adjoining 
 fragments of the rock. The results agree very closely. 
 In all there are fourteen results on basic rocks : 
 Deccan basalts and traps (6), . ;','. 4'3 
 Giant's Causeway basalt (5), . . . .5*8 
 Basalt, Mellenifjord, Greenland, . . .. 5'0 
 " Whinstone," Balfour Bore, Fifeshire (from a 
 depth of 3000 feet), 3'8 
 
 Mean, 4'9 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 43 
 o 
 
 On granites there are six results : 
 
 Leineter granite, Bally knock en (2), . . .5*4 
 Mourne granite, Newry, Co. Down, . . . 3'7 
 Aberdeen granite (3) (from depths of 80 to 270 ft. 
 
 in Kemnary Quarry), 3*2 
 
 Mean, . . . 4'1 
 Syenite, Plauen, . 6'8 
 
 And the following from the very ancient rocks of 
 
 Sutherlandshire, Scotland : 
 
 Lewisian mica schist, . . . . . '.6*3 
 gneiss, . . . 3'8 
 
 ,, hornblende schist, . . . .8*0 
 
 Mean, . . 5 '7 
 
 Twelve specimens of recent volcanic racks : 
 Lava, Crater, Kilauea, 1875, . . .6*2 
 Scoria, Stromboli, . . . . . . .6*0 
 
 Augite andesite, Val di Lachi, Lipari, . .2*4 
 
 Trachyte, Monte Olibano, Campi Phlegraei (2), . 10 '6 
 Obsidian, Ascension Island, . . . .2*5 
 
 Lava, Rocci Rossi, Lipari (2), . . . . 8'0 
 
 Bomb, Vulcano (2), 7 '6 
 
 Olivine trachyte, Ischia (2), . . . . 9'4 
 Pumice, Chimborazo, near Quito, . . . 4'8 
 Lava, Vesuvius, 1855 (2), . ; . . . 19 '2 
 Bomb, Martinique, 1902 (weathered), . . .2*0 
 Pumice, Krakatoa, 6 '9 
 
 Mean, . . . 7'1 
 
 To these must be added a large number of rocks 
 which enter into the composition of the Simplon 
 massif, and which are believed to be of igneous origin ; 
 and others again which enter into the St. Gothard and 
 the Finsteraarhorn massifs. These are in every case 
 
44 RADIOACTIVITY AND GEOLOGY. 
 
 tunnel specimens, some of those from the Simplon 
 coming from depths beneath the surface of 2135 metres 
 (7005 feet). 
 
 [Reserving particulars till we come to consider 
 specially the results obtained in the case of these 
 two great tunnels, it is sufficient to say here that in 
 the case of the Simplon some difficulty was experienced 
 from what may be called sporadic radium. Possibly 
 the cause of this has to do with the facts that not only 
 are many of these rocks rather above the average in 
 radioactivity, but they have been exposed to extreme 
 metamorphic actions which may have led to a segre- 
 gation of the uranium. Some, bearing evident uranium 
 minerals, were met with, so that a low and a high mean, 
 the latter embracing a few very exceptional results, 
 appear to arise out of the experiments. Further 
 experiments are in progress to determine, if possible, 
 the most probable average. Such extreme variations 
 were not noticed in the St. Gothard rocks. The 
 Finsteraarhorn granite is included in the mean of 
 the St. Gothard igneous rocks. 
 
 Simplon (32) ; schists and gneisses, mean, low, . 7*6 
 
 (32); high,. 9-1 
 
 St. Gothard (32) ; granites, gneisses, and schists, 
 
 mean, 5'1 
 
 The mean of all the igneous rocks examined by the 
 author (92 distinct specimens), taking the low mean of 
 the Simplon rocks, is 6'2. This may be somewhat 
 unduly raised owing to the seemingly rather abnormal 
 results obtained with the Simplon materials. Setting 
 aside for the present the entire Simplon series of igneous 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 45 
 
 rocks, the mean of 60 rocks is 5'5. These 60 results, 
 taken along with those of Strutt and Eve on similar 
 materials, give a general mean of 4-2 obtained on 92 
 specimens of various igneous rocks. 
 
 It would be impossible to predict with any certainty 
 the relative radioactive states of the sediments and 
 the parent igneous rocks. So far as the purely detrital 
 rocks are concerned, the question would turn, of course, 
 on how far the uranium is retained in the debris of 
 the parent rock, and on the weathering qualities of 
 certain minerals which appear to be very often 
 specially radioactive ; for instance, zircon. There is 
 little doubt that high results occasionally met in 
 rocks of the detrital class are due to the preservation 
 of this mineral. Evidently we could not safely 
 predict the result where such uncertain factors enter ; 
 but, reasoning from the behaviour of other heavy metallic 
 substances, the probability would incline in favour of 
 the uranium being in considerable part removed in 
 the general disintegration of the rock. Thus we should 
 expect a lowered radioactivity in the case of such rocks. 
 The precipitated sediments present a very different 
 problem, and one which experiment alone could solve. 
 Here the radioactivity must depend on the abstraction 
 of uranium along with the rock-materials in the 
 process of deposition, and clearly the radioactivity 
 of the ocean is concerned in the genesis of such 
 materials. The uranium in the ocean and its sedi- 
 ments is indeed a factor of such considerable impor- 
 tance in the consideration of the whole question of 
 radioactive denudation, seeing that the whole or the 
 
46 RADIOACTIVITY AND GEOLOGY. 
 
 greater part of it must have come from the land, that 
 we will state what is known on the subject before 
 passing on to the sediments. 
 
 The waters of the ocean cover five-sevenths of the 
 earth's surface to a mean depth of about 3 '8 kilometres, 
 and represent, of course, far the most abundant of the 
 surface-materials open to our investigations. A con- 
 siderable number of results have accordingly been 
 obtained in order to arrive at some conclusion as to the 
 probable mean radioactivity. Quantities of the water 
 from 1500 to 3000 c.cs., in most cases concentrated to 
 about 'half bulk by evaporation in a laboratory into 
 which radioactive preparations had never come, were 
 used for testing. The results obtained are for the greater 
 number published with fuller particulars elsewhere. 1 
 
 Radium per c.c. 
 
 Coast round Ireland (5), 0'034 
 
 Atlantic Ocean, Madeira to Bay of Biscay (5), 0'017 
 
 Atlantic, New York to Ireland, Arctic current, 0*016 
 
 ,, ,, ,, ,, Gulf Stream, . 0'014 
 
 ,, Mid Atlantic, . O'Oll 
 
 ,, 260 miles W. of 
 
 Ireland, . O'OOS 
 
 lat. 00', long. 3126'W., . . . 0*038 
 
 Atlantic, South ; lat. 16 54' S., long. 3749' W., 0'007 
 
 Mediterranean (2), O'OOS 
 
 Black Sea, 0'007 
 
 Arabian Sea, lat. 1040'N., long. 580'E., . 0'027 
 Indian Ocean to Mediterranean 
 
 Sandheads to Madras, 0'004 
 
 Off Colombo, . . 0-007 
 
 ,, ,, Minicory to Sokstoa, 0*004 
 
 Red Sea, off Jiddah, 0'009 
 
 1 Joly, Phil Mag., March, 1908, p. 385. 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 47 
 
 The mean of the twenty-four determinations, many 
 of which are individually the results of several experi- 
 ments, is 0'017 x 10~ 12 grams per cubic centimetre. 
 
 These results are obtained by modes of treatment 
 varied in such a manner as would best guard against 
 any systematic experimental error. Sea-waters are 
 only safely dealt with on addition of pure redistilled 
 hydrochloric acid, which lessens greatly the risk of a 
 precipitate forming in the process of concentration. 
 The variability of the results is considerable. To what 
 is this due ? It would seem unlikely that retention 
 of the emanation can account for the lower results, 
 for most of them were confirmed by a second boiling, 
 and in some cases on three or four repetitions of the 
 experiment. The second boiling generally gives a some- 
 what increased yield of emanation, but there is seldom 
 any subsequent rise. Again, the matter was inves- 
 tigated by adding to a sea-water a certain known 
 small quantity of radium, and seeking if the emanation 
 corresponding to it could be completely withdrawn. 
 The result was quite satisfactory. This experiment 
 does not indeed prove conclusively that a residuum of 
 emanation might not remain over in all sea-waters 
 after ebullition ; but if so, we would expect this residuum 
 to be fairly constant in amount, and the differences 
 from experiment to experiment have still to be 
 explained. Nothing is more astonishing than the fact 
 that these small amounts of emanation can, in any 
 case, be withdrawn. The volume of the emanation per 
 gram of radium is but 0*585 cub. mms. 1 Thus there are 
 
 1 Rutherford and Geiger, Proe. It. ., 81 A., 173. 
 
48 RADIOACTIVITY AND GEOLOGY. 
 
 but 9 billionths of a cubic millimetre in a litre of 
 average sea-water ! 
 
 It is, perhaps, premature to say much as to the 
 possible cause of the variations. The most plausible 
 explanation seems to be the removal of radioactive 
 matter from the surface-waters by precipitation under 
 the action of decaying organic life and sulphur 
 bacteria; so that according to the local organic con- 
 ditions and it is known that these vary considerably 
 under the influences of surface-currents, etc. a large 
 variation of the radium near the surface might arise. 
 The investigation of water from the deeper parts of 
 the ocean has yet to be carried out, and will be of 
 interest. 
 
 There is evidently a large quantity of radium in the 
 ocean. Taking the mass of the ocean as 1*452 x 10 18 
 tonnes, there must be about 20 x 10 9 grams, or about 
 20,000 tons of radium, in its waters. 
 
 It is of interest to note in connexion with the radio- 
 activity of sea-water that Eve 1 has found that the 
 ionization of the atmosphere over the central parts of 
 the Atlantic is comparable with that over the land, 
 even in the calmest weather, when it cannot be 
 supposed that the emanation, which, doubtless, is 
 responsible, could have been blown from the con- 
 tinents. This fact appears to be traceable to the 
 radioactivity of the water, and the facility with which 
 emanation may be supposed to escape from it. The 
 bubbling of air through liquids containing a trace of 
 radium results in loading the air with the emanation. 
 
 i Eve, Phil. Mag., 6, xiii., p. 248. 
 
RADIUM IN THE EARTH'S SURFACE -MATERIALS. 49 
 
 Hence every bubble rising through the water, or 
 mingled with it in rough weather, brings emanation to 
 the surface. Again, it is certain that the emanation 
 diffuses out at the surface ; and as this is continually 
 being renewed by currents and disturbances of every 
 kind, the effect must be maintained continually at its 
 maximum. It must result from these causes that 
 the ocean, although not nearly so radioactive as 
 the land, must give up its emanation more freely, 
 and hence the observed ionization appears accounted 
 for. 
 
 The condition of the deep-sea deposits fully supports 
 the observations on the radioactivity of the ocean. 
 From materials supplied to the author, mainly by 
 Sir John Murray, seventeen various samples of oceanic 
 deposits have been examined. 1 
 
 Blue Mud. "Challenger." Off E. coast of N. America. 
 
 Lat. 38 34' N., long. 72 10' W. 1240 fathoms. 
 
 Radium 3'1. 
 Terrigenous Mud. From " mud volcano," coast of Arakan, 
 
 Bay of Bengal. Surface. Radium 2 '9. 
 Green Sand. Ss. "Albatross." Locality unknown. Radium 
 
 4-5. 
 Globigerina Ooze. Off W. coast of Ireland. 570 fathoms. 
 
 Radium 6 "6. 
 Globigevina Ooze. Central N. Atlantic. Lat. 48 17' N., 
 
 long. 39 49' W. 2493 fathoms. Radium 7*0. 
 Globigerina Ooze. "Challenger." Middle of S. Atlantic. 
 
 Lat. 21 13' S., long. 14 2' W. 1990 fathoms. Radium 
 
 67. 
 
 1 Joly, Phil. Mag., July, 1908, p. 190. 
 E 
 
50 RADIOACTIVITY AND GEOLOGY. 
 
 Globigerina Ooze. " Challenger." Pacific Ocean, W. of S. 
 
 America. Lat. 38 6' S., long. 82 2' W. 1825 fathoms. 
 
 Radium 7 '4. 
 Globigerina Ooze. "Albatross." Central Pacific. Lat. 22 
 
 11' S., long. 133 21' W. 2042 fathoms. Radium 8'0. 
 Calcareous Mud. "Albatross." E. of Society Islands. Lat. 
 
 21 4' 6" S., long. 133 1' 2" W. 2225 fathoms. Radium 
 
 22-2. 
 Red Clay. " Albatross." N. Pacific, W. of Central America. 
 
 Lat. 10 38' N., long. 105 47' 6 W. 1955 fathoms. 
 
 Radium 13 '0. 
 Red Clay. "Challenger." N. Atlantic, off coast of Africa. 
 
 Lat. 24 20' N., long. 24 28' W. 2740 fathoms. Radium 
 
 15-4. 
 Red Clay. "Challenger." Central Pacific, near region of 
 
 Radiolarian Ooze (as under). Lat. 13 28' S., long. 
 
 149 13' W. 2350 fathoms. Radium 52 '6. 
 Radiolarian Ooze. "Challenger." Central Pacific. Lat. 
 
 3 48' S., long. 152 56' W. 2600 fathoms. Radium 
 
 22-8. 
 (From this sample the magnetic particles had been 
 
 abstracted.) 
 Radiolarian Ooze. "Challenger." Central Pacific. Lat. 
 
 7 25 S., long. 152 n 15' W. 2750 fathoms. Radium 
 
 50-3. 
 Manganese Nodule. Locality same as that of last ooze. 
 
 Radium 24 '0. 
 Manganese Nodule. "Albatross." Pacific, off W. coast of 
 
 S. America. Lat. 8 29 '5 S., long. 85 35' '6 W. 2370 
 
 fathoms. Radium 21 '0. 
 
 The diatom oozes have not yet been examined. 
 The great quantity of radium spread over the ocean 
 floor is well shown when we enter the above results 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 51 
 
 against the extension of the several sediments as 
 determined by Sir John Murray : 
 
 TJ j- Extension : millions 
 
 Radium. of square miles. 
 
 Globigerina Ooze, . . 7*2 . . 49 '5 
 Radiolarian Ooze, . .367 . . 2 '5 
 Red Clay, .... 27'0 . . 51 '5 
 
 It is established on satisfactory evidence that the 
 more slowly collecting deposits are those of non- 
 calcareous character the red clay and the radiolarian 
 ooze ; the red clay in some cases being almost devoid 
 of calcareous matter, and collecting at an extremely 
 slow rate. The evidence for this is the amount of 
 unburied and even fossil remains which are brought 
 up with such deposits. The foregoing results show 
 that these deposits are richer in radium than the more 
 rapidly growing calcareous sediments. This would 
 follow if we supposed the rain of calcareous and 
 siliceous tests acted as a dilutant. It is of interest 
 to note in this connexion that the process of extraction 
 of lime salts from the ocean by organic actions must, 
 to a considerable extent, exclude the uranium in the 
 waters, as indeed might be anticipated. The curious 
 calculation of Bischof, 1 that an oyster requires the lime 
 from some 27,000 to 76,000 times its weight of sea- 
 water, would establish this fact; for if the uranium 
 present in this bulk of water was taken into the shelly 
 limestones, their radioactivity would be much greater 
 than experiment shows. 
 
 1 Bischof, Elements of Chemical and Physical Geoloyy, vol. i., p. 180, 
 English translation. 
 
 2 
 
52 RADIOACTIVITY AND GEOLOGY. 
 
 The following table shows that radioactivity and 
 the percentage of calcareous matter in these deposits 
 stand in an inverse ratio : 
 
 Calcium 
 
 Carbonate Radium, 
 per cent. 
 
 Globigerina Ooze. "Chall."338, . 92'24 . 6'7 
 
 296, . 64-34 . 7'4 
 
 Red Clay, . 5, . 12'00 . 15'4 
 
 . 276, . 28-28 . 52'6 
 
 Radiolarian Ooze, . 272, . 10 '19 . 22 -8 
 
 274, . 3-89 . 50-3 
 
 The percentages of calcium carbonate are from the 
 report of the " Challenger " Expedition. The red clay 
 fourth from the top in the table might appear ex- 
 ceptional to the others, having too high a percentage 
 of calcium carbonate for the radium content. It is 
 probable that this is a case of recent change in the 
 character of the deposit, the antiquity being shown by 
 the number of sharks' teeth and manganese nodules 
 brought up. Thus the specimen is a mixed one, so to 
 speak, there being a lower part rich in calcareous 
 matter, which acts to dilute the red clay more recently 
 forming above. Several such cases of recent change 
 are recorded by Murray and Renard. 1 
 
 The view that the radium on the earth's surface 
 might be of extra-terrestrial origin 2 suggested the 
 desirability of examining the cosmic particles long ago 
 identified by Sir John Murray as widely distributed 
 over the ocean floor, and, as it were, concentrated 
 in the deposits of very slow growth. If the theory 
 
 1 Report Scientific Results, " Challenger " Expedition. 
 
 2 Joly, Nature, January 24, 1907, p. 294. 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 53 
 
 has any reality, we should expect that such bodies 
 would reveal exceptional richness in uranium, and, of 
 course, in radium. A tube of magnetic particles, 
 among which the cosmic particles would be included, 
 received from Sir John Murray, was accordingly 
 tested for radium. These magnetic particles had been 
 withdrawn from material raised from a depth of 
 2300 fathoms in the North Pacific. The quantity 
 available was only 1-1 gram. The result was a radium 
 content, with difficulty read, of 0*6. Had there been 
 present even one rich uranium-bearing particle, so 
 small a result would not have been obtained. The 
 theory referred to therefore lacks any support which 
 this experiment might afford. 
 
 "We have seen in an earlier chapter that it is to the 
 genetic connexion between uranium and radium that 
 we look for the maintenance and renewal of the latter. 
 It is thus of importance, in relation to oceanic radium, 
 to establish, where practicable, the existence of the 
 parent substance, and thus obtain assurance that the 
 radium is not exotic in character, and the radioactivity 
 a passing property. In the majority of rock-investi- 
 gations this is impossible, owing to the minuteness of 
 the quantities of uranium involved, and the lack of 
 character which the uranium spectrum exhibits. But 
 in these cases the conditions assure us that, unless there 
 be some other parent to radium, the uranium must be 
 there ; for how else has the radioactivity been main- 
 tained through past ages ? The oceanic conditions 
 lead us to a similar conclusion, so far as the waters 
 of the ocean are concerned. In the great volume of 
 
54 EADIOACTIVITY AND GEOLOGY. 
 
 the ocean we have estimated not less than 20,000 
 tons of radium. This cannot have been supplied in a 
 period so brief as to be included in historical times. 
 Yet, unless the uranium is present, the whole amount 
 of radium must be renewed in a few thousand years 
 by the rivers. In order to accomplish this, however, 
 the rivers must be much richer in radium than the 
 ocean. We can acquire an idea of what their radium 
 content must be. The waters of the ocean possess 
 a total mass of 14 x 10 18 tonnes. The amount of 
 water annually discharged by the rivers of the world 
 has been estimated by Murray at 27,191 cubic kilo- 
 metres, or about 27 x 10 18 tonnes. Now a-sVo part of 
 the radium is transformed each year, so that if the rivers 
 are effective in keeping up the supply, they must in 
 one year send into the ocean an amount of radium 
 equal to ^ of what is present in its waters. But 
 the quantities of ocean-water and river-discharge per 
 annum are as 50,000 to 1, or, in other words, the rivers 
 must take 50,000 years to replenish the ocean. They 
 should, therefore, be richer in radium in the ratio of 
 50,000 to 2540, or as, say, 20 to 1. There is no evidence 
 of river- water possessing such richness. An examination 
 of Nile water afforded a radioactivity of but 0*0042, 
 which is about one-fourth the oceanic, and water from 
 the Kio de la Plata showed 0-0052. 
 
 Again, we are obviously in the difficulty of accounting 
 for the uranium corresponding to the radioactivity of 
 the ocean if not conveyed to the sea. For in this case it 
 must be in the detrital sediments, and these should show 
 the concentrated radioactivity of ages of denudation. 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 55 
 
 This they do not, but in point of fact are less radioactive 
 than the igneous rocks a state which is in harmony 
 with one conclusion only : that the uranium released 
 from the rocks by denudation is carried into the 
 ocean. 
 
 While this reasoning seems unassailable as regards 
 the ocean, we cannot apply it rigorously to the sediments 
 accumulating beneath. It might be that a certain pre- 
 cipitation of radium would give to the surface parts of 
 these deposits a temporary high radioactivity. 
 
 Against this view we have the fact that such ancient 
 deep-sea deposits as are open to our examination show 
 considerable radioactivity. The chalk and greensand 
 are instances. The former must have been a relatively 
 fast-formed deposit composed almost entirely of sub- 
 stances extracted from solution ; its radioactivity com- 
 pares well with that of Globigerina Ooze. A specimen 
 of green sandstone showed a somewhat higher radio- 
 activity than a similar sediment recently dredged. 
 Plainly in these cases the uranium must have been 
 precipitated along with the other materials. 
 
 As further evidence on this point, in the case of a 
 red clay showing the high radioactivity of 544, Professor 
 Emil Werner has recently estimated the uranium, using 
 a colorimetric method. In 8*42 grams of the clay sub- 
 mitted to Werner, the quantity of uranium should be 
 1*3 milligrams. The estimate afforded from 0'6 to 0*7 
 milligrams. But the separation of the uranium is 
 probably incomplete. 1 
 
 If we take it that the richness in radium of the 
 
 1 Joly, Phil. Mag., July, 1908, p. 196. 
 
56 RADIOACTIVITY AND GEOLOGY. 
 
 oceanic sediments is on the average no greater than 
 that which we observe in the Globigerina Oozes and 
 this is probably a low estimate the quantity of radium 
 involved is enormous. We can calculate with consider- 
 able security the aggregate mass of such oceanic deposits 
 as are derived from dissolved materials; i.e. the cal- 
 careous oozes and muds, the diatomaceous oozes, the 
 radiolarian oozes, and much of the mass of the red 
 clays. 
 
 The elements of this seemingly insoluble problem are 
 really simple. We know that the sediments in the 
 ocean once in solution are all ultimately derived from 
 the igneous rocks ; we also know, as the average result 
 of numerous analyses of both igneous and detrital sedi- 
 mentary rocks, that about 33 per cent, of the former 
 pass into solution in the process of their conversion into 
 the latter. Now of these substances going into solution, 
 most are again precipitated from the ocean, or extracted 
 by organic agency more or less rapidly, forming, in fact, 
 the sub-oceanic sediments. The salts of sodium alone 
 are neither precipitated nor extracted. 1 This is owing 
 to their very high solubility. The recognition of this 
 fact gives us a clue to many great generalizations in 
 the history of solvent denudation. Among other facts, 
 it serves to tell us what total amount of original igneous 
 rock must have been broken up by denudation during 
 the course of geological time. 2 This knowledge is 
 obtained directly from the chemical analyses showing 
 the average loss of sodium from igneous rocks when 
 
 1 Joly, Trans. Royal Dublin Society, vol. vii., ser. ii., 1899, p. 23, 
 et seq. 2 Joly, ibid., p. 46. 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 57 
 
 becoming sedimentary rocks, and the chemical analyses 
 which give us the quantity of sodium in the sea. 
 
 Thus we find that the average igneous rock contains 
 3*39 per cent, of soda and the average sedimentary rock 
 1-30 per cent. 1 The loss has, therefore, been 
 
 3-39 - 1-30 x ~- = 2-52 percent. 
 
 in the process of conversion. Now the sodium in the 
 ocean when converted into the equivalent oxide 
 amounts to 21 x 10 15 tonnes. The mass of the parent 
 igneous rock is therefore : 
 91 x 1 15 
 
 " .-0 X 10 = 83 ' 3 X 1Q16 t0nneS ' 
 a Oft 
 
 About 33 per cent, of this parent igneous rock 
 passes into solution ; and this amount would represent 
 all the substances now in the sediments and still in 
 solution in the ocean, but for the fact that some part of 
 what has been precipitated in the ocean in past times 
 is no longer to be reckoned among the deep-sea deposits, 
 having been, in fact, elevated into dry land. Such are 
 the limestones and some other deposits which, com- 
 paratively, are insignificant in amount. Van Hise 2 
 estimates the bulk of the calcareous rocks as amounting 
 to about 5 per cent, of the total sedimentaries on the 
 land. Now, as the insoluble detrital sediments are 67 per 
 cent, of the total parent igneous rocks, there exist 
 
 83-3 x 10 16 x -- = 55-8 x 10 16 
 tonnes of such detrital rocks, by far the larger part of 
 
 * Clarke, The Data of Geochemistry. 
 
 2 Van Hise, Treatise on Metamorphism, p. 940. Somewhat higher 
 estimates have been made by others. 
 
58 RADIOACTIVITY AND GEOLOGY. 
 
 which is on the land. According to Van Rise's estimate, 
 then, the mass of detrital rocks plus the mass of 
 calcareous rocks is obtained from the equation : 
 
 M x ^ 
 M- ~ = 55-8 x 10 16 tonnes ; 
 
 from which M = 587 x 10 16 tonnes, and 5 per cent, of 
 this is the amount of the calcareous rocks that is, 
 2-9 x 10 16 tonnes. 
 
 Hence, from 33 per cent, of the mass of the parent 
 igneous rock i.e., from 27'5 x 10 16 tonnes we must 
 take 2-9 x 10 16 tonnes, in order to allow for the lime- 
 stones now elevated on the dry land. This leaves 
 24-6 x 10 16 tonnes as the mass of the sediments on the 
 floor of the ocean plus the mass of dissolved elements 
 in the ocean. Taking the mass of the ocean as 
 1-45 x 10 18 tonnes, and the dissolved substances therein 
 as 3*5 per cent., the deduction for matter in solution is 
 51 x 10 16 tonnes, leaving finally about 19'5 x 10 16 tonnes. 
 This is the mass of sediment precipitated on the ocean 
 floor. The area of the ocean is estimated at 367 x 10 12 
 square metres. If the mean specific gravity of such 
 deposits as we are dealing with be 2 '5, their bulk 
 amounts to 8 x 10 16 cubic metres nearly, and the 
 average depth of precipitated sediments over the ocean 
 floor would be about 218 metres, assuming that the rate 
 of deposition had everywhere been alike throughout 
 geological time. The latter supposition is, of course, 
 imaginary. 
 
 It is probably safe to assume that the mean 
 radioactivity of this vast quantity of sediment is no 
 greater than that of Globigerina Oozes. Accepting 
 
KADIUM IN THE EAKTH'S SURFACE- MATERIALS. 59 
 
 6 x lO^ 12 grams of radium per gram as the radioactivity, 
 we arrive at the result that there are 117 x 10 6 tonnes 
 of radium, or well over one million tons, spread over 
 the ocean floor. This is the record of the accumulated 
 work of denudation throughout geological time : the 
 quantity in the ocean waters is, as we have seen, 
 relatively small. The result not only serves to bring 
 before us a perspective view of the great phenomena 
 with which we are dealing, but supports the finding 
 on the sedimentary rocks, which appears to show that 
 there is an appreciable difference in the radioactivity 
 of the sedimentary and igneous surface-materials. 
 
 
 
 The detrital rocks can, as already observed, owe only 
 a small part of their radium to the waters in which 
 they have been deposited. The finer sediments of this 
 class owe more to this source, probably, than those 
 coarser ones which are practically composed entirely of 
 discrete particles of older rocks. It is, of course, 
 impossible to draw a hard and fast line between this 
 group of rocks and those derived mainly by precipita- 
 tion of dissolved substances. The following belong 
 mainly to the detrital group : 
 
 Welsh Slate (4), 5'8 
 
 Devonian Slate, Valencia, Co. Kerry, . . 3*6 
 Carbonaceous Shale, Burnlip Bore, Coatbridge, 
 
 Fifeshire. Depth, 1173 ft., . . . . 8'7 
 Carbonaceous Shale, Burnlip Bore, Coatbridge, 
 
 Fifeshire. Depth, 1168 ft., . . . .67 
 Carbonaceous Sandstone, Burnlip Bore, Coat- 
 bridge, Fifeshire. Depth, 1095 ft., . . 2'9 
 Keuper Sandstone, 2*0 
 
60 RADIOACTIVITY AND GEOLOGY. 
 
 Argillaceous Shale, Balfour Bore, Fifeshire. 
 
 Depth, 2124 ft., 3'6 
 
 Argillaceous Shale, Balfour Bore, Fifeshire. 
 
 Depth, 2568 ft., 3 '5 
 
 Argillaceous Shale, Balfour Bore, Fifeshire. 
 
 Depth, 3420ft., 37 
 
 Arkose Sandstone, Balfour Bore, Fifeshire. 
 
 Depth, 3804 ft., 4'1 
 
 Carbonaceous Shale, Balfour Bore, Fifeshire. 
 
 Depth, 4404 ft., 3'6 
 
 Quartz Grit, Balfour Bore, Fifeshire. Depth, 
 
 4530ft., .5-1 
 
 Sandy Mud from bed of R. Amazon, Pard, . 3*2 
 The mean of these results is 44. 
 Some of these, as will be seen, come from considerable 
 depths. The shales contain plant remains, and are 
 probably of fresh-water origin. 
 
 The following belong to the second group of sedi- 
 mentary rocks, and must have derived their radium 
 mainly or entirely from the ocean : 
 
 Limestone, Carboniferous, Burnlip Bore. Depth, 686 ft., 3'0 
 Balfour Bore. 2442ft., 10'9 
 
 ,, Ordovician, Trenton Valley, New York, . .3*2 
 
 ,, Carboniferous, Armagh, . . - . . 3'4 
 
 ,, Lias, Vaihingen, Wurtemberg, > . . 3'0 
 
 ,, Solenhofen, Bavaria, . ... . 2^2 
 
 Dolomite, Zechstein, Osterhagen, Harz, . . . .3*2 
 
 Chalk, Marsupites, A ; . . 4'2 
 
 Coral Rock (recent), Funafuti Bore. Depth, 736-748 ft., 1-5 
 
 815-822 ft., 2-5 
 844-866 ft., 1-5 
 874-936 ft., 1-3 
 
 Gypsum, Keuper, Westphalia, 6'9 
 
 Green Sandstone, Werl, Westphalia, . . . . 4'9 
 The mean of these results is 3 '7. 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 61 
 
 Additional to the 27 rocks of the above tables may 
 be quoted experiments on 19 rocks belonging to the 
 sedimentary class and of Mesozoic age, from the 
 St. Gothard tunnel. They are considerably altered by 
 metamorphic action. They are cited more particularly 
 further on. Again, 17 rocks of the Simplon tunnel 
 which have been examined for radium are of the sedi- 
 mentary class, and of Trias-Jura age. The means in 
 each case are: 
 
 Mesozoic Sediments, St. Gothard (19), . , 4'2 
 Trias- Jura Sediments, Simplon (17), . / 6*9 
 
 Thus a total of 63 rocks of sedimentary origin have 
 been examined, giving the general mean of 4*7 ; or, if 
 the Simplon results be excluded for reasons already 
 given, the mean is 4*3. 
 
 Even when exceptionally high results are excluded, 
 the average radioactivity of the igneous rocks, accor- 
 ding to the present experiments, must be taken as 
 approximating to 5 and of sedimentary rocks as near 4. 
 For purposes of calculation further on, it has been 
 thought well to take even lower values, partly because 
 the general mean of all known results would fall below 
 the above numbers, partly because we have to extend 
 our estimates to materials which we cannot with 
 certainty affirm to be represented on the surface. 
 
 It may be well to emphasize the fact that in selecting 
 rocks for examination only unweathered specimens 
 should be taken. Between the weathered rock and the 
 final sandy residue containing but little radium it is 
 only a matter of degree. Thus the sand of the 
 Arabian Desert afforded but 0'4 as the radium content ; 
 
62 RADIOACTIVITY AND GEOLOGY. 
 
 and in experiments made by other observers such 
 substances as clays and soils invariably show the 
 effects of weathering in removing the uranium and 
 radium. Where our object is to find a value for the 
 radium- content of the average crust-materials of the 
 earth, it is evident that washed-out superficial residues, 
 although in themselves of interest, should not be 
 included. 
 
 Perhaps the most important problem of the future 
 will be to find if any indication of diminishing radio- 
 activity, as we go down in the crust, is to be detected. 
 So far nothing definite on this point can be inferred. 
 In parts the Deccan basalt shows a low radium-content ; 
 but the average is not lower than that of other rocks 
 of the kind. The central St. Gothard rocks, as will later 
 be seen, show the lowest radioactivities met with. 
 There may be significance in this. But the whole 
 question is at present hampered with the difficulty 
 that we know but little as to the origin of the igneous 
 rocks. We do not know the depths from which they 
 have been brought ; and it may be that their appear^ 
 ance at the surface is connected with a radioactivity 
 greater than what generally prevails in the regions, 
 beneath or within the crust, from which they originate. 
 
 The widespread occurrence of radium in the rocks 
 affords an explanation of emanation not only in spring- 
 waters, but in air drawn from soils and in the air 
 of caves. Extending this conclusion, we see in the 
 existence of gaseous substances among the series of 
 radioactive elements a natural explanation of the 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 63 
 
 radioactivity of the atmosphere, and, in part, of its 
 state of ionization. The radioactive gaseous elements 
 being in every known case substances of rapidly changing 
 character, we recognize here the influence of a continual 
 diffusion and convection of gases from the soils and 
 rocks in making good the transformation of atmospheric 
 emanation into solid forms. The experiments of Elster 
 and Geitel 1 and others have shown that these solids 
 may be collected by exposing negatively electrified 
 wires to the air. Such electrified wires become, in fact, 
 coated by active deposits which may be removed by 
 solvents or by friction, as in the case of the emanations 
 of thorium and radium. (See Chapter X.) 
 
 Again, there appears to be little doubt that the 
 occurrence of helium in springs, in natural gases, and 
 in the atmosphere (in which according to Eamsay it 
 exists to the extent of four parts in a million) must be 
 ascribed to the general diffusion of unstable elements 
 in the surface-crust of the earth. For there is no 
 longer any question of the identity of helium with 
 the alpha ray. This subject is dealt with in another 
 connexion (Chapter XI,. Here it is sufficient to call 
 attention to the confirmatory bearing of these facts 
 upon the results of the investigations recorded in the 
 foregoing pages. 
 
 The presence of thorium as a generally diffused rock- 
 constituent has yet to receive the attention it deserves. 
 Its emanation has been detected under various conditions 
 both in the atmosphere and in underground waters ; 
 
 1 Elster and Geitel, Phys* Zeit., ii., p. 590, and iii., p. 76, 1901. 
 
64 RADIOACTIVITY AND GEOLOGY. 
 
 and the experiments of Strutt 1 would appear to show 
 that this element and its radioactive descendants are 
 not without influence in supplying helium to the 
 rocks. 
 
 It is possible that the aggregate radiothermal energy 
 from this source may be not inconsiderable compared 
 with that derived -from the uranium family of elements. 
 
 A remarkable phenomenon by which the presence 
 of radium may be detected by visual observation occurs 
 in certain rocks. In brown mica (most generally biotite), 
 in cordierite, and in some other minerals, small, circular, 
 coloured spots, known as pleochroic haloes, have long 
 been recognized by petrologists. These coloured areas 
 are invariably found associated with a minute, centrally 
 placed crystal, either of zircon, or more rarely of 
 apatite. 
 
 The properties of the haloes are remarkable. In the 
 case of biotite the medium in which the halo occurs is 
 itself strongly pleochroic. A section of biotite taken 
 transverse to the cleavage, when viewed by transmitted 
 polarized light, darkens and brightens as the section is 
 rotated over the polarized beam ; the absorption being 
 greatest when the plane of vibration of the ray is in 
 the plane of cleavage of the mineral. A section which 
 is taken parallel to the cleavage and viewed by trans- 
 mitted polarized light shows only a feeble change of 
 colour when turned round respecting the ray. The 
 remarkably distinct cleavage properties of mica enable 
 these sections to be readily distinguished under the 
 
 1 Strutt, Proc.lt. S., vol. Ixxv., pp. 88, 312 ; also vol. Ixxx., p. 5C. 
 
PLATE III. 
 
 Photograph of a pleochroic halo in a rock-section; magnified 300 
 diameters ; light polarized. The halo-sphere extends across flakes 
 of biotite mutually inclined, as is shown by the unequal absorp- 
 tion of the light. 
 
OF THE 
 
 UNIVERSITY 
 
 F 
 
RADIUM IN THE EARTH'S SURFACE- MATERIALS. 65 
 
 microscope ; the former showing fine longitudinal stria- 
 tions, the latter none. 
 
 A halo occurring in this medium shows all the 
 above pleochroic properties intensified. Viewed in a 
 section across the cleavage of the mica the halo may 
 become quite opaque when the polarized ray vibrates 
 in the plane of cleavage, and, again, may practically 
 disappear when the vibrations are at right angles to 
 this position ; the central zircon being then most plainly 
 defined and showing its characteristic strongly refracting 
 properties. If the halo is observed on a mica section 
 taken parallel to the cleavage, there is a comparatively 
 slight change in the absorption when the ray or section 
 is rotated. 
 
 It is evident that these haloes are really spherical in 
 shape. For no matter in what direction the section is 
 made, they still preserve a circular form, only departing 
 from this if the central particle is elongated; in this 
 case the halo, following the extension of the particle, 
 becomes ellipsoidal : but when the zircon is a minute 
 speck of roughly cubical form, the halo is found to be 
 a circle of remarkable truth in every plane of section. 
 
 The mode of formation of these haloes has long been 
 an enigma, and many investigations have been made in 
 the hope of throwing light on their origin. It has been 
 found that when the rock is highly heated the haloes 
 disappear. Chiefly owing to this last observation, they 
 have been ascribed by some petrologists to organic 
 colouring matter. 
 
 A very little consideration should suffice to dispose 
 of the view that they can be due to any diffused or 
 
 F 
 
66 RADIOACTIVITY AND GEOLOGY. 
 
 aggregated colouring substance. For diffusion or aggre- 
 gation could not possibly proceed at an equal rate across 
 and along the cleavage of a medium possessing the 
 structure of mica. In the plane of cleavage this mineral 
 is a highly pervious substance, as is evinced by the 
 intercalations of minerals, such as hematite, which 
 have been introduced in solution, and also by the 
 facility with which alterations take place in the 
 direction of cleavage. The direction transverse to the 
 cleavage is, on the other hand, one of remarkable 
 impermeability, as every observation testifies. It is 
 safe to affirm that no process of diffusion, whether 
 gaseous or liquid, nor yet any process of aggregation, 
 could take place in such a medium without being 
 influenced by the difference of physical properties 
 which the wonderful cleavage gives rise to. 
 
 Now, not only do we find that the haloes extend their 
 boundaries uninfluenced by the crystalline cleavage, but 
 cases are not uncommonly met with where the halo 
 extends from one flake of mica to another quite unin- 
 fluenced by any mutual orientation of the flakes. This 
 happens when the originating zircon is situated near 
 the boundary of a mica crystal, and a second crystal of 
 mica is sufficiently near to be affected. The photograph 
 (Plate III.) shows such a case. The magnification is 300 
 diameters, and the light is polarized. The lower dark 
 part of the halo is in a crystal of biotite so turned 
 to the polarized rays that the absorption is nearly 
 complete. The upper part of the halo, being in a 
 differently oriented section of mica, shows a less 
 degree of absorption. The central zircon is clearly 
 
RADIUM IN THE EARTH'S SURFACE-MATERIALS. 67 
 
 seen. The rock here is a granite a rock in which 
 haloes are common. 
 
 In the case of the substance cordierite or iolite we 
 are dealing with a medium of very different crystallo- 
 graphic properties from those of mica. There is not the 
 remarkable cleavage, nor is the pleochroism conspicuous 
 in thin sections. The substance possesses, however, 
 pleochroic properties which in a small hand specimen 
 are easily seen without the use of a polarizer; the 
 crystal most generally appearing blue in one direction 
 and straw-yellow in a perpendicular direction. 
 
 Haloes found in cordierite are not so conspicuous as 
 in biotite. They are visible on certain sections of the 
 mineral as pale-yellow stains surrounding the zircon 
 enclosure. Kotated over the polarized rays, these 
 stains brighten and darken. 
 
 There would certainly be less difficulty in ascribing 
 the colouration in this case to the diffusion of a pig- 
 ment. The colouration disappears when the rock is 
 heated, as in the case of biotite. 
 
 That these haloes are caused by the radioactive pro- 
 perties of the centrally placed mineral particle, admits 
 of little doubt. 1 The evidence may be summarized as 
 follows : (a) They seem to be invariably associated with 
 the presence of a central substance of radioactive cha- 
 racter. Some petrologists affirm that they occur with 
 zircon only. But both zircon and apatite, according to the 
 observations of Strutt, 2 are minerals of high radio- 
 activity, containing, in every case examined, unusually 
 large amounts of radium. Allanite is said to form, 
 
 1 Joly, Phil. Mag., March, 1907, p. 381. 0. Miigge, CentralbL fur 
 Min., July, 1907. 2 Strutt, Proc. R.S., vol. Ixxviii., p. 150. 
 
 F 2 
 
68 RADIOACTIVITY AND GEOLOGY. 
 
 sometimes, the central mineral : the author finds that 
 this mineral is, in some cases at any rate, strongly radio- 
 active. 1 (6) Theories of a pigment, etc., diffusing from 
 the central particles are not tenaole, as explained above. 
 (c) There is exact correspondence between the radius 
 of the halo and the distance to which the alpha radia- 
 tion of radium would penetrate effectively, i.e., before 
 its velocity is so reduced that its ionizing properties 
 disappear. The halo appears never to exceed appre- 
 ciably a certain maximum radial dimension of 0*05 
 millimetre. The average of a number of measurements 
 was 0*04 millimetre. Smaller haloes are due generally 
 to the section under observation being taken off the 
 centre of the sphere, and, possibly, are occasionally due 
 to a feeble source of radioactivity, so that the visible 
 effect is confined to a region wherein the radiations are 
 sufficiently dense. Kutherford has shown that thirteen 
 layers of aluminium, each '00031 centimetre thick, 
 reduce the velocity of the most energetic alpha radia- 
 tions of radium to a degree rendering them incapable of 
 effecting photographic actions or of exciting phospho- 
 rescence. The aggregate thickness of aluminium is here 
 0'04 millimetre, and the density of biotite (27 to 3'1) 
 differing but little from that of aluminium (2 -7), we 
 see that the range of chemical activity of an alpha ray 
 is in close correspondence with the radius of the halo. 
 Such a coincidence should really be conclusive, for on 
 any hypothesis of diffusion, etc., there need be no radial 
 limits that we can define, (d) Colouration of like tint 
 can be produced in cordierite 2 and biotite 3 by exposing 
 
 1 Joly, loc. oil. 2 0. Miigge, Centralbl. fur Min., July, 1907. 
 
 3 Joly, Nature, Oct. 10, 1907 p. 589. 
 
EADIUM IN THE EARTH'S SURFACE-MATERIALS. 69 
 
 these substances to the rays from specks of radium. 
 The areas so coloured are pleochroic. In the case of 
 mica the pleochroism is feeble, for the colouration 
 thus artificially produced is viewed transverse to the 
 cleavage surface, which in any case is a direction of 
 feeble pleochroism. 
 
 It is true that the evidence from the experimental 
 development of colouration is weakened by the proba- 
 bility that the change is here to be ascribed to the 
 action of the more penetrating rays. Tims we may be 
 dealing with the ]3 and y radiations which are known 
 to colour many media. It is possible that the deep 
 brown of certain micas much spotted by haloes may be 
 due to these penetrating radiations. The coincidence 
 of the radius with the effective range of the alpha ray, 
 on the other hand, renders it practically certain that 
 this particular radiation is responsible for the halo. It 
 is probable that no true artificially formed halo has 
 yet been produced. It may be added that Zirkel 1 
 records the occurrence of a grey marginal layer around 
 the zircons of the Ehenish basalt. This layer is 0'05 
 millimetre thick, and is doubtless of similar origin to 
 the halo. The examination of this layer for helium 
 would be of interest. 
 
 What the halo is we do not know. Helium must be 
 stored therein, but in very minute quantities. The 
 presence of pleochroic haloes in rocks affords a visible 
 proof of the sporadic and unequal distribution of 
 radium to which we have already called attention. 
 
 1 Zirkel, ' Uber Urausscheidungen in Rheinischen Basalten," p. 52. 
 
CHAPTEE IV. 
 
 UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 
 
 UNSATISFACTORY STATE OF THE SUBJECT OF UNDERGROUND TEMPERA- 
 TURE ESTIMATES OF THE AVERAGE GRADIENT IMPORTANCE OF THE 
 GRADIENT SIGNIFICANCE AND USE OF THE CONDUCTIVITY THB 
 
 CONDUCTIVITIES OF ROCKS VARIATIONS OF CONDUCTIVITY WITH 
 
 TEMPERATURE AND WITH CHANGE OF STATE APPLICATION TO CAL- 
 CULATE THE HEAT ESCAPING FROM THE EARTH APPLICATIONS TO 
 
 BORINGS AND TUNNELS OUR KNOWLEDGE INSUFFICIENT THE BAL- 
 
 FOUR BORE THE SIMPLON TUNNEL : TEMPERATURES ENCOUNTERED 
 
 EXPLANATIONS INADEQUATE COMPARISON WITH TEMPERATURES 
 
 IN THE CENTRAL ST. GOTHARD CONDUCTIVITY OF THE SIMPLON 
 ROCKS THE RADIOACTIVITY OF THE SIMPLON ROCKS DOWNWARD 
 EXTENSION OF THE ROCKS REQUIRED TO ACCOUNT FOR THE ESCAPING 
 HEAT THE GEOLOGICAL STRUCTURE OF THE ST. GOTHARD VARIA- 
 TIONS OF GRADIENTS IN THE ST. GOTHARD THE DISTRIBUTION OF 
 RADIUM SUGGESTS AN EXPLANATION THE GRADIENTS AND THE 
 RADIOACTIVITY COMPARED. 
 
 THE subject of underground temperature is one of 
 difficulty and obscurity. Much observational matter 
 has been collected which can only be partially sifted 
 from erroneous and misleading results, the pursuit of 
 the subject being often left to those who are quite 
 unqualified for the task. Observations are seldom 
 made for their own sake, but under conditions which 
 render them of secondary moment; whereas the sub- 
 ject is of such capital importance that investigations 
 specially directed to the end in view would richly 
 repay even large expense. 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 1 
 
 As the subject stands, there is grave doubt as to the 
 reliability and significance of many of the published 
 results, and much diversity of opinion exists as to 
 what observations are specially deserving of weight. 
 One fact appears clear, that, whether owing to local 
 differences of conductivity or other causes, there is 
 considerable variation in the recorded gradients. In 
 mountain regions the circulation of water under 
 gravity plays a considerable part, as the highly scien- 
 tific observations made in the St. Gothard and Simplon 
 tunnels have definitely proved. Sollas has suggested 
 that in some cases the abnormal gradients may be 
 associated with the distribution of molten matter 
 beneath the crust. 1 
 
 The following are some of the values, accepted by 
 authorities on the subject, as fair averages of the results 
 on the rise of temperature downwards : 
 
 Prestwich, . . . . 1 C. in 24 '3 metres 
 Lord Kelvin, , . ' i ,, 27 '5 ,, 
 
 Schardt, 2 .- * . -/,;V 32'0 
 
 Geikie, 3 : ^' ; . - ' ' ; 27'0 to 32'4 metres 
 British Assoc. Committee, . ,, 32 '4 metres 
 Clarence King, . . . 38 "9 ,, 
 
 The mean of the values here collected is only a little 
 above that adopted by the British Association Com- 
 mittee. Accordingly, in the applications which come 
 later on, we take the average gradient as 32 metres 
 for 1 C. 
 
 1 Sollas, The Age of the Earth, p. 18. 
 
 2 Schardt, Verhandl. Schweizerischen Naturf. Gesellsch., 1904, 87, 
 p. 204. 
 
 3 Geikie, Text-look of Geology, vol. i., p. 64. 
 
72 EADIOACTIVITY AND GEOLOGY. 
 
 The importance of the gradient from the geological 
 point of view is the information which, taken along 
 with the conductivity, it affords as to the flow of heat 
 to the earth's surface. 
 
 The reader who is not versed in the subject should 
 endeavour to be quite clear on the connexion between 
 the gradient and the heat-flow. The consideration of 
 an analogous case may help. The flow of water through 
 a uniform, narrow tube that is, the number of cubic 
 centimetres passing any cross-section per second may 
 be estimated if we know the difference of pressure at two 
 points, say, one metre asunder ; and also the effect of 
 unit-pressure in driving water through unit-length of 
 the tube. Let the pressures be measured by the 
 equivalent head of water. Then if the difference of 
 hydrostatic pressure at points one metre apart is, say, 
 ten centimetres, and if we know by experiment that a 
 difference of hydrostatic pressure of one centimetre at 
 points one centimetre asunder, produces a flow of, say, 
 five cubic centimetres per second, then we conclude that 
 as the pressure gradient of ten centimetres per metre is 
 one-tenth that which produces five cubic centimetres 
 per second, the flow will only be the one-tenth part, 
 or one-half a cubic centimetre per second. 
 
 Now the gradient of pressure in the case of the tube 
 corresponds to the gradients of temperature we have 
 already discussed as prevailing in the rocks; and the 
 experimentally determined flow under the condition of 
 unit-pressure gradient corresponds to the conductivity 
 of the particular rock. In the latter case that which 
 flows is heat, measured in so many calories or therms 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 73 
 
 per second. When we know how many calories flow 
 through a cross-section of one square centimetre for 
 unit gradient that is, a difference of temperature of 
 1 C. per centimetre and also know the gradient 
 prevailing in the rock, we have only to express the 
 latter as such a fraction of a degree per centimetre, 
 and then multiply by the conductivity, and we have 
 the flow in calories per square centimetre. The calorie 
 here is the quantity of heat required to raise one gram 
 of cold water through one degree centigrade. 
 
 We see, then, that the indications of the gradient 
 are dependent upon the conductivity of the particular 
 class of rock in which the observations are made. 
 Thus there might be cases in which the gradient was 
 very different, but the flow of heat might be the same. 
 Again, the gradients might be alike, but the flow of 
 heat different. These principles will be required in 
 connexion with various matters later on. 
 
 The thermal conductivity is again a very variable 
 property; and we can only make general use of it 
 by seeking some mean value. Eocks are what would be 
 described as bad conductors. Under the conditions of 
 gradient mentioned i.e., 1 C. per centimetre the flow 
 in calories per square centimetre per second is in 
 nearly every case some number in the third place of 
 decimals. 
 
 The following are the most important determinations 
 selected from the results published by the British 
 Association Committee, 1 and in the Landholt-Bornstein 
 
 1 Everett, C.G.S. System of Units. 
 
74 RADIOACTIVITY AND GEOLOGY. 
 
 Tabellen. The rocks are dry : moisture tends to increase 
 the conductivity : 
 
 Sandstone, , . . . f 0'0055 
 
 Micaceous flagstone, . . . . 0*0053 
 
 Slate, 0-0048 
 
 Marbles, Limestone, and Dolomite, . . 0'0051 
 
 Caen stone, 0*0043 
 
 Chalk, . 0-0025 
 
 Clay slate, . 0'0027 
 
 Calcareous sandstone, 0'0021 
 
 Granite, . . . . . . . 0*0053 
 
 Whinstone, Trap rock, and Mica- schist, . 0'0038 
 Basalt, . . . .... 0-0067 
 
 Syenite, . . . * . . . . < . 0'0044 
 
 Glass, . .. . . . . . 0-0017 
 
 
 
 The mean of these values is for the igneous materials 
 0*0042, and for the sedimentary 0*0041. For approxi- 
 mate calculations the average 0-004 sufficiently nearly 
 represents the results on rocks generally. 
 
 A very important question is whether the con- 
 ductivity increases or diminishes at high temperatures. 
 There is insufficient information on the subject. 
 K. Weber has found that gneiss, which has a con- 
 ductivity of 0*00578 at 0, falls in conductivity to 
 0-00416 at 100 C. ; x and Lees has found, on the other 
 hand, that the conductivity of glass increases by 0'0025 
 per cent, per degree between 35 and 60 C. 2 Metals 
 appear to diminish in conductivity with rise of tem- 
 perature; but alloys, on the other hand, improve in 
 
 1 Landholt-Bornstein, Tabellen. 
 
 2 Lees, Phil. Trans., 1898, A, 399. 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 75 
 
 conductivity. 1 Such results as we possess on con- 
 ductivity, traced through the solid and into the liquid 
 state, would rather lead to the view that a fluid rock- 
 magma must possess a lower conductivity still than 
 the solid. Thus Lees found that ice falls in conductivity 
 from the temperature of liquid air upwards, and shows 
 a considerable diminution in the liquid state. Similarly 
 in the case of aniline the conductivity solid is much 
 greater than in the liquid state. In the case of 
 glycerine the change is small, but still one of diminu- 
 tion. 2 Barus has obtained similar results in the case 
 of thymol. 3 
 
 Knowing the conductivity, it is easy to estimate the 
 amount of heat which must annually escape at the 
 earth's surface. 
 
 We have taken a mean gradient of 32 metres per 
 degree, and a mean conductivity of O004 for the average 
 rock. Expressing the gradient as the rise in tempera- 
 ture in a distance of one centimetre measured down- 
 wards, we have 
 
 x 0-004 = 1-25 x 10- 
 
 as the quantity of heat in calories which is conveyed 
 to the surface per second per square centimetre of the 
 earth's surface. If we multiply by the number of 
 seconds in a year (31*5 x 10 6 ), and by the area of the 
 earth in square centimetres (51 x 10 17 ), we finally arrive 
 
 1 Lees, Proc. R. S. 80, A. 143. 2 Ibid. 
 3 Barus, Am. Journ. Sc. t July, 1892. 
 
76 RADIOACTIVITY AND GEOLOGY. 
 
 at the total annual escape of heat from the earth. It 
 comes out at 2-0 x 10 20 calories. 
 
 Of course this must be regarded as only an approxi- 
 mation. The gradient beneath the oceans is unknown ; 
 we assume that it does not differ much from that 
 which prevails (with variations) over the land surface. 
 For the purpose of arriving at general views the result 
 is probably close enough to the truth. 
 
 We have now to consider the principles which guide 
 us in estimating the temperature effects produced by 
 radium as a widely diffused rock-constituent. 
 
 We have already seen that the heat evolved by one 
 gram of radium and its associated radioactive elements is 
 closely 5 f 6 x 10" 2 calories per gram of radium per second; 
 that is to say, one calorie every eighteen seconds. This 
 is, indeed, a rapid rate of evolution of energy from 
 so small a quantity of matter. When, however, we 
 come to deal with billionths of a gram of radium per 
 gram of rock, of course the absolute quantity of heat 
 evolved in any small volume of the rock is minute. 
 On the other hand, in the case of the geological 
 problems, there is practically unlimited time for its 
 accumulation. 
 
 The accumulation is favoured by the nature of the 
 circumstances. We are not dealing, it will be noticed, 
 with an isolated quantity of radium deposited at a 
 particular depth from the surface, but with materials 
 containing radium disseminated more or less uniformly 
 throughout their bulk. Hence the rock whether 
 igneous or sedimentary is continually producing heat 
 
UNDERGPwOUND TEMPERATURE AND RADIOACTIVITY. 77 
 
 throughout every cubic centimetre of its volume. 
 Thus, at any level beneath the surface, the heat 
 produced in a particular cubic centimetre of the rock 
 can only escape in virtue of this little mass of rock 
 being hotter than the cubic centimetre of material 
 immediately above it. But in this also heat is being 
 developed, thus checking the escape of that which is 
 developed beneath; and this reasoning applies all the 
 way from base to top of the entire layer of rock : so 
 that if no heat is escaping below, the maximum 
 temperature must be at the base, and this basal 
 temperature gives place to a lesser temperature as we 
 go upwards at first changing- slowly, and then rather 
 more rapidly near the surface. 
 
 When the question is treated mathematically, the 
 equilibrium temperature 9, at any depth x from the 
 surface, is found by the equation 1 : 
 
 where D is the total depth of the radioactive layer 
 supposed neither to gain nor lose heat at the base, 
 and to retain a constant temperature at the immediate 
 surface ; K is the conductivity of the particular rock ; 
 h is the constant of heat-production of radium per 
 second ; and q the amount of radium contained in one 
 cubic centimetre of the rock. 
 
 At the very base of the layer, x becomes equal 
 to D, and the basal or maximum temperature is 
 
 See Appendix A. 
 
78 RADIOACTIVITY AND GEOLOGY. 
 
 The first equation tells us that in order to determine 
 the temperature at any particular depth in the layer, 
 we must know the value of D or the total depth of 
 the radioactive layer. This might indeed be evident 
 from what we have already said about the temperature 
 at every level influencing the temperature beneath, 4 ;id, 
 of course, in sending heat to the surface, influencing 
 also the temperature above it. The second equation 
 tells us the very important fact that the maximum 
 temperature attained increases with the square of the 
 depth of the radioactive layer. Thus, if equal quantities 
 of radium were contained in two different radioactive 
 strata, that stratum which possessed it most diffusedly 
 distributed that is which had less per cubic centimetre 
 would have the higher basal temperature ; for the 
 depth of the stratum is necessarily greater than that 
 in which the radium is more concentrated. This, if 
 at first appearing contradictory, will be understood 
 when it is remembered that the general effect of the 
 more diffuse distribution is to remove the radioactive 
 materials further from the surface, and so enable the 
 heat to be the better retained. 
 
 Applying these facts to the problem of interpreting 
 the radioactivity of the surface-materials in terms of 
 the gradient of temperature downwards, or, in other 
 words, of endeavouring to connect the radioactivity 
 with the underground temperature, we see that our 
 deepest borings and tunnels are still too high up in the 
 crust to enable us to pronounce with any certainty on 
 the effect of radium met with in the rocks. The local 
 radioactivity in itself can produce little effect on the 
 temperature of the rocks. 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 79 
 
 An example will make this clear. The Simplon 
 tunnel runs for a considerable distance at a depth, say, 
 of 2,000 metres from the surface. The radioactivity of 
 the rocks would appear to be about 19 x 10" 12 grams per 
 cubic centimetre, according to the experiments. Now, 
 if we suppose that this radioactivity extends no further 
 down than the level of the tunnel, the radium in the 
 whole 2,000 metres would produce a temperature due 
 to radioactivity of only 5'3 C. in the tunnel, this being 
 the basal temperature of the layer. 
 
 We see from this how unlikely it is that volcanic 
 temperatures can be brought very near the surface 
 by the radioactivity of the rocks in the upper crust; 
 for this involves a temperature of some 1200 being 
 so produced. To bring about such a temperature, as 
 a basal temperature, at a depth of, say, 7 kilometres 
 from the surface, there must be a radioactivity through- 
 out the 7 kilometres of over 340 x 10~ 12 grams per cubic 
 centimetre. This is quite in excess of what prevails in 
 any known considerable rock- mass. And as lavas, so far 
 as examined, show no very excessive radioactivity, there 
 can be no support for this theory of volcanic origin. 
 
 The gradients met with in deep borings, as already 
 observed, vary considerably. The variations are doubt- 
 less in most cases due to variations in the rock-con- 
 ductivity. But in some very erratic cases this may 
 not be so, and it may be found possible to establish 
 some connexion with the radioactivity of the local rocks. 
 The majority of researches in this direction must, how- 
 ever, probably prove unfruitful. Consider in this 
 connexion some radium measurements made in the 
 
80 
 
 RADIOACTIVITY AND GEOLOGY. 
 
 case of the Iklfour Bore in Fifeshire, which is carried 
 through Carboniferous shales and sandstones to a depth 
 of 4534-5 feet (1383 metres). The boring passes through 
 a succession of beds changing frequently in character ; 
 the most continuous rock being a coarse, diabase- 
 like layer some 500 feet thick. The shales contain 
 vegetable fossils, and are probably of fresh-water 
 origin. Throughout these materials, wherever tested, 
 save in one c|se, there is a notable sameness in 
 radioactivity : 
 
 - 
 
 Depth from 
 surface : 
 feet. 
 
 Gradient : 
 metres 
 per degree 
 Centigrade. 
 
 Radio- 
 activity. 
 
 Black argillaceous rock, . 
 
 2124 
 
 45-3 
 
 3-6 
 
 Grey limestone, 
 
 2442 
 
 427 
 
 10-9 
 
 Argillaceous, fossilif erous, 
 
 2568 
 
 42-7 
 
 3-5 
 
 Whinstone, 
 
 3000 
 
 42-7 
 
 3-8 
 
 Argillaceous rock, . ,, 
 
 3420 
 
 53-5 
 
 3-7 
 
 Arkose sandstone, . 
 
 3804 
 
 34-0 
 
 4-1 
 
 Black shale, 
 
 4404 
 
 437 
 
 3-6 
 
 Quartz grit, 
 
 4530 
 
 43-7 
 
 5-1 
 
 It is probable that the varying gradients which prevail 
 at the levels from which the samples are taken are 
 referable almost entirely to the differing conductivi- 
 ties of the beds : the radioactivity only affecting the 
 temperatures in quite a secondary manner. 
 
 In the case of the two great tunnels, the Simplon and 
 the St. Gothard, there is evidence of the intervention of 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 81 
 
 radioactivity in affecting the temperatures encountered 
 in making the tunnels. This evidence is, however, only 
 to be obtained by assuming and the assumption is 
 doubtless quite legitimate the downward continuation 
 of the radioactivity observed at the levels of the tunnels. 
 Tht, temperatures, which are so essential to any scientific 
 discussion of the subject, were read in the rocks when 
 the headings were being made. It has been found that 
 the means taken to keep down the temperature in the 
 progress of the works soon result in a fall of the 
 rock-temperatures, so that the data to be of any real 
 value must be obtained before this artificial source of 
 cooling becomes effective. Nothing can exceed the 
 pains devoted in both cases to this difficult work. 
 Its value is incalculable, not only from the scientific 
 but from the practical point of view. 
 
 The high temperatures met with in the Simplon, as 
 all know, nearly put an end to the project, although 
 the actual difficulty was more that arising from the 
 quantity of heat which entered in the torrents of hot 
 water than from the temperature of the rocks. The 
 facts in outline are these : For a stretch of some 
 seven or eight kilometres the mean depth of the 
 tunnel below the surface of the mountain is about 
 1700 metres. At the north end of this stretch, the 
 rocks attain the temperature of about 55, and this at 
 the south end sinks to 35. The temperature 5 5 '4 was 
 the highest encountered. It was not at the deepest part 
 of the tunnel, but at a point somewhat to the north of 
 it. 
 
 How unexpected such temperatures were, may be 
 G 
 
82 KADIO ACTIVITY AND GEOLOGY. 
 
 shown by the predictions of the best authorities when 
 the tunnel was still unattempted. Stapff distinguished 
 at once as engineer and as geologist had in the case 
 of the St. Gothard made a scientific study of the dis- 
 tribution of rock-temperature, and would rightly be 
 regarded as the most competent authority of the time. 
 His prediction for the Simplon was a maximum tem- 
 perature of 47 C. He was, however, alone in making 
 so high a prediction. Stockalper, also of St. Gothard 
 fame, foretold 37 at a depth of 2000 metres from 
 the surface. Heim thought 38 to 39 the probable 
 maximum. 1 
 
 These predictions were, doubtless, influenced by the 
 accepted views as to gradients in mountains. The 
 gradient actually attained at the hottest part of the 
 tunnel would not be excessive for a boring made in the 
 plains. Thus, if we take it that 50 C. was reached at 
 1600 metres below the mountain surface at C., the 
 gradient is no more than the average value of 32 metres 
 per degree. However, previous experience of gradients 
 in steep mountains would lead to the belief that the 
 gradients in such cases are influenced by the convexity 
 and steepness of the ground. In the case of the 
 St. Gothard, the gradient for the major part of the 
 tunnel was 46*6 metres, 2 and in the case of the Mont 
 Cenis tunnel it has been estimated at 427 metres. 3 In 
 
 1 Schardt, Verhandl Schweizerischen Naturf. Oeselkch. 1904. 87, 
 Jahresversammlung, p. 204. 
 
 Stapff, Trans. North of England Mining and Mech. Engineers, 
 xxxiii., 19 et seq. 
 
 3 Prestwich, Proc. E. S., xli., p. 1 et seg* 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 83 
 
 the last case the neglect of observations during the 
 progress of the headings deprives the estimate of much 
 of its reliability. Osmond Fisher, indeed, contends 
 with much force that the convexity of the ground 
 cannot exercise a marked influence on the gradients. 1 
 However, the fact remains that the temperatures 
 observed must be regarded as above those generally 
 experienced in previous cases, and, therefore, as 
 abnormal on the evidence of the, perhaps, small 
 number of observations of reliable character. 
 
 In explanation of the observed facts, it was pointed 
 out that the north end of the tunnel, where the 
 temperature was high, is exceptionally dry. The 
 conductivity of the rock is, therefore, less, and there is 
 no circulating water to carry off the heat ; while the 
 opposite conditions prevail at the south end. This is, 
 of course, a physical explanation of why the gradients 
 at the south end are low, and gives no reason for the 
 high temperatures at the north end. The circulation of 
 water is, in fact, a condition tending to disguise the 
 true flow of heat at the south end if the average 
 conductivity of rock is assumed where in truth it does 
 not apply. The question really turns on the con- 
 ductivity of the rock where the temperatures were 
 highest ; we want to know whether these temperatures 
 indicate an abnormal heat-flow or not. The next 
 explanation is, therefore, more to the point: the 
 direction of the schistosity is appealed to. It is well 
 known that considerable differences exist in the 
 conductivity of certain rocks according as the heat-flow 
 
 1 Phyric* of the Earth'* Crust, 2nd ed., p. 222. 
 G2 
 
84 ItADIOACTIVITY AND GEOLOGY. 
 
 takes place across the schistose cleavage or along the 
 cleavage. The explanation would be a good one if the 
 schistosity was actually horizontal. In point of fact, 
 the sections show it as pointing at a high angle to 
 the horizontal at stretches near the north end where 
 the temperature already nearly reached its maximum. 1 
 It cannot be doubted that the whole explanation of the 
 very remarkable difference between the gradients in 
 the central reaches of the Simplon and the St. Gothard 
 tunnels is not fully explained in the angle of schistosity 
 of the rocks in the former, nor yet, probably, in the 
 conductivities, as we shall presently see. 
 
 In the case of the St. Gothard, the temperatures 
 observed were of the most interesting character. In 
 the central parts of the tunnel the mean gradient 
 comes out as 46*6 metres per degree. These rocks are 
 gneisses and schists of varying character. At the north 
 end, where the tunnel passes through granite the 
 same granite that enters into the massif of the 
 Finsteraarhorn the gradient steepens to 20*9 metres. 
 This last gradient, exceptionally steep even for a level 
 plain, will again claim our attention. It is desirable 
 to first confine our considerations to the temperature 
 conditions where they are apparently least affected by 
 special circumstances in the two mountains. Failing 
 actual comparative measurements of the conductivities, 
 we must assume that in the central St. Gothard there 
 is actually a lesser flow of heat than prevails in the 
 Simplon massif. We assume that in those stretches 
 of both tunnels where there is no circulation of water 
 
 1 Schardt, loc. cit., and Schmidt, Eclogie Geol. Helvetia, ix., No. 484. 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 85 
 
 in the rocks, and both rocks are of much the same 
 character, the differing gradient must represent a real 
 difference iii the quantity of heat escaping to the 
 surface. In support of this assumption, it is to be 
 observed that there appears no good reason to explain 
 a difference in gradients of 32 and 46'6 metres, or 
 nearly 50 per cent., on any quality of the rocks. 
 Both are rocks of schistose and gneissic character, 
 highly compressed, and not cooled by circulating water. 
 The schistosity is more vertical, apparently, in the 
 St. Gothard than in the Simplon; but this cannot 
 create so great a difference in conductivity. Thus 
 Zollinger 1 gives the conductivities in kilogram-calories 
 per metre per hour of the schists and gneiss of the 
 Simplon tunnel, taken across and with the cleavage, 
 as 2'28 and 3*02 respectively ; in the cases of calcareous 
 schists and limestone, as 2 '10 and 3 '26 ; and in the case 
 of granite 1'94 across the stratification, and 2'49 in the 
 line of stratification. These reduced to the units 
 previously used the centimetre, gram, and second 
 give : 
 
 Gneiss and schists across cleavage, . . 0'0063 
 
 along . 0-0084 
 
 Calcareous schists 0'0058 
 
 Limestone, 0'0090 
 
 Granite across stratification, .... 0'0054 
 
 ,, along .... 0-0069 
 
 As schistosity is oriented in the tunnel, only a fraction 
 of the difference of conductivity in the several cases 
 
 1 Discussion on Mr. Fox's paper : Minutes, Proceedings Institute Civil 
 Engineers, 168, p. 53. 
 
86 RADIOACTIVITY AND GEOLOGY. 
 
 can be supposed to affect the flow of heat. It is also 
 evident that there is nothing in the absolute values 
 of the conductivities to account for the high tempera- 
 tures in the Simplon ; the conductivities are rather 
 above the average values. 
 
 It would appear, however, that in the radium-content 
 of the Simplon, contrasted with that of the St. Gothard, 
 a possible explanation is forthcoming. The following 
 summary embodies the results of experiments on the 
 radioactivities of the Simplon rocks. A detailed 
 statement of the individual results on the 49 rocks 
 investigated is not necessary, and would hardly be 
 desirable in view of the fact that a revision of the 
 averages arrived at may be necessary (see p. 44). 
 
 Jurassic and Triassic altered sediments, . . 6*4 
 Crystalline schists, partly Jurassic and Triassic, 
 partly Archaean, . . . . . . 7'3 
 
 Monte Leone gneiss and primitive gneiss, . .6*3 
 Schistose gneiss a fold from beneath, . .6*5 
 Antigorio gneiss, . . . . . .6*8 
 
 This subdivision of the Simplon rocks is that of 
 Professor Schardt. 
 
 The mean of the individual experiments is 7*1 x 10~ 12 
 grams per gram, and if some exceptionally high readings 
 be substituted for low ones obtained in the same rock, 
 the mean rises to 97 x 10~ 12 . 
 
 Now it will be presently seen that the experiments 
 on the St. Gothard rocks do not give such high values 
 unless in the granite at the north end of the tunnel. 
 The gradient, according to Stapff, 1 declines from the 
 
 1 Stapff, loc. eit. 
 
^ 
 " s ^i 
 
 -*I 
 
 1* 
 
 "V <J 
 
 ^ b 
 
 ^a 
 
 ; 
 
 *> Q 
 
 3 
 
 !- 
 
 c 
 
 r 
 
 f : 
 
 3 
 
 g 
 
 tJi 
 
 5 * 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 87 
 
 point 3550 metres measured from the north end to 
 4050 metres, after which it keeps a very low value 
 to the south end of the tunnel. Water-circulation 
 appears, however, to affect the gradients at the south 
 extremity, giving rise to the lowest gradients anywhere 
 observed in the tunnel. This applies probably to the 
 last 3 kilometres. If we take the radium determin- 
 ations from the beginning of the true St. Gothard 
 massif to the distance 11,000 metres from the north 
 end, we have the radium readings corresponding to the 
 central low gradients. A glance at the accompanying 
 chart, Plate IV., where the radium values are plotted 
 as a full line, and the gradients as a dotted line, will 
 more fully explain. The mean of these radium readings 
 is 3*3. In this mean we have apparently a record 
 of local radioactivity which we may compare, with 
 security against causes of special disturbance, with 
 the Simplon results. We find, in fact, the low radio- 
 activity associated with the low gradients, and the 
 high radioactivity with the high gradients. 
 
 We have already seen that without making assump- 
 tions as to the downward extension of the materials 
 examined so near the surface, we cannot expect to 
 connect our observations with the gradients. It is of 
 interest to estimate what downward extension of the 
 two rock-masses must be assumed to exist, always 
 supposing them to maintain the observed mean radio- 
 activities, in order to account for the temperature 
 observations. Such an estimate is simple, if we suppose 
 the conductivities alike throughout, and that therefore 
 the differing gradients represent a different heat-flow 
 
88 EADIOACTIVITY AND GEOLOGY. 
 
 to the surface. If now this difference of heat-flow is 
 due to the different radioactivities of the rocks, then 
 obviously there must be some distance to which if both 
 masses extend the superior heat-flow from the Simplon 
 becomes accounted for. This will be evident when it 
 is remembered that each metre downward increases 
 the superiority of the Simplon in respect of the total 
 radium transmitting heat to the surface. 
 
 It has been explained that the product of the 
 gradient and the conductivity is a quantity of heat 
 flowing to the surface ; accordingly, it follows that if 
 J2 and r be the radium content per cubic centimetre 
 of the Simplon and the St. Gothard rocks respectively, 
 and G and g the gradients in each case, and K the 
 conductivity, then when the rocks extend to a depth 
 sufficient to produce the difference of heat-flow by the 
 difference of total radium, we have X(E - r)h = K(G-g), 
 where X is the required depth, and li the heat-constant 
 of radium. Inserting the values of the radioactivities per 
 cubic centimetre (19 x 10~ 12 , and 8 x 10~ 12 ), and taking 
 JTas 4 x 10" 3 , and the gradients as -^^ and -^-^ 
 respectively, the value of X comes out as 7 kilometres. 
 That is to say, a downward extension of 7 kilometres 
 will establish the observed difference in gradient. 
 
 In the length of the St. Gothard tunnel a re- 
 markable variation of gradient was found to exist 
 between the northern sections and the central and 
 southern parts. As already stated, the gradient near 
 the north end is as high as 20*9 metres. In point of 
 fact, a study of the rocks through which the tunnel is 
 carried shows that the region in which this excessive 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 89 
 
 gradient was found is geologically distinct from the 
 St. Gothard massif. Entering at Goeschenen, the 
 tunnel begins in the compact granite which is re- 
 garded as belonging to the Finsteraar massif. This 
 granite, in parts gneissic, at a distance from the 
 entrance of about 1800 metres, gives place to rocks 
 of sedimentary origin, much altered, and of Triassic 
 and Jurassic age. The sediments take the form of 
 a downward fold or syncline, caught between the 
 Finsteraar granite and the rocks of the St. Gothard 
 massif proper. The downward extension of this 
 synclinal fold cannot be great. It is petrographically 
 continuous with the basin of the Ursern Thai, and 
 is known as the Ursernmulde. For two kilometres 
 about, the tunnel traverses these altered sediments, 
 which are of very varied character, before entering 
 the highly metamorphosed rocks of the St. Gothard 
 massif rocks which appear to be really of unknown 
 origin, but are generally regarded as igneous. Many 
 varieties of schists and gneiss enter into this complex, 
 which extends for about 7500 metres. The last section 
 is run through the Tessinmulde for a distance of 
 3000 metres. These materials are again sedimentary 
 in origin, and are mesozoic in age. (See Frontispiece.) 
 
 Stapff, in describing the variations in gradient in 
 the northern, central, and southern parts of the tunnel, 
 writes : 
 
 "They [the isotherms] show irregularities on the 
 south side, which clearly depend on cold springs; they 
 bend down rapidly, and then run smoothly inclined 
 beneath the water-filled section of the mountain. 
 
90 RADIOACTIVITY AND GEOLOGY. 
 
 Other local irregularities can be explained by the 
 decomposition of the rock; but there is no obvious 
 explanation of the rapid increase in the granite rocks 
 at the northern end of the tunnel (2000 metres) ; and 
 it is probably to be attributed to the influence of 
 different thermal qualities of the rock on the 
 coefficient of increase. For the rest these 2000 metres 
 of granite belong to the massif of the Finsteraarhorn, 
 and, geologically speaking, they do not share in the 
 composition of the St. Gothard. Perhaps these two 
 massifs belong to different geological periods (as 
 supposed for geological reasons long ago). What 
 wonder, then, if one of them be cooler than th 
 other?" 1 
 
 The suggestion here appears to be that some part 
 of the original heat has been retained by the granite 
 at the northern end of the tunnel. Prestwich modified 
 this suggested explanation by referring the heat in 
 the granite to the dynamical source arising from the 
 movement of the Finsteraar granite into its existing 
 position. 2 
 
 The results of the examination of the rocks through- 
 out the tunnel for radium seem to offer an explanation 
 of less hypothetical character. It is only necessary to 
 assume that the measurements of radium in the granite 
 indicate the radioactive character of the rock in its 
 downward extension beneath the in-folded Ursernmulde. 
 
 The chart showing the gradients and the measure- 
 ments renders evident to the eye that at the north end 
 
 1 Stapff, loe. cit., p. 30. 2 Prestwich, loc. cit., p. 44. 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 91 
 
 the radioactivity, which is not specially high at the 
 entrance, increases rapidly after the first kilometre to 
 8 and ultimately to 14, just before the granite dips 
 beneath the syncline. As already observed, the Ursern 
 rocks can only go down a little way, and can therefore 
 have but little influence on the supply of radioactive 
 heat; and the gradients where the tunnel traverses 
 these rocks must be referable to heat emanating from 
 beneath them. Stapff shows in a detailed table of 
 gradients beneath the plain of Andermatt that the 
 gradients continue to rise to a point about 2500 metres 
 from the north end, after which there is a rapid 
 fall, so that when the St. Gothard rocks are met the 
 gradient has fallen to nearly the low mean of the 
 central section. Here the radioactivity of the rock 
 changes to values about 3, rising, indeed, for short 
 lengths, but again sinking, and ultimately reaching 
 values only a little over 1. At the south end, where 
 the syncline of the Tessin rocks meets the St. Gothard, 
 a revival of radioactivity is indicated by the experi- 
 ments ; but in the Tessinmulde the radioactivities fall 
 again. At this end of the tunnel the wetness of the 
 rocks referred to by Stapff must remove the heat by 
 convection, and no stress can be placed on the 
 radioactivity. 
 
 Setting out the mean radioactivities of the different 
 geological subdivisions of the tunnel, we have : 
 
 Granite of Finsteraarhorn, 7*7 
 
 Sediments of Ursernmulde, 4*9 
 
 St. Gothard Mamrif, 3*9 
 
 Sediments of Tessinmulde, . . 3'4 
 
92 RADIOACTIVITY AND GEOLOGY. 
 
 The central section, however, if treated as a separate 
 thermal province, would show a mean radioactivity of 
 3-3 only. 
 
 The measurements which yield these results were 
 carried out after long experience on this work had 
 been acquired. True it is that random specimens of 
 the tunnel rocks would probably show here and 
 there variations from the individual measurements ; 
 but unless a much larger number than fifty-one 
 samples are dealt with, more representative informa- 
 tion is improbable. A circumstance, quite accidental 
 in character, which arose during the progress of the 
 research is confirmatory of its representative character. 
 A first set of observations was made on thirty-six 
 rock-chips kindly given by the trustees of the British 
 Museum. The chart so determined left many gaps, 
 as well as readings which called for substantiation. 
 Fifteen additional samples were obtained in order to 
 complete, and, it was hoped, confirm, the chart. "When 
 the fresh samples were examined, it was found, in 
 every case but one, that the results agreed with the 
 previous neighbouring determinations, or, in other 
 words, in one case only was any departure from the 
 original chart necessitated. This is the reading 6-5 
 among the Tessin rocks. This seems evidence that 
 accidental errors had not affected the experiments, 
 and that the samples are representative in their 
 indications. 
 
 It is unfortunate that we have no means of assuring 
 ourselves as to the radioactive state of the granite 
 beneath the Ursern rocks. But if in lieu of this 
 
UNDERGROUND TEMPERATURE AND RADIOACTIVITY. 93 
 
 knowledge we accept the last available reading in 
 the granite as indicative of its radioactive state, and 
 proceed as before to investigate the downward exten- 
 sion of rocks, having the radioactivities of 14 and 3*3, 
 which will account for the differing heat-flows from 
 the central and northern parts of the tunnel, we find 
 that about 6 kilometres would suffice the heat being 
 conducted in an approximately vertical direction, and 
 the conductivities of the two rock-masses being 
 assumed alike. If some lesser radioactivity be 
 assigned to the granite, so as to embrace a larger 
 number of experiments, then the downward extension 
 required, of course, becomes greater. It will be 
 noticed that the downward extension arrived at for 
 the central St. Gothard agrees with that arising from 
 the comparison of the Simplon and St. Gothard rocks. 
 The foregoing pages give some idea of the difficulties 
 and uncertainties attending this branch of research. 
 There seems, however, to be a probability that a con- 
 nexion between radioactivity and earth gradients 
 has here been established. It is to be hoped that 
 future work will enable us to pronounce upon this 
 with certainty; for the matter is not only one of 
 scientific interest, but of practical bearing on engineer- 
 ing problems of the nature and magnitude of that 
 of the Simplon tunnel. 
 
CHAPTEE V. 
 
 THE INSTABILITY OF THE EARTH'S CRUST. 
 
 PHENOMENA OF MOUNTAIN-BUILDING THEIR RHYTHMIC CHARACTER 
 PRIMARY CONDITION : THE COMl'RESSIVE STRESS IN THE CRUST 
 THE BABBAGE-HERSCHEL EXPLANATION INADEQUATE THE RADIO- 
 THERMAL EFFECTS IN THE GREAT DEPOSITS INCREASE OF THESE 
 
 EFFECTS DUB TO THE RADIOACTIVITY OF THE UPPER CRUST 
 CALCULATIONS FOR SOME SPECIAL CASES FURTHER RADIOACTIVE 
 HEATING ARISING FROM LATERAL COMPRESSION' A RHYTHMIC CHAR- 
 ACTER NECESSARILY INVOLVED THE GREAT GEOSYNCLINE8 TRACE- 
 ABLE TO RADIOACTIVITY OF THE SEDIMENTS: SO ALSO THE ATTENDANT 
 EARTHQUAKE AND VOLCANIC PHENOMENA INFLUENCE ON GEOLOGICAL 
 HISTORY. 
 
 IN the deposition of sediments their uplifting into 
 mountain chains their subsequent removal and their 
 re-deposition elsewhere there is a continual convective 
 movement of radioactive materials on the surface of the 
 earth. Around the margins of all the great continents, 
 where the sedimentation necessarily takes place to the 
 greatest extent, the result of this convection is the 
 accumulation of vast amounts of radioactive materials '> 
 and we have now to ask if this phenomenon may not 
 entail consequences which can only be ascribed to the 
 presence of the attendant radioactive energy. 
 
 The radioactivity of the secondary rocks is an 
 assured fact arising out of the experiments under- 
 taken to determine the amounts of radium in the 
 surface materials of the earth. Whether ancient or 
 modern, fresh or metamorphosed, the sediments in all 
 
THE INSTABILITY OF THE EARTH'S CRUST. 95 
 
 cases exhibit radioactivity. Those that are detrital in 
 origin owe their uranium directly to the parent igneous 
 rocks; those which have been precipitated from the 
 ocean take it from the waters. We are here in pos- 
 session of a fact the consequences of which we are 
 clearly entitled to pursue wherever the accumulation 
 of these rocks finds place, and even if their aggregate 
 growth amounts to depths measured in miles. 
 
 It has long been known to geologists that a mountain 
 is not a mass of piled-up igneous rocks, but (save in the 
 case of volcanic cones) is composed of a complex of 
 sedimentary rocks ridged up, folded, and often hori- 
 zontally over-thrust, by intense compressive forces. 
 The great mountain ranges of the world, without 
 exception, originated in the disturbance of sedimentary 
 materials. 
 
 The mountain-forming movements have invariably 
 taken place where the accumulation has been greatest. 
 As we proceed away from the mountain we find the 
 sediments rapidly thin out; and become relatively 
 insignificant in depth where the continental surface 
 remains undisturbed. This character appears, too, to 
 have been persistent from the earliest times to the 
 most recent, plainly revealing that in the fact of 
 sedimentation the chain of events which results in 
 the mountain is inaugurated. 
 
 The part played by igneous materials in these dis- 
 turbances of the crust is only revealed by the discovering 
 hand of denudation. When the foundations of the 
 mountains are uncovered, we find that within, and like 
 a supporting core, the underlying plastic rocks have 
 
96 RADIOACTIVITY AND GEOLOGY. 
 
 also been upraised, following the upward movement of 
 the sediments, and often injected into their folds, forced 
 into cracks, and in many cases intensely metamorphosed 
 by the heat, pressure, and shearing to which they were 
 subjected. 
 
 Vast thicknesses of sediments are involved in these 
 events. Indeed, whatever be the connexion between 
 the deposition and upheaval, this element of great depth 
 of accumulation seems a necessary condition, and must 
 enter as a factor in the physical process involved. The 
 mountain can only arise where the geosyncline is deeply 
 filled by long ages of sedimentation. 
 
 The evidence gathered by geologists all over the 
 world respecting the magnitude of these mountain- 
 building sediments is uniformly impressive. There is, 
 doubtless, the possibility of exaggerated measurements 
 in some cases, but with every allowance the thicknesses 
 are measured in miles. We might give up many pages 
 to transcripts of such records ; but a few facts must 
 suffice to convey an idea of what is repeated, with little 
 more than variations of locality, all over the earth. In 
 the New "World the broad structural features are, 
 perhaps, more simple than in the Old. In the Pro- 
 terozoic era of the North American continent the 
 sediments were probably collected at a period when the 
 crust of the earth was more readily disturbed than now. 
 Three great sedimentary upheavals occurred : Huronian, 
 Animikean, and Keweenawan ; each followed by vast 
 intervals of denudation, in which the upheaved crust 
 was again worn down. To these intervals a duration 
 as great as that of deposition has been ascribed. The 
 
THE INSTABILITY OF THE EARTH'S CRUST. 97 
 
 removal of the sediments during these eras of denuda- 
 tion may perhaps be set off against errors affecting the 
 estimates of thickness. Figures up to 18,000 feet have 
 been cited for the depth of sediments originally taking 
 part in the first of these systems, 14,000 feet for the 
 second, and up to 50,000 feet for the third. Consider- 
 able amounts of igneous materials arising from beneath 
 are associated with these accumulations a feature 
 which is, apparently, hardly so conspicuous in the sedi- 
 mentary accumulations of more recent times. In later 
 ages North American geological history moved on a still 
 vaster scale. Dana, 1 in describing the fundamental 
 characters of mountain-building phenomena, brings 
 before us some of these great geological events in 
 graphic language: 
 
 " A mountain range of the common type, like that to 
 which the Appalachians belong, is made out of the 
 sedimentary formations of a long preceding era ; beds 
 that were laid down conformably, and in succession 
 until they had reached the needed thickness; beds 
 spreading over a region tens of thousands of square 
 miles in area. The region over which sedimentary 
 formations were in progress in order to make, finally, 
 the Appalachian range, reached from New York to 
 Alabama, and had a breadth of 100 to 200 miles, and 
 the pile of horizontal beds along the middle was 40,000 
 feet in depth. The pile for the Wahsatch Mountains 
 was 60,000 feet thick, according to King. The beds 
 for the Appalachians were not laid down in a deep 
 
 1 Dana, Manual of Geology, 3rd ed., p. 794. 
 H 
 
98 RADIOACTIVITY AND GEOLOGY. 
 
 ocean, but in shallow waters, where a gradual sub- 
 sidence was in progress ; and they at last, when ready 
 for the genesis, lay in a trough 40,000 feet deep, 
 filling the trough to the brim. It thus appears that 
 epochs of mountain-making have occurred only after 
 long intervals of quiet in the history of a continent." 
 
 On the western side of North America the work of 
 mountain-building was, indeed, on the grandest scale. 
 For long ages and through a succession of geological 
 epochs sedimentation had proceeded so that the accumu- 
 lations of Palaeozoic and Mesozoic times had collected 
 in the geosyncline formed by their own ever increasing 
 weight. The site of the future Laramide range was in 
 late Cretaceous times occupied by some 50,000 feet of 
 sedimentary deposits ; but the limit had apparently been 
 attained, and at this time the Laramide range, as well 
 as its southerly continuation into the United States, 
 the Eockies, had their beginning. Chamberlin and 
 Salisbury 1 estimate that the height of the mountains, 
 developed in the Laramide range at this time was 
 20,000 feet, and that, owing to the further elevation 
 which has since taken place, from 32,000 to 35,000 
 feet would be their present height if erosion had not 
 reduced them. Thus on either side of the great 
 continent we have the same forces at work, throwing 
 up mountain ridges where the sediments had formerly 
 been shed into the ocean. 
 
 In the Himalayas an immense mass of sediments, 
 laid down on the floor of the ancient Tethys, was at the 
 
 1 Chamberlin and Salisbury, Geology, 1906, iii., 163. 
 
THE INSTABILITY OF THE EARTH'S CRUST. 99 
 
 end of the Eocene period first disturbed. Conditions 
 of quiescent deposition since the Upper Carboniferous 
 thus came to an end, and a succession of mountain- 
 building movements was inaugurated in North India 
 the final movements extending into Pliocene times 
 and the sedimentation continuing over those areas 
 not yet uplifted above the sea- level. The masses of 
 sediment involved in these prolonged eras of deposition 
 and partial upheaval are now, in view of the almost 
 incredible erosion which has since occurred, hardly 
 determinable, but must be enormous. The Alps and 
 the Pyrenees share in this history. Nor would the 
 list of such crustal movements be complete save in the 
 enumeration of every great range upon the earth. 
 
 These great events are of a rhythmic character ; the 
 crust, as it were, pulsating under the combined influences 
 of sedimentation and denudation. The first involves 
 downward movements under the gathering load, and 
 ultimately a reversal of the movement to one of 
 upheaval; the second factor, which throughout has 
 been in operation as creator of the sediments, then 
 intervenes as an assailant of the newly raised moun- 
 tains, transporting their materials again to the ocean, 
 when the rhythmic action is restored to its first phase, 
 and the age-long sequence of events must begin all 
 over again. 
 
 It has long been inferred that compressive stress in 
 the crust must be a primary condition of these move- 
 ments. The work required to effect the upheavals must 
 be derived from some pre-existing source of energy. 
 The phenomenon intrinsically one of folding of the 
 
 H2 
 
100 RADIOACTIVITY AND GEOLOGY. 
 
 crust suggests the adjustment of the earth-crust to a 
 lessening radius ; the continual loss of heat from the 
 surface of the earth affording an explanation of this 
 shrinkage, and the assumption of compressive stress in 
 this folding envelope naturally arising as a harmonious 
 part of the general theory. The compressive stresses 
 must, however, be confined to the upper few miles of 
 the crust, for, in fact, the downward increase of tem- 
 perature and pressure must soon confer fluid properties 
 on the medium, and slow tangential compression must 
 result in hydrostatic pressure rather than directed 
 stresses. Thus the folding visible in the mountain 
 range, and the lateral compression arising therefrom, 
 are effects confined to the upper parts of the crust. 
 
 The energy, then, which uplifts the mountain is a 
 surviving part of the original gravitational potentional 
 energy of the crust itself. It must be assumed that 
 the crust in following downwards the shrinking sub- 
 crustal magma, as this lessens its volume in the process 
 of congelation and crystallization, develops immense 
 compressive stresses in its materials, vast geographical 
 areas being involved. When folding at length takes 
 place along the axis of the elongated syncline of 
 deposition, the stresses find relief probably for some 
 hundreds of miles, and the region of folding now 
 becomes compressed in a transverse direction. As an 
 illustration, the Laramide range, according to Dawson, 
 represents the reduction of a surface-belt 50 miles 
 wide to one of 25 miles. The marvellous translatory 
 movements of crustal folds from south to north arising 
 in the genesis of the Swiss Alps, which recent research 
 
THE INSTABILITY OF THE EARTH'S CRUST. 101 
 
 has brought to light, is another example of these move- 
 ments of relief, which continue to take place perhaps 
 for many millions of years after they are initiated. 
 
 The result of this yielding of the crust is a buckling 
 of the surface which on the whole is directed upwards ; 
 but depression also is an attendant, in many cases at 
 least, on mountain upheaval. Thus we find that the 
 ocean floor is depressed into a syncline along the entire 
 western coast of South America; a trough always 
 parallel to the ranges of the Andes. The downward 
 deflection of the crust is of course an outcome of the 
 same compressive stresses which elevate the mountain. 
 
 The fact that the yielding of the crust is always 
 situated where the sediments have accumulated to the 
 greatest depth, has led to attempts from time to time 
 at establishing a physical connexion between the one 
 and the other. The best-known of these theories 
 is that of Babbage and Herschel. This seeks the 
 connexion in the rise of the geotherms into the 
 sinking mass of sediment and the consequent increase 
 of temperature of the earth-crust beneath. It will be 
 understood that as these isogeotherms lie at a uniform 
 distance from the surface of the earth, unless where 
 special variations of conductivity may disturb them, the 
 introduction of material pressed downwards from above 
 must result in these materials partaking of the temper- 
 ature proper to the depth to which they are depressed. 
 In other words, the geotherms rise into the sinking 
 sediments, always, however, preserving their former 
 average distance from the surface. The argument is 
 that as this process undoubtedly involves the heating 
 
102 RADIOACTIVITY AND GEOLOGY. 
 
 up of that portion of the crust which the sediments 
 have displaced downwards, the result must be a local 
 enfeeblement of the crust, and hence these areas become 
 those of least resistance to the stresses in the crust. 
 
 When this theory is examined closely, we see that 
 it only amounts to saying that the bedded rocks, which 
 have taken the place of the igneous materials beneath, 
 as a part of the rigid crust of the earth, must be less 
 able to withstand compressive stress than the average 
 crust. For there has been no absolute rise . of the geo- 
 therms, the conductivities of both classes of materials 
 differing but little. Sedimentary rock has merely taken 
 the place of average crust-rock, and is subjected to the 
 same average temperature and pressure prevailing in 
 the surrounding crust. But are there any grounds for 
 the assumption that the compressive resistance of a 
 complex of sedimentary rocks is inferior to one of 
 igneous materials ? The metamorphosed siliceous sedi- 
 ments are among the strongest rocks known as regards 
 resistance to compressive stress ; and if limestones have 
 indeed plastic qualities, it must be remembered that 
 their average amount is only some 5 per cent, of the 
 whole. Again, so far as rise of temperature in the upper 
 crust may affect the question, a temperature which 
 will soften an average igneous rock will not soften 
 a sedimentary rock, for the reason that the effect of 
 solvent denudation has been to remove those alkaline 
 silicates which confer fusibility. 
 
 When, however, we take into account that in the 
 radium content of the sediments there is a source of 
 heat competent to bring the geotherms closer to the 
 
THE INSTABILITY OF THE EARTH'S CRUST. 103 
 
 surface in the region of deposition than elsewhere in 
 the surrounding crust, the difficulties disappear. We 
 can calculate what these heating effects will amount to. 
 Let us take first a simple case : a succession of beds 
 such as Dana refers to, 40,000 feet in thickness that 
 is, somewhat over 12 kilometres. Looking at the 
 results already recorded for sedimentary materials, we 
 see that 4 x 10" 12 grams per gram is not too large an 
 estimate of the average radioactivity. This is 11 x 10~ 12 
 per cubic centimetre. Then we find for the basal 
 temperature of the layer of sediment (see p. 77): 
 
 tf - n x f"; ^ 1(H x 144 XKH-- lire. 
 
 2 x 4 x 10 3 
 
 That is, there will be an elevation of about 111 
 above that which similar materials under the same 
 conditions, but without radioactive ingredients, would 
 experience. Thus the final position of the isogeo- 
 therms is no longer alike in the sediments and 
 elsewhere in the crust, as necessarily arises in the 
 Babbage-Herschel theory, but there has been an upward 
 movement due to radioactivity. If the gradients 
 be supposed similar to what they now are say, 
 30 metres to 1 degree the upward shift has been 
 3330 metres at the base of the layer. 
 
 It would be quite comprehensible that even this 
 amount of heating would appreciably weaken the 
 rigid part of the crust over this area. For the heating 
 effect will extend downwards, and the temperature will 
 rise a corresponding amount in the depressed under- 
 lying parts. To make this more apparent, suppose 
 
104 RADIOACTIVITY AND GEOLOGY. 
 
 there was as much as 40 kilometres of the crust rigid : 
 then the geotherm say of 800 originally denning 
 the downward limit of rigidity, moves up 3 kilometres, 
 and the thickness of the rigid part falls to 37 kilo- 
 metres, and there is, consequently, a reduction of 
 7| per cent, in the effective resistance, supposing all 
 other factors alike. When the compressive stress in the 
 crust over this region of the earth, increasing with the 
 lapse of time, attains to nearly the limiting resistance, 
 the flexure would, almost certainly, begin here, and 
 once begun, there would be not only mechanical 
 reasons for its continuance, but accelerative radioactive 
 effects would arise from a source to be referred to later. 
 Thus the deposition of the sediments would, under the 
 conditions of a shrinking world, involve their own 
 upheaval. 
 
 The problem of the influence of radium in mountain 
 elevation is, however, not so simple as given above: 
 the actual effects produced are at once more striking 
 and more complex. It will be shown in a later 
 chapter that there must be over the earth's surface a 
 specially rich radioactive layer, and that such radio- 
 activity as exists beneath it must extend downwards 
 in materials relatively poor in uranium. The depth of 
 the uranium-rich upper part of the crust can be guessed 
 at from the necessity of reconciling its properties with 
 certain facts ; and the result of our guess is that it 
 must extend to a depth of between 10 and 15 kilo- 
 metres. Along with the existence of this layer the 
 important fact must be held in view that the basal 
 temperature of any radioactive layer increases as the 
 
THE INSTABILITY OF THE EARTH'S CRUST. 105 
 
 square of its thickness. Thus the effect of the sedi- 
 ment collecting upon the normal radioactive covering 
 of the earth will be to produce a total thickness 
 of radioactive materials made up of the normal 
 layer and of the sediments greater than exists else- 
 where, and the temperature at the base of the under- 
 lying normal layer will be so much greater than it was 
 before, as the square of the combined thickness of 
 layer and sediments is to the square of its thickness 
 elsewhere. That is, if the thickness of the normal 
 layer is, say, 12 kilometres, and its basal temperature 
 so far as due to radium is ti, and sediments are laid 
 down upon it, making it, say, 22 kilometres, the 
 basal temperature t* is obtained by the proportion 
 ti : t z : : 12 2 : 22 2 . We assume the conductivity and 
 radioactivity of the sediments the same as prevails in 
 the normal layer. 
 
 The last assumption is not completely justified on the 
 experiments. The error of assuming the radioactivities 
 to be alike will, however, be unimportant. In any 
 case we will take a value for the common radioactivity 
 considerably below the mean of all existing experi- 
 ments, mindful of the fact that we really do not know 
 how far down the surface radioactivity is carried at its 
 full value. Judging from the indications of igneous 
 rocks brought to the surface, probably from considerable 
 depths, there is no reason to suppose a falling off. We 
 accept, however, for our calculations the radioactivity 
 3*0 x 10~ 12 grams per gram, or 84 x 10~ 12 grams per 
 cubic centimetre for the entire complex layer made up 
 
106 RADIOACTIVITY AND GEOLOGY. 
 
 of sediments and the underlying radioactive materials. 
 The precise figure we adopt in our calculations is of 
 secondary importance, for the margin is wide within 
 which we may suppose it to vary (according as future 
 researches may decide), and the deductions now to be 
 considered still retain their fundamental significance. 
 
 Let us consider the cases of sedimentary deposits 
 amounting to 4, 6, 8, 10, 12, and 14 kilometres. To 
 these thicknesses we add the inferred depth (12 kilo- 
 metres) of the normal layer. Here again it must be 
 observed that wide variations in our assumption will 
 not seriously affect the result. The depth taken above 
 for purposes of calculation has been arrived at on 
 a basis distinct from the present investigation. It is 
 certain that we may assume a lesser or a greater depth, 
 varying it through a wide range, and still the teaching 
 of the result will remain the same. 
 
 The total depths are, then, 16, 18, 20, 22, 24, 26 
 kilometres. "We take the average conductivity as 
 4 x 10~ 3 . It will now conduce to clearness if we 
 follow the calculation in the case of one of these 
 assumed depths ; recollecting that all present the 
 same problem. 
 
 We easily find on the equation for (p. 77) that 
 the temperature due to radioactivity at the base of 
 the 22-kilometre layer (which we select) is 285. 
 Now what we want to find is the weakening of the 
 crust arising from this radiothermal temperature : that 
 is, how much weaker the crust is here than it is else- 
 where. In other words, the first consideration is : 
 
THE INSTABILITY OF THE EARTH'S CRUST. 107 
 
 what is the thermal state of the crust, assumed prac- 
 tically free from overlying sediment, elsewhere at 
 this depth ? The temperature generally prevailing at 
 22 kilometres involves, of course, the gradients down- 
 wards from the surface. Here we recall the mean 
 gradient of 32 metres, assuming that the temperature 
 conditions are like those of to-day. This is, however, 
 the observed surface gradient, and we know that this 
 surface gradient cannot apply to the deeper parts of 
 the crust, owing to the disturbing effects of the radium 
 in the normal layer. Below this layer the gradient 
 must be less. We desire to distinguish between the 
 heating effects of the radium near the surface and the 
 heat due to conducted interior heat. The simplest 
 way of investigating this and it is accurate enough 
 for our purpose is to deduct the heat out-put of the 
 normal layer per square centimetre of the earth's 
 surface from the average heat-emission of the earth. 
 A simple calculation shows that 0'564 x 1O 6 must be 
 deducted from the 1*250 x 10" 6 calories previously 
 arrived at (p. 75) as the out-flow per square centimetre 
 per second. The balance of 0*686 x 10~ 6 gives us the 
 interior heat-gradient (p. 73 et seq.). It comes out as 
 1 in 58*4 metres or, say, 58 metres. 
 
 We are now in a position to determine the tem- 
 perature of the geotherm at places unaffected by 
 sedimentation at the depth of 22 kilometres. The 
 radium in the upper 12 kilometres creates a basal rise 
 of nearly 85, which must affect all points beneath 
 this level. The out-flowing interior heat gives rise 
 to 379 as calculated on the gradient of 58 metres; 
 
108 RADIOACTIVITY AND GEOLOGY. 
 
 and therefore the temperature at the depth of 22 kilo- 
 metres must be 379 + 85, that is 464. 
 
 Keturning now to the new conditions, when, of the 
 22 kilometres, 10 are composed of sediment and 12 
 of the normal radioactive layer, we find a radium 
 temperature of 285, and to this must be added the 
 temperature due to the interior heat. At this level 
 we have found this to be 379. The total temperature 
 is therefore 664. We have thus found that the effect 
 of the radioactive stratum of sediments placed on the 
 normal radioactive layer has been to raise the tem- 
 perature at the depth of 22 kilometres (where the 
 rise will have reached the maximum) from 464 to 
 664 ; that is, by 200. 
 
 This, of course, is an upward displacement of the 
 geotherms. By how much are they displaced ? The 
 slope or gradient of temperature is at points deep 
 down 1 in 58 metres, and the temperature, at the 
 depth of 22 kilometres, has been raised 200. This 
 increase would only be gained elsewhere by going 
 down 11 '6 kilometres: and hence this has been the 
 elevation of the geotherms beneath the radium-rich 
 materials. 
 
 This calculation is made for each of the sedimentary 
 accumulations already selected. We can best com- 
 pare and estimate the effects of the different depths 
 of deposit by finding the upward shift of some one 
 geotherm. We take that having the temperature of 
 800, assuming that this approximately represents the 
 softening temperature of the crust rocks. Possibly a 
 ower temperature would be better chosen. Now, 
 
THE INSTABILITY OF THE EARTH'S CRUST. 109 
 
 normally this geotherm lies at about 40 kilometres 
 down, and we find, finally, the results : 
 
 Thickness of 
 Sedimentary Deposit. 
 
 Resulting Rise of 
 Isogeotherms. 
 
 Weakening of Earth's 
 Crust as defined by the 
 Rise of the Geotherm 
 at 40 kilometres. 
 
 Kilometres. 
 
 Kilometres. 
 
 Kilometres. 
 
 4 
 
 3-8 
 
 40 to 36-2 
 
 6 
 
 6-2 
 
 40 to 33-8 
 
 8 
 
 87 
 
 40 to 31 -3 
 
 10 
 
 11-6 
 
 40 to 28-4 
 
 12 
 
 14-8 
 
 40 to 25-2 
 
 14 
 
 18-2 
 
 40 to 21 -8 
 
 Looking at these figures we see that even the 4- 
 kilometre deposit involves a reduction of the effective 
 strength of the earth's crust by 10 per cent. 
 
 Such results are, of course, approximate only. 
 More cannot be claimed for them than that they 
 represent roughly the magnitude of the effects it is 
 sought to investigate. They show that as the sedi- 
 ments grow in depth there is a rising of the geotherm 
 of plasticity whatever its true temperature may be 
 gradually reducing the thickness of that part of the 
 upper crust which is bearing the simultaneously in- 
 creasing compressive stresses. Below this geotherm 
 long-continued stress resolves itself into hydrostatic 
 pressure ; above it (there is, of course, no sharp line of 
 demarcation) the crust accumulates elastic energy. 
 The final yielding and flexure occur when the resis- 
 tant cross-section has sufficiently diminished. It is 
 probable that there is also some outward hydrostatic 
 
110 RADIOACTIVITY AND GEOLOGY. 
 
 thrust over the area of rising temperature, which 
 assists in determining the upward throw of the folds. 
 
 When yielding has begun in any geosyncline, and 
 the materials are faulted and overthrust, there results 
 a considerably increased thickness. As an instance, 
 consider the piling up of sediments over the existing 
 materials of the Alps, which resulted from the com- 
 pressive force acting from south to north in the 
 progress of Alpine upheaval. Schmidt of Basel has 
 estimated that from 15 to 20 kilometres of rock 
 covered the materials of the Simplon as now exposed, 
 at the time when the erogenic forces were actively at 
 work folding and shearing the beds, and injecting 
 into their folds the plastic gneisses coming from 
 beneath. 1 The lateral compression of the area of 
 deposition of the Laramide, already referred to, 
 resulted in a great thickening of the deposits. Many 
 other cases might be cited ; the effect is always in 
 some degree necessarily produced. 
 
 If time be given for the heat to accumulate in the 
 lower depths of the crushed-up sediments, here is 
 an additional source of increased temperature. The 
 piled-up masses of the Simplon might, even on a low 
 estimate of the radioactivity, have occasioned a rise 
 due to radioactive heating of from 300 to 500 ; and 
 if this be added to the interior heat, a total of from 
 800 to 1100 might have prevailed in the rocks now 
 exposed at the surface of the mountain. Even a lesser 
 temperature, accompanied by the intense pressure 
 conditions, might well occasion the appearances of 
 
 1 Schmidt, EC. Geol. Helvetia, vol. ix., No. 4, p. 590. 
 
THE INSTABILITY OF THE EARTH'S CRUST. Ill 
 
 thermal metamorphism described by Weinschenk, and 
 for which, otherwise, there is difficulty in accounting. 1 
 
 This increase upon the primarily developed tempera- 
 ture conditions takes place concurrently with the 
 progress of compression ; and while it cannot be taken 
 into account in estimating the conditions of initial 
 yielding of the crust, it adds an element of instability, 
 inasmuch as any progressive thickening by lateral 
 compression results in an accelerated rise of the 
 geotherms. It is possible that time sufficient for 
 these effects to develop, if not to their final, yet to a 
 considerable extent, was available. The figures in the 
 case of the Simplon plainly show that rock-melting 
 temperatures would be brought about at but a little 
 distance below the present surface. The viscous move- 
 ments of siliceous materials, and the out-pouring of 
 igneous rocks which often attend mountain elevation, 
 would find an explanation in such temperatures. 
 
 There is no more striking feature of the part here 
 played by radioactivity than the fact that the rhythmic 
 occurrence of depression and upheaval succeeding each 
 other after great intervals of time, and often shifting 
 their position but little from the first scene of sedimen- 
 tation, becomes accounted for. The energy, as we have 
 already remarked, is in fact transported with the 
 sediments the energy which determines the place of 
 yielding and upheaval, and ordains that the mountain 
 ranges shall stand around the continental borders. 
 Sedimentation from this point of view is a convection 
 of energy. 
 
 1 Weinschenk, Congres Geol. Internal, 1900, i., p. 332. 
 
112 RADIOACTIVITY AND GEOLOGY. 
 
 When the consolidated sediments are by these and by 
 succeeding movements forced upwards into mountains, 
 they are exposed to denudative effects greatly exceeding 
 what affects the plains. Witness the removal during 
 late Tertiary times of the vast thickness of rock 
 enveloping the Alps. Such great masses are hurried 
 away by ice, rivers, and rain. The ocean receives them ; 
 and with infinite patience the world awaits the slow 
 accumulation of the radioactive energy beginning 
 afresh upon its work. The time for such events 
 appears to us immense, for millions of years are 
 required for the sediments to grow in thickness, 
 and the geotherms to move upwards ; but vast as it is, 
 it is but a moment in the life of the parent substance, 
 whose atoms, hardly diminished in numbers, pursue 
 their changes while the mountains come and go, 
 and the rudiments of life develop into its highest 
 consummations. 
 
 The broad features of the denuded earth are grand in 
 their simplicity. A geosyncline borders the Pacific, in 
 past times the recipient of vast sedimentary accumula- 
 tions. It follows the wall of mountains standing up 
 on the western side of North and South America for a 
 distance of some 8000 miles. It reappears at New 
 Zealand, and passing north of Australia contains the 
 festoons of islands which skirt Malaysia and eastern 
 Asia. A branch diverging to the west lies along the 
 equator, and bends northward, taking in the Himalayas, 
 the Alps, and the Pyrenees. In short, the geosynclines 
 lie where the oceans meet the continents, or did so in 
 the past, and conserve the sediments near the land. 
 
THE INSTABILITY OF THE EARTH'S CRUST. 113 
 
 They embrace the greatest mountain ranges on the 
 earth, and now hold the greater part of the material 
 eroded from the land throughout the long ages of 
 Mesozoic denudation. 
 
 It seems impossible to escape the conclusion that in 
 this great development of ocean-encircling areas of 
 deposition and crustal folding, the heat of radioactivity 
 plays a part. The geosynclines were a necessary 
 outcome of the permanency of the continents, and 
 originated in the age-long accumulation of the denuded 
 radioactive energy. We recognize, therefore, in the 
 movements of the sediments not only an influence 
 localizing and accelerating crustal movements, but one 
 which, in subservience to the primal distribution of 
 land and water, has determined some of the greatest 
 geographical features of the globe. 
 
 It is no more than a step to show that bound up with 
 the radioactive energy are most of the earthquake and 
 volcanic phenomena of the earth. The association of 
 earthquakes with the great geosynclines is well known. 
 The work of De Moutessus showed that over 94 per 
 cent, of all recorded shocks lie in the geosynclinal belts. 
 There can be no doubt that these manifestations of 
 instability are the results of the local weakness and 
 flexure which originated in the accumulation of energy 
 denuded from the continents. Similarly we may view 
 in volcanoes phenomena referable to the same funda- 
 mental cause. The volcano was, in fact, long regarded 
 as more intimately connected with earthquakes than it, 
 probably, actually is ; the association being regarded in 
 a causative light, whereas the connexion is more that of 
 
 I 
 
114 RADIOACTIVITY AND GEOLOGY. 
 
 possessing a common origin. The girdle of volcanoes 
 around the Pacific and the earthquake belt coincide. 
 Again, the ancient and modern volcanoes and earth- 
 quakes of Europe are associated with the geosyncline 
 of the greater Mediterranean, the Tethys of Mesozoic 
 times. There is no difficulty in understanding in a 
 general way the nature of the association. The earth- 
 quake is the manifestation of rupture and slip, and, as 
 Suess has shown, the epicentres shift along that fault- 
 line where the crust, has yielded. 1 The volcano marks 
 the spot where the zone of fusion is brought so high 
 up in the fractured crust that the melted materials are 
 poured out upon the surface. 
 
 In a recent work on the subject of earthquakes 
 Professor Hobbs writes : " One of the most interesting 
 of the generalizations which De Montessus has reached 
 as a result of his protracted studies, is that the earth- 
 quake districts on the land correspond almost exactly 
 to those belts upon the Globe which were the almost 
 continuous ocean basins of the long Secondary era of 
 geological history. Within these belts the sedimentary 
 formations of the crust were laid down in the greatest 
 thickness, and the formations follow each other in re- 
 latively complete succession. For almost or quite the 
 period of this long era it is therefore clear that the 
 ocean covered these zones. About them the formations 
 are found interrupted, and the lacunae indicate that the 
 sea invaded the area only to recede from it, and again at 
 some later period to transgress upon it. For a long 
 time, therefore, these earthquake belts were the sea 
 
 1 Suess, The Face of the Earth, vol. ii., chap. ii. 
 
THE INSTABILITY OF THE EARTH J S CRUST. 115 
 
 basins the geosynclines. They became later the rising 
 mountains of the Tertiary period, and mountains they 
 are to-day. The earthquake belts are hence those 
 portions of the earth's crust which in recent times have 
 suffered the greatest movements in a vertical direction 
 they are the most mobile portions of the earth's crust" 1 
 The italics are Professor Hobbs's. 
 
 Whether the movements attending mountain eleva- 
 tion and denudation are a connected and integral part 
 of those wide geographical changes which result in 
 submergence and elevation of large continental areas, 
 is an obscure and complex question. We seem, indeed, 
 according to the views of some authorities, hardly in 
 a position to affirm with certainty that such wide- 
 spread movements of the land have actually occurred, 
 and that the phenomena are not the outcome of 
 fluctuations of oceanic level ; that our observations go 
 no further than the recognition of positive and 
 negative movements of the strand. 
 
 However this may be, by far the greater part of 
 denudation during geological time has been done on the 
 mountain ranges. There is apparently an accelera- 
 tive principle involved in this fact, tending to increase 
 the magnitude of mountain ranges with the advance 
 of geological time. It is, in short, indisputable 
 that the erogenic movements which uplift the hills 
 have been at the basis of geological history. To them 
 the great accumulations of sediments which now form 
 so large a part of continental land are mainly due. 
 There can be no doubt of the fact that these move- 
 
 1 Hobbs, Earthquakes, p. 58. 
 I 2 
 
116 RADIOACTIVITY AND GEOLOGY. 
 
 ments have swayed the entire history, both inorganic 
 and organic, of the world in which we live. 
 
 To sum the contents of this chapter in the most 
 general terms, we find that in the conception of de- 
 nudation as producing the convection and accumula- 
 tion of radio-thermal energy the surface features of 
 the globe receive a new significance. The heat of the 
 earth is not internal only, but rather a heat- source 
 exists at the surface, which, as we shall see, cannot 
 prevail to the same degree within ; and when the con- 
 ditions become favourable for the aggregation of the 
 energy, the crust, heated both from beneath and from 
 above, assumes properties more akin to those of its 
 earlier stages of development, the secular heat-loss 
 being restored in the radioactive supplies. These 
 causes of local mobility have been in operation, 
 shifting somewhat from place to place, and defined 
 geographically by the continental masses undergoing 
 denudation, since the earliest times. 
 
CHAPTER VI. 
 
 THE INSTABILITY OF THE OCEAN FLOOR. 
 
 DARWIN'S THEORY OF CORAL-ISLAND FORMATION EXTENT OF UN- 
 STABLE AREAS CONDITIONS OF DEEP-SEA DEPOSITION MURRAY'S 
 
 THEORY FUNAFUTI THE SEDIMENTS AROUND CORAL ISLANDS THE 
 
 LAWS OF SEDIMENTATION : THEIR GEOLOGICAL IMPORTANCE THE 
 RADIOACTIVE STATE OF THE SEDIMENTS INCREASE OF RADIOTHERMAL 
 
 EFFECT WITH DILUTION TIME OF ACCUMULATION PERMANENCY OF 
 
 THE OCEAN BASINS THE EXISTENCE OF SUBMERGED CONTINENTS 
 INCONSISTENT WITH THE DATA OF SEDIMENTATION RATE OF DEPOSI- 
 TION OF DEEP-SEA SEDIMENTS THE BARBADOS DEPOSITS RATE OF 
 COLLECTION OF THE CHALK EVIDENCE OF VARIATION IN RATE OF 
 ACCUMULATION CONCLUSION AS TO POSSIBLE WEAKENING OF THE 
 CRUST BENEATH OCEANIC DEPOSITS EFFECTS OF THE INSTABILITY ON 
 THE MEAN SEA-LEVEL AND THE TRANSGRESSION OF THE OCEAN UPON 
 THE LAND. 
 
 IN the case of mountain elevation as discussed in 
 Chapter V, the conditions necessary for the accumula- 
 tion of radioactive heat undoubtedly exist, and tem- 
 peratures adequate to affect the local stability of the 
 crust must be attained. The whole theory is of the 
 simplest kind. Given a stressed crust and a local 
 source of heat applied above while the normal heat of 
 the earth flows upward from beneath, and the area 
 where these conditions exist must necessarily become 
 the first place of yielding and flexure ; as naturally as 
 the rupture of the chain occurs at the weakest link. 
 Under these conditions accordingly, we find associated 
 with great accumulations of radioactive sediments 
 
118 RADIOACTIVITY AND GEOLOGY. 
 
 bending and fracture of the crust, with the attendant 
 phenomena of earthquake and volcanoes. The cause and 
 the result are both before us. 
 
 There exist, however, on the surface of the earth 
 areas where indeed the results of instability restless- 
 ness and vulcanicity appear evidently displayed, but 
 the demonstration of the adequate cause is at present 
 denied to us, and possibly may always be so. It is of 
 interest to examine how far conjecture as to the cause 
 in these cases may be guided by the knowledge at our 
 disposal. 
 
 Darwin long ago urged that in the facts connected 
 with the coral islands or atolls there is evidence of 
 instability of the earth's crust. On the hypothesis of 
 downward movement he proposed to reconcile the 
 existence of these steeply ascending structures of coral 
 with the limitation which confines the operations of 
 the reef-building polyps to depths of some fifteen 
 or twenty fathoms of water. The volcanic cone, then 
 doubtless active and casting its ashes around, had first 
 to rise above or near to the surface of the sea. Later 
 this must sink, affording, however, a foundation suitable 
 to the needs of the reef-builders. And the patient 
 architects, ever keeping pace with the downward move- 
 ment of the cone, had maintained at the sea's surface 
 a visible monument of the volcano which once had 
 been. 
 
 Admittedly, Darwin's theory involves movements 
 of the ocean floor in many places and over areas of 
 great extent. Nor are the movements downwards only, 
 but also upwards, where we see the once submerged 
 
THE INSTABILITY OF THE OCEAN FLOOR. 119 
 
 coral standing high above sea-level. We must con- 
 clude that waves of flexure or of buckling affected 
 the earth's crust over these oceanic areas for long 
 periods of time, but periods which probably should be 
 referred to the comparatively recent past. 
 
 The areas involved are really enormous. The south- 
 equatorial coral region extends nearly 7000 miles in 
 latitude, and from 1000 to 1500 miles in longitude 
 an area about three times as great as that of Australia. 
 The Maldive Archipelago of coral islands in the Indian 
 Ocean is relatively small about 23,000 square miles. 
 These and other coral islands exhibit everywhere the 
 same characteristics. They everywhere emerge not 
 from the deepest ocean, but from plateaux seldom or 
 never over 2000 fathoms beneath the surface. They 
 appear to rise from waters teeming with that minute 
 surface-life which floors the ocean with organic 
 remains. 
 
 The leading features of oceanic deposition must here 
 be recalled to mind. The shallower parts of the 
 ocean, to depths of about 2000 fathoms, accumulate 
 sedimentary materials which never reach the deeper 
 areas. The reason of this resides in the soluble 
 nature of the calcareous tests of the dead foraminifera, 
 coccoliths, &c., which contribute so largely to the sub- 
 oceanic deposits. On these minute objects the waters 
 act after the death of the tissues, slowly restoring the 
 calcium carbonate to a state of solution. This action, 
 inappreciable near the surface, increases in the depths 
 with the increase of pressure. For a double reason, 
 then, the test disappears before it reaches the greatest 
 
120 KADIOACTIVITY AND GEOLOGY. 
 
 depths : the increasing solvent power of the water with 
 its dissolved carbon dioxide, and the slower progress 
 to the bottom of the dwindling particle, thereby 
 ensuring longer exposure to the solvent actions. The 
 endless shower of particles vanishes before it attains 
 the greatest depths, much as a shower of snow in the 
 upper atmosphere might dissolve to rain before the 
 lower and warmer layers of the atmosphere were passed 
 through. Hence it follows that on certain deep-lying 
 parts of the ocean-floor the growth of sediment is 
 extremely slow, and on other shallower parts the rate 
 of growth is relatively fast. 
 
 Sir John Murray founded upon these facts a 
 theory of coral-island formation which, unlike Darwin's, 
 did not necessarily involve a subsiding sea-bottom. 
 According to Murray's view (and A. Agassiz arrived at 
 similar conclusions), the foundation of the island is a 
 submarine bank (it may be a volcanic cone) which has 
 been extended upwards by the accumulation of ooze, 
 till the depth beneath the surface is so much reduced 
 that the reef -builders find thereon a suitable habitat. 
 Here the active agent is the sedimentary growth, and 
 the coral polyp and lime- secreting algae bring their 
 edifices to the surface without any crustal movement 
 being involved. 
 
 The claims of this ingenious alternative led to the 
 boring of the coral island Funafuti by Sollas and 
 David. The boring, which was carried to a depth of 
 185 fathoms by David, met with no foundational layer 
 of chalk or calcareous ooze. This should appear at 
 about 30 fathoms down if the foundation of the coral 
 
THE INSTABILITY OF THE OCEAN FLOOR. 121 
 
 rock indeed rests upon ooze, and there had been no ver- 
 tical movement. But as only true coral rock was 
 encountered the entire 1100 feet of descent, clearly 
 subsidence must be admitted. Although the true 
 foundation was not reached by the boring, Darwin's 
 view that there has been a subsidence is in this case 
 established. The coral rock now at a depth below sea- 
 level of 1100 feet must once have been at or near the 
 surface. 
 
 The beautiful map of oceanic deposits, which science 
 owes mainly to the work of Murray and Renard, shows 
 that the areas on which the coral islands stand are, 
 without exception, floored by the globigerina ooze, and, 
 as already remarked, from 1000 to 2000 fathoms 
 beneath the surface. Neighbouring areas sink to 
 3000 fathoms and more, and are covered with the 
 red clay, or with radiolarian ooze. From these deep 
 regions no coral islands spring. It is known that the 
 growth of these two last-named deposits is of extreme 
 slowness. In truth, the calcareous supplies being cut 
 off, no materials beyond those furnished by silica- 
 secreting organisms, and such volcanic dust as is carried 
 through the atmosphere or through the water, and the 
 meteoric dust from space, reach these remote parts of 
 the world. Hence they become the charnel house of 
 unburied remains; some, possibly, of even Tertiary 
 age ; others, the harder parts of the remains of recent 
 cetaceans, reaching their present position probably at 
 long-separated intervals of time. The strange man- 
 ganese nodules also grow here, apparently from slowly 
 decomposing mineral debris, and collecting considerable 
 
122 RADIOACTIVITY AND GEOLOGY. 
 
 quantities of uranium. That the dredge often comes 
 up laden with these unburied remains is proof of 
 the paucity of sedimentary materials reaching these 
 depths. 
 
 There is no subject in the domain of physical geology 
 more fascinating than those refined but obscure laws of 
 sedimentation which preserve to the continents their 
 waste of minute detritus, so that they collect age after 
 age in the shallow waters around the land, instead of 
 being borne to the remote parts of the ocean. In these 
 laws we recognize an important factor in mountain- 
 building, although only a contributory one. For it 
 appears certain that without their intervention all the 
 finest debris from the continents would have been 
 scattered far from the land, and the sediments of the 
 great synclines of the earth proportionally - reduced 
 in volume. The amount so deducted might be one 
 quarter of the whole, or even more. So great a loss 
 must not only have resulted in a less mean elevation 
 of the continents above sea-level (unless compensatory 
 effects intervened), but also in lesser radiothermal 
 effects in the sediments ; delaying all the great events 
 of mountain elevation and vulcanicity, which in their 
 sum represent a large part of the dynamical history of 
 the surface crust. 
 
 The experimental demonstration of these funda- 
 mental laws of sedimentation is easy. Measure out 
 equal volumes of sea- water and distilled water in glass 
 beakers. Stir into each an equal amount of finest 
 kaolin or clay, and leave at rest. In a few hours, the 
 sea-water is again limpid, while the fresh water for 
 
w 
 
 INSTABILITY OF THE OCEAN FLOOR. 123 
 
 days and even weeks may keep its cloudiness. The 
 dissolved salts are evidently responsible for the clear- 
 ing, seeing that the conditions are alike in all other 
 respects. Again, we may vary the experiment. We 
 make up solutions of sodium chloride and of magne- 
 sium chloride in the same proportions as these salts 
 exist in sea-water. We now find that the magnesium 
 chloride clears more rapidly, although there is much 
 less of this salt present than there is of chloride of 
 sodium. And if we once more compare these two salts, 
 but this time make up quantities proportional to the 
 molecular weights of the salts, so that equal numbers 
 of molecules are present in each solution, the difference 
 is still more marked in favour of the magnesium 
 chloride. Lastly we compare the sedimentation in a 
 solution of sugar with that in distilled water, and find 
 little or no difference. 
 
 The fundamental fact to be learned from these 
 experiments, and which has been confirmed by scores 
 of others of the most varied kind, is that the salt 
 must be ionized in order to bring about the pre- 
 cipitation of the clay. Such solutions conduct elec- 
 tricity as the ionized air in the electroscope conveys a 
 current, i.e. in virtue of the presence of free electric 
 charges on certain of the atoms and dissociated 
 molecules ; both positively and negatively charged 
 ions being necessarily present in equal numbers or 
 in equal electric quantity. In some way the free 
 electric charge causes the suspended particles of clay 
 to adhere one to another, and so they fall to the 
 bottom ; the viscous opposition of the water being less 
 
124 RADIOACTIVITY AND GEOLOGY. 
 
 effective on the aggregated particles. Sugar will not 
 act, because sugar solution contains no ions. The 
 magnesium is more effective than the sodium, because 
 the divalent metallic ion of magnesium carries a 
 double ionic charge, whereas the sodium ion is mono- 
 valent, and carries but the single ionic quantity of 
 electricity. 1 
 
 xThis effect, so important in geological history, and 
 not without influence on the central oceanic deposits 
 which we have been discussing, is, then, dependent 
 on the fact that sea-water is rich in ions, divalent 
 and monovalent. And these laws operate every- 
 where throughout the ocean. They hasten the finest 
 particles, derived from broken- up organisms and atmo- 
 spheric dust, on their journey to the bottom ; they 
 effectually secure the isolation of the central oceans 
 from the land sediments; and, not improbably, they 
 operate to secure the precipitation of ions of high 
 valency, such as those of dissolved uranium, along with 
 the falling sediments. The precipitation of the metallic 
 ion has, indeed, in some cases been experimentally 
 proved. 2 And this brings us back from our digression 
 if indeed it be a digression to the radioactive 
 condition of the sediments in the unstable oceanic 
 areas. 
 
 From what has already been stated in a former 
 chapter, when dealing with the radioactivity of the 
 
 1 "Whetham, Phil. Mag., xlviii., p. 74 ; Joly, Sci. Pree. Royal Dublin 
 Society, ix., p. 325, Compte rendu Congres GeoL Internat., 1900, p. 1 
 et seq., and Trans. R.D.S., vii., 391. 
 
 2 Linder and Picton, Chem. Soc. Journal, 1895. 
 
THE INSTABILITY OF THE OCEAN FLOOR. 125 
 
 oceanic sediments, it would appear that the precipita- 
 tion of the uranium occurs all over the ocean. Near 
 the land it becomes entangled with the finer detrital 
 sediments ; further out it is associated with the chalky 
 oozes derived from pelagic life; and in the deeper 
 central seas it reaches the red clays. Its varying degree 
 of concentration will then mainly depend on the abun- 
 dance of non-radioactive materials falling along with 
 it to the bottom, and acting as diluent. The slowly 
 collecting red clays and radiolarian oozes attain a 
 radioactivity from 30 to 50: the fast-forming globi- 
 gerina ooze, from 6 to 8 ; and the near-shore deposits 
 (in harmony with the finding on the land sediments), 
 from 3 to 5. 
 
 Can we associate the instability of certain areas with 
 the radioactivity of the sediments piled thereon ? At 
 first sight it might be thought that if this was so the 
 instability should be most in evidence where the radio- 
 activity was highest. But, in truth, the conditions 
 requisite for the attainment of a high temperature 
 at the base of a sedimentary deposit show that this 
 inference is not justified. It was shown (p. 77) that 
 the basal temperature was greater as the square of the 
 depth of the deposit, so that a given amount of radium 
 highly diluted with inert materials, and therefore con- 
 tained in a deep layer of feeble radioactivity per unit of 
 volume, will give rise to a higher temperature beneath 
 than the same amount of radium concentrated in a 
 shallow deposit, although the radioactivity is, of course, 
 in the latter case greater. Now these are just the rela- 
 tive conditions prevailing in the globigerina ooze and in 
 
126 RADIOACTIVITY AND GEOLOGY. 
 
 the red clay. The former has a lower radioactivity, but 
 not necessarily a lesser total quantity of radium. It 
 is highly dilute, and therefore the contained radium is 
 more deeply buried. The latter with a greater radio- 
 activity keeps its radium nearer the surface. The first 
 distribution favours the retention of the heat; the 
 second facilitates its escape. 
 
 Another condition also enters to enhance the thermal 
 effects beneath the chalky deposit the lesser conduc- 
 tivity of such deposits. We are dealing with what is 
 essentially chalk. This is a substance of exceptionally 
 low conductivity, the recorded determinations generally 
 assigning 0-002 or 0'003 as the thermal conductivity : 
 i.e., one-half or three-fourths of what may be commonly 
 ascribed to sedimentary materials. 
 
 Thus there are present all the conditions favour- 
 able to a high basal temperature a high radioactivity 
 (6 to 8), a low conductivity, and a rapid rate of deposi- 
 tion. The question then naturally turns on the thickness 
 of sediment which may be attained in this part of the 
 ocean. The great subject of the deep-sea deposits is 
 here presented to us with a new interest. Can we 
 obtain no guidance as to what may be the thickness of 
 these deposits over the disturbed areas of the ocean 
 floor ? 
 
 The time during which the accumulation may have 
 been progressing seems indeterminate, and, for all we 
 know, may have extended throughout the whole of 
 geological history. This leads us to consider for a 
 little the doctrine of the permanency of the oceans and 
 continents. 
 
THE INSTABILITY OF THE OCEAN FLOOR. 127 
 
 It has been shown that the permanence of the great 
 oceanic depressions may be involved in the larger 
 dynamics of the earth ;* and it is for future investi- 
 gations to determine how far lost continents, of the 
 magnitude which some geologists seem to favour, can, 
 consistently with this important result, be supposed to 
 come into existence and again disappear. But there is 
 an independent line of evidence to show that continents 
 comparable in magnitude with those we know, and of 
 any comparable longevity, cannot have existed. The 
 argument is based on the facts of solvent denudation. 
 
 The sodium in the ocean is the key to the history 
 of solvent denudation. Every connected fact we know 
 supports the view that this was derived from the 
 igneous rocks of the earth by the processes of weather- 
 ing and solution progressing throughout geological time. 
 The total of it has been preserved, practically in its 
 entirety, in the ocean, wherein the chloride has accu- 
 mulated to such an extent that if spread over the 
 earth's surface in a uniform layer, it would cover it to 
 a depth of 112 feet. Contrasted with so great a mass, 
 the quantities of rock-salt which have been abstracted 
 from the sea by evaporation at various times in the 
 past, are minute, and, as a source of error in our 
 calculations, may be neglected. 
 
 Now it is found that the quantity of oceanic sodium 
 tallies with the sediments as we find them upon the 
 existing continents : that is, the estimated bulk of the 
 sediments is just about what would account for the 
 salinity of the ocean, assuming that these sediments 
 
 'Love, Rep. British Assoc., 1907, and Phil. Trans. .S., 207, p. 171. 
 
128 RADIOACTIVITY AND GEOLOGY. 
 
 were derived from igneous rocks having the same 
 average composition as those we now see upon the 
 surface. The last assumption is so probable that it 
 need not be further discussed. But if the known 
 sediments are such as agree with the record of denu- 
 dation which the ocean gives us, there cannot be yet 
 other continents each with its own burden of sedi- 
 ments now hidden beneath the ocean. True, our estimate 
 of sediment upon the existing land-surface may not be 
 very accurate, but it can hardly be supposed miscalcu- 
 lated to the extent of the sedimentation of continents 
 comparable in magnitude with those we are dealing 
 with. The figures show that so far from there being 
 any indication of missing sediment, it is the other way, 
 and we are confronted with an embarras de richesse 
 which accounts for the marine sodium, and leaves 
 sedimentary material over. 
 
 Estimates of the amount of sediment on the earth 
 have been made by several authorities. The early 
 estimates were all very high. Dana thought they 
 might be taken as not exceeding an average of 5 miles 
 (8 kilometres) in depth over the continents. Mellard 
 Eeade modified an originally very high estimate to 
 one of about the same magnitude as Dana's, stating 
 his conclusion that the total deposits would cover the 
 entire earth to a depth of one mile (1*61 kilometre). 
 Van Hise, 1 reconsidering the estimates of these earlier 
 writers, states, as a guess, that the sediments which 
 exist in the zone of katamorphism, and have always 
 
 1 Van Hise, A Treatise on Metamorphitm, p. 939. 
 
THE INSTABILITY OF THE OCEAN FLOOR. 129 
 
 been in this zone (i.e., have not been thermally meta- 
 morphosed into different petrological materials) may be 
 taken as on an average 2 kilometres over the continents. 
 Prior to this estimate of Van Hise, the author had, 
 for reasons which need not be cited, modified Mellard 
 Keade's estimate to one which may be represented by 
 saying that the purely detrital sediments would cover 
 the continents to the depth of 1/1 mile (1*77 kilometre), 
 including an allowance of 10 per cent, for coastal deposits. 
 This estimate deals with those sediments with which 
 alone we are now concerned. It will be seen that it is 
 lower than any other available estimate, for the omitted 
 limestones are not probably more than 5 per cent., and, 
 besides, Van Hise's estimate is admittedly a restricted 
 one. 
 
 A layer of sediment 1*77 kilometre thick spread over 
 the area of the continents (14,456 x 10 4 square kilo- 
 metres) would possess a mass of about 64 x 10 16 tonnes. 
 And from the known facts of solvent weathering we 
 are entitled to conclude that this mass is about 67 per 
 cent, of the parent igneous rock from which it was 
 derived. This gives to the parent rock a mass of 
 95-5 x 10 16 tonnes. 
 
 The estimated amount of detrital sedimentaries 
 must include about 8*3 x 10 15 tonnes of the sodium 
 oxide, Na 2 0. This we conclude from the results of the 
 analyses of typical sedimentary rocks given by Clarke 
 in his Data of Geochemistry. The sodium in the 
 ocean when converted into the equivalent mass of the 
 oxide affords 21 '0 x 10 15 tonnes. It is evident that in 
 
 K 
 
130 RADIOACTIVITY AND GEOLOGY. 
 
 the soda of the estimated sediments, added to the soda 
 deduced from oceanic sodium, we have, apparently, the 
 total soda which entered into the composition of the 
 original igneous rock. We accordingly restore it, and 
 rind that the result is 3*07 per cent, of soda. 
 
 Let us now compare this result with the average soda 
 content of igneous rocks. According to tables given by 
 Clarke, 1 the average soda content of these rocks is 
 3*39 per cent. The agreement is close, considering the 
 nature of the problem. It is noteworthy, however, 
 that there is a deficiency of sodium. What should we 
 infer from this ? The only reasonable way of effecting 
 a reconciliation is to reduce the amount of parent 
 igneous rock, or, in other words, to reduce the estimate 
 of the sediments. When we do so, we can of course 
 bring about agreement (p. 37). Thus we find that the 
 estimated sediments on the continents are in excess of 
 the requirements of solvent denudation, as revealed by 
 the oceanic record. How, then, can it be assumed that 
 the sedimentary burden of whole continents is beneath 
 the sea ? Nor does it appear possible to assail the 
 truth of the oceanic record, unless on the possible 
 but improbable supposition that there was a large 
 amount of original sodium in the ocean not derived 
 from the rocks. But if we take this view, we limit 
 still more the amount of sodium which we may restore 
 to the igneous rocks, and so necessitate a further 
 reduction in the estimate of the sediments. 
 
 The theory of lost continents has often been upheld 
 in order to account for sedimentary accumulations in 
 
 1 Clarke, The Data of Geochemistry, p. 31. 
 
THE INSTABILITY OF THE OCEAN FLOOR. 131 
 
 certain lands, which accumulations are, so to speak, 
 fatherless. To postulate whole continents in order to 
 supply the parentage is to get into the serious diffi- 
 culty we have discussed above. That former continents 
 of any antiquity or magnitude are not hidden beneath 
 the waves seems certain, unless the estimates of sedi- 
 ments are quite erroneous. The argument, of course, 
 does not touch on the former existence of those cause- 
 ways which biological history requires as the highways 
 of organic migration. 
 
 Our point here is, however, the essential antiquity 
 of the great oceanic depressions which embrace the 
 coral islands, and the greatness of the time during 
 which sediments may have accumulated in those areas. 
 It would appear that we may in no niggardly spirit 
 deal with that reserve of time which geological science 
 has laid apart for those who lawfully prefer their 
 demands. 
 
 As regards the rate of deposition, it seems assured 
 that this varies enormously with the nature of the 
 deposit and the surface conditions, as well as with the 
 conditions of depth. Even siliceous deposits may 
 attain relatively rapid rates of growth where favoured 
 by abundance of dissolved siliceous materials carried by 
 the great rivers into the ocean. 
 
 Quite possibly this last condition has contributed to 
 the amount of siliceous ooze which is presented to 
 our inspection on the re-elevated island of Barbados ; 
 this island being not very far removed from the 
 drainage discharge of a great continent. These deposits 
 are, however, from their character, undoubtedly oceanic 
 
 K2 
 
132 KADIOACTTVITY AND GEOLOGY. 
 
 sediments, and the facts which they seem to convey are 
 of particular interest. The history of the island as 
 revealed in its sediments is remarkable. Underlying 
 the deep-sea deposits are sandy clays and sandstones 
 of relatively shallow-water character. Above the deep- 
 sea beds are coral rocks of surface formation. The 
 island must therefore have gone down to the depths for 
 a period and re-emerged bearing with it the accumula- 
 tions which gathered during the period of submergence. 
 As it approached the surface, the reef-builder settled 
 upon it, and covered the soft oceanic sediments with a 
 protecting cover of coral. Thus we have in its rocks a 
 fairly complete record of the recent sedimentary history 
 of a small patch of the ocean floor. 
 
 In their investigation of the island Jukes-Browne and 
 Harrison obtained measurements of the deposits which 
 show that during the period of submergence a maximum 
 of about 300 feet of sediments of oceanic type were 
 laid down. 1 Their section on Mount Hillaby is as 
 follows : 
 
 Grey volcanic mudstones, .... 25 feet. 
 Very fine-grained argillaceous earths, red, 
 
 pink, and yellowish, often mottled, 60 ,, 
 
 Chalky earths and marls, . . . . 25 ,, 
 Calcareo-siliceous earths, with layers of 
 
 pumiceous sand, . . ,, . 45 ,, 
 
 Pure siliceous radiolarian earths, . . 130 ,, 
 Calcareo-siliceous earths, passing down into 
 
 chalky earths, 40 
 
 Total, . . 325 feet. 
 1 Jukes-Browne and Harrison, Q.J.G.S. xlviii., p. 210. 
 
THE INSTABILITY OF THE OCEAN FLOOR. 133 
 
 The Scotland beds, of shallow - water origin, come 
 immediately beneath. The writers show by other 
 measurements that the real thickness is somewhat 
 greater. Omitting the uppermost bed as being one of 
 quite indeterminate rate of deposition, we have 300 
 feet of oceanic materials, mainly red clays (60 ft.) 
 and radiolarian ooze (130 ft.) ; the other beds being 
 seemingly an admixture of such sediments with some 
 calcareous matter. Whatever the geological position 
 of the Scotland beds may be, they are certainly not 
 oceanic in origin; and again the rock immediately 
 above is coral rock, and therefore marks the re-ascent 
 of the submerged area to the surface. 
 
 We could, of course, form some idea of the rate 
 of deposition of these oceanic beds if we knew the 
 duration of submergence. Here we have to face con- 
 flicting views as to the precise geological period in 
 which the submergence took place. Jukes-Browne and 
 Harrison believe the beds collected probably during 
 Pliocene and partly, perhaps, in Pleistocene times. 
 J. W. Spencer 1 in a more recent paper ascribes 
 these deposits to Eocene times. Sollas 2 estimates the 
 maximum thickness of Eocene sediments as 12,000 feet. 
 The rate of deposition if taken at one foot in a century 
 would ascribe 1*2 million of years for the duration of 
 this period. This rate is probably too great. One 
 third of a foot as the mean rate of detrital sedi- 
 mentation seems to accord better with the evidence 
 as to the duration of geological time and the estimated 
 
 1 Spencer, Q.J.G.S. Iviii., p. 304 et teg. 
 ~ Solias, The Age of the Earth, p. 14. 
 
134 RADIOACTIVITY AND GEOLOGY. 
 
 total maximum sedimentation of all the geological 
 periods. On this rate the Eocene period must have 
 lasted 3 -6 millions of years. The whole of the Pliocene, 
 following the same mode of estimation, would amount 
 to 1*5 million years, and the Pliocene, Pleistocene, and 
 Kecent together to 2*7 millions of years. We see, then, 
 that the period of submergence should be something 
 between one million and three million years. These 
 limits would give rates of deposition of the Barbados 
 sediments from 9 to 3 millimetres in a century. 
 
 These rates appear very slow, but in point of fact they 
 are too high to be generally assumed for such deposits. 
 We see this when we reckon that if geological time 
 extended 100 million years, a rate of 5 millimetres in a 
 century would accumulate to 5 kilometres a much 
 greater depth than is allowable on our former estimate 
 of the total amount of oceanic deposits. 
 
 It is, however, quite certain, as already remarked, 
 that the rate of accumulation of oceanic sediments 
 varies enormously. The chalk attained a thickness of 
 from 1200 to 1500 ft. in British Cretaceous seas; and 
 this is a true oceanic deposit. The period of deposition 
 was a relatively brief one. Huxley inferred that the 
 rate of collection of this ooze was as much as 1 inch in 
 a year ; but Sollas has shown that this rate is certainly 
 excessive, and believes that 1 inch in 40 years would be 
 more nearly correct. 1 Sir John Murray and Mr. Peake 2 
 have given reasons for supposing that in parts of the 
 
 1 Sollas, The Age of the Earth, p. 160. 
 
 3 Sir John Murray and R. E. Peake, Royal Geographical Society 
 Suppl. Papers, 1904, p. 21. 
 
THE INSTABILITY OF THE OCEAN FLOOR. 135 
 
 Atlantic the Globigerina ooze collects on the cables at 
 a rate of as much as 10 inches in a century. These 
 estimates are in no case, of course, assured to us as cor- 
 rect ; but it is apparent that the rate of accumulation of 
 the calcareous ooze must be very much greater than 
 that of the red clay, which cannot be more than a milli- 
 metre or two in a century. This lack of knowledge will 
 certainly be remedied with the progress of science. 
 There should be no insurmountable difficulty either as 
 regards natural obstacles or costliness, in effecting 
 direct measurements of the rate of collection of the 
 faster-growing deposits. 
 
 We are, at the present time, left with only this much 
 guidance as regards the rate of sedimentation pro- 
 gressing over the coral island plateaus, that it may be 
 very considerable, and certainly is very considerable if 
 at all resembling in rate that with which the chalk 
 was laid down in the Cretaceous seas. Thus there is 
 nothing improbable in the view that great accumulations 
 of sediment exist around the coral islands. We appear 
 restricted neither in our demands upon time nor as to 
 rate of deposition of course within limits. And we 
 are assured that the radioactivity is more favourable 
 for the attainment of larger radio-thermal effects than 
 in ordinary sediments. If the rate of collecting had 
 been only 5 millimetres in a century over a period 
 considerably short of the probable duration of geolo- 
 gical time, the depth of sediment might well amount 
 to 4 kilometres, which, on the assumptions discussed 
 in Chapter V, involves a reduction of effective strength 
 amounting to nearly 10 per cent. But as the radio- 
 
136 RADIOACTIVITY AND GEOLOGY. 
 
 activity of the chalky deposit is more than double that 
 assumed for the sediments in preparing the table, and, 
 again, as the conductivity is less for the sedimentary 
 part of the total layer, the result tabulated must fall 
 considerably short of the actual effect in the present 
 case. Probably not less than 20 per cent, should be 
 taken as the reduction in effective resistance and thick- 
 ness of the crust due to an accumulation such as is 
 here supposed. And, without improbable demands on 
 the limiting total of the suboceanic sediments, the 
 area of weakness might be sufficiently extended to 
 determine a wide -spread upheaval of the central 
 Pacific floor. 
 
 We have now in outline reviewed the conditions, both 
 those which are known to prevail, and those which may 
 reasonably be supposed to prevail, in the coral seas; 
 and if we are right, the deeper sedimentary accumula- 
 tions of the central ocean must play an important part 
 in the movements of the crust. But we cannot stop 
 here: the consequences of these movements must be 
 considered, and, such as they are, they must be 
 accredited to the influence of uranium when its energy 
 is focussed, as it were, upon particular areas of the 
 stressed and precarious crust of the earth. It may now 
 turn out that those transgressions of the seas upon 
 the continents, which in the past were of world-wide 
 extent, in so far that they seem to have simultaneously 
 submerged many lands, are traceable to these oceanic 
 movements, or at least partially traceable thereto. If 
 the buckling of the sea-floor may be defined by the 
 estimates which Darwin and others have made of the 
 
THE INSTABILITY OF THE OCEAN FLOOR. 13? 
 
 recent settlement indicated by the coral islands, then 
 this view seems more than mere hypothesis, for the rise 
 and fall of the sea upon the land by some hundreds of 
 feet is assured even if the continents be supposed to 
 have remained rigidly resistant and fixed in their 
 present position under the increased load. 
 
 The boring of Funafuti has shown that this island 
 has subsided at least 900 or 1000 feet in recent times. 
 For this the evidence seems quite irrefragable. But 
 the boring which proves this extends but a small 
 fraction of the probable depth of the coral. The pene- 
 tration of the drill has hardly been more than a tenth 
 or a twelfth part of the elevation of the island above the 
 sea-floor. That the coral ends just beneath the boring 
 is unlikely. We may fairly conclude that a much 
 greater downward movement than 1000 feet has 
 actually occurred. Darwin concluded from the evi- 
 dence of the soundings that the whole subsidence 
 cannot be considered less than 6000 feet, and believed 
 that it began in Tertiary times. There is, in the case 
 of the quasi-coral island of Barbados, evidence of a 
 subsidence and re-elevation in Tertiary times of not 
 less than 10,000 feet. Jukes-Browne and Harrison 
 believe that in its case we may safely assume between 
 12,000 and 18,000 feet. 1 
 
 Now, if we accept Darwin's estimate of 1000 fathoms 
 for the general subsidence of the floor of the Coral Sea, 
 the change in the capacity of the Pacific basin would 
 have affected the distribution of land and water very 
 considerably. It is easy to show, similarly, that if there 
 
 1 Jukes-Browne and Harrison, loe. cit., p. 201. 
 
138 RADIOACTIVITY AND GEOLOGY. 
 
 was an elevation of this amount from the present level, 
 extensive areas of the existing continents would be 
 covered. Taking the upheaved area at 2 x 10 7 square 
 kilometres, the general level of the ocean would be 
 raised about 340 feet, assuming a transgression of the 
 waters to this depth upon five per cent, of the land area. 
 The actual area of submerged land would be greater, 
 but the average depth of submergence less than 340 
 feet. This depth would suffice for the deposition of the 
 chalk. 1 
 
 We do not here seek to account for the origin of the 
 surface stress which is responsible for these movements, 
 nor yet to account for such compensatory motions of 
 the crust as probably occurred. "We know such move- 
 ments have taken place in past times; and whether 
 they be referable to secular contraction or to adjust- 
 ments of the lithosphere such as Love has suggested, 2 
 or to the operation of both causes, is outside the scope 
 of our present subject. 
 
 The whole matter is a relatively small one, although 
 to the earth's inhabitants so important. Imagine a 
 globe 2 metres in diameter : the movement of the sea- 
 floor we have assumed would be represented on such a 
 globe by the radial displacement of an area upon its 
 surface through about the one-fourth part of a milli- 
 metre, that is, about the one-hundredth part of an inch, 
 or, say, the thickness of a visiting-card ! The move- 
 
 1 Jeffreys, Presidential Address, British Association, 1877 ; also 
 Chamberlin and Salisbury, Geology, vol. iii., p. 150. In the above 
 estimate the existing area of the ocean is taken as 36,708 x 10*, and the 
 present land surface as 14,456 x 10 4 square kilometres. 
 
 2 Love, he. cit., p. 239 et seq. 
 
THE INSTABILITY OF THE OCEAN FLOOR. 139 
 
 ments in the geosynclines and the buckling of the 
 ocean-floor are, indeed, trifling details of the earth's 
 vicissitudes. 
 
 Those who take pleasure in the contemplation of the 
 means various in detail, alike in result by which the 
 ends of nature are often accomplished, will reflect on 
 the fact that the foregoing views have led us to ascribe 
 two dynamical episodes of the crust fateful to organic 
 history if petty in themselves to the convective 
 movements of uranium on the surface of the earth : 
 the one restricted to the continental margins, owing 
 to the solid nature of the materials involved ; the 
 other coming into existence in the remotest parts 
 of the ocean-floor, the dissolved elements receding 
 indefinitely from the land. In each case the con- 
 vective and cumulative conditions have to be fulfilled ; 
 but the physical processes responsible are widely 
 different. 
 
CHAPTER VII. 
 
 EADIOACTIVITY AND MOUNTAIN STRUCTURE. 
 
 PROBABLE PERIODS OF TIME INVOLVED IN THE GENESIS OF THE ALPS 
 EVIDENCE OF HIGH TEMPERATURE IN ALPINE ROCKS THESE 
 CONDITIONS UNACCOUNTABLE WITHOUT THE INTERVENTION OF 
 BADIOTHERMAL ACTIONS RECENT VIEWS ON ALPINE TECTONICS 
 THE ORIGIN OF THE PREALPES AND OF THE GREAT LIMESTONE 
 ALPS THE DEFERLEMENT OF THE RECUMBENT FOLDS TEMPERA- 
 TURES PROBABLY ATTAINED THEORY OF THE HORST UNTENABLE 
 THE FLUID OR VISCOUS CONDITIONS SUFFICIENT TO ACCOUNT FOR 
 THE RECUMBENCE OF THB FOLDS AND FOB THE DEFERLEMENT 
 SKETCH OF THE STAGES OF DEVELOPMENT. 
 
 AN intelligent observer of the geological changes 
 progressing in southern Europe in Eocene times would 
 have seen little to inspire him with a premonition of the 
 events then developing. The Nummulitic limestones 
 were being laid down in that enlarged Mediterranean 
 which at this period, save for a few islands, covered 
 most of south Europe. Of these stratified remains, as 
 well as of the great beds of Cretaceous, Jurassic, 
 Triassic, and Permian sediments beneath, our hypo- 
 thetical observer would probably have been regardless ; 
 just as to-day we observe, with an indifference born 
 of our transitoriness, the deposits rapidly gathering 
 wherever river discharge is distributing the sediments 
 over the sea-floor, or the lime-secreting organisms are 
 actively at work. And yet it took but a few millions 
 of years to uplift the deposits of the ancient Tethys; 
 
RADIOACTIVITY AND MOUNTAIN STRUCTURE. 141 
 
 pile high its sediments in fold upon fold in the Alps, 
 the Carpathians, and the Himalayas; and exposing 
 them to the rigours of denudation at altitudes where 
 glaciation, land-slip, and torrent for ever prevail 
 inaugurate a new epoch of sedimentation and upheaval. 
 In the case of the Alps, to which we wish now 
 specially to refer, the chief upheaval appears to have 
 been in Oligocene times, although movement continued 
 to the close of the Pliocene. There was thus a period 
 of some millions of years within which the entire 
 phenomena were comprised. Again availing ourselves 
 of Professor Sollas' computations, we may sum the 
 maximum depths of sedimentary deposits of the 
 geological periods concerned as follows : 
 
 Pliocene, . . 5,000 feet, 
 
 Miocene, . ,, 9,000 ,, 
 
 Oligocene, . . 12,000 
 
 Eocene, V . 12,000 
 
 and assuming that the orogenic forces began their work 
 in the last quarter of the Eocene period, we have a 
 total of 29,000 feet as some measure of the time which 
 elapsed. At the rate of one-third of a foot in a century 
 these deposits could not have collected in less than 
 87 millions of years. It would appear that not less 
 than from 6 to 8 millions of years were consumed in 
 the genesis of the Alps before constructive movements 
 finally ceased. 
 
 The progress of the earth- movements was attended 
 by the usual volcanic phenomena. The Oligocene and 
 Miocene volcanoes extended in a band marked by the 
 Auvergne, the Eifel, the Bohemian, and the eastern 
 
142 RADIOACTIVITY AND GEOLOGY. 
 
 Carpathian eruptions ; and, later, towards the close of 
 the movements in Pliocene times, the south border 
 regions of the Alps became the scene of eruptions such 
 as those of Etna, Santorin, Somma (Vesuvius), &c. 
 
 We have referred to these well-known episodes with 
 two objects in view : to recall to mind the time- 
 interval involved, and the evidence of intense crustal 
 disturbance, both dynamic and thermal. According to 
 views explained in a previous chapter, the energetic 
 effects of radium in the sediments and upper crust 
 were a principal factor in localizing and bringing 
 about these results. We propose now to inquire if, 
 also, in the more intimate structure of the Alps, the 
 radioactive energy may not have borne a part. 
 
 What we see to-day in the Alps is but a residue 
 spared by denudation. It is certain that vast thick- 
 nesses of material have disappeared. Even while con- 
 structive effects were still in progress, denudative forces 
 were not idle. Of this fact the shingle accumulations 
 of the Molasse, where, on the northern borders of the 
 Alps, they stand piled into mountains, bear eloquent 
 testimony. In the sub-Apennine series of Italy, the 
 great beds of clays, marls, and limestones afford evi- 
 dence of these destructive processes continued into 
 Pliocene times. We have already referred to Schmidt's 
 estimate that the sedimentary covering must have 
 in places amounted to from 15,000 to 20,000 metres. 
 The evidence for this is mainly tectonic or struc- 
 tural ; but is partly forthcoming in the changes which 
 the materials now open to our inspection plainly reveal. 
 Thus it is impossible to suppose that gneissic rocks 
 
RADIOACTIVITY AND MOUNTAIN STRUCTURE. 143 
 
 can become so far plastic as to flow in and around the 
 calcareous sediments, or be penetrated by the latter 
 as we see in the Jungfrau and elsewhere unless 
 great pressures and high temperatures prevailed. And, 
 according to some writers, the temperatures revealed 
 by the intimate structural changes of rock-forming 
 minerals must have amounted to those of fusion. The 
 existence of such conditions is supported by the 
 observation that where the crystallization is now the 
 most perfect, the phenomena of folding and injection 
 are best developed. 1 These high temperatures would 
 appear to be unaccountable without the intervention 
 of radiothermal effects ; and, indeed, have been regarded 
 as enigmatic by observers of the phenomena in question. 
 A covering of 20,000 metres in thickness would not 
 occasion an earth-temperature exceeding 500 C. if the 
 gradients were such as obtain in mountain regions 
 generally ; and 600 is about the limit we could ascribe 
 to the purely passive effects of such a layer in elevating 
 the geotherms. 
 
 Those who are still unacquainted with the recently 
 published observations on the structure of the Alps 
 may find it difficult to enter into what has now to be 
 stated ; for the facts are, indeed, very different from the 
 generally preconceived ideas of mountain formation. 
 Nor can we wonder that many geologists for long held 
 back from admitting views which appeared so extreme. 
 Eeceptivity is the first virtue of the scientific mind ; 
 but, with every desire to lay aside prejudice, many 
 felt unequal to the acceptance of structural features 
 
 1 Weinschenk, C. JR. Congret Geol., 1900, p. 321 et seq. 
 
144 RADIOACTIVITY AND GEOLOGY. 
 
 involving a folding of the earth-crust in laps which lay 
 for scores of miles from country to country, and the 
 carriage of mountainous materials from the south of 
 the Alps to the north, leaving them finally as Alpine 
 ranges of ancient sediments reposing on foundations 
 of more recent date. The historian of the subject will 
 have to relate how some who finally were most active 
 in advancing the new views were at first opposed to 
 them. In the change of conviction of these eminent 
 geologists we have the strongest proof of the con- 
 vincing nature of the observations and the reality of 
 the tectonic features upon which the recent views are 
 founded. 
 
 The lesser mountains which stand along the northern 
 border of the great limestone Alps, those known as the 
 Prealpes, present the strange characteristic of resting 
 upon materials younger than themselves. Such moun- 
 tains as the remarkable-looking Mythen, near Schwyz, 
 for instance, are weathered from masses of Triassic 
 and Jurassic rock, and repose on the much more recent 
 Flysch. In sharp contrast to the Flysch scenery, they 
 stand as abrupt and gigantic erratics, which have 
 been transported from the central zone of the Alps 
 lying far to the south. They are strangers petrolo- 
 gically, stratographically, and geographically, 1 to the 
 locality in which they now occur. The exotic materials 
 may be dolomites, limestones, schists, sandstones, or 
 rocks of igneous origin. They show in every case traces 
 of the severe dynamic actions to which they have been 
 subjected in transit. The igneous, like the sedimentary, 
 
 1 De Lappareot, Traits de Geologic, p. 1785. 
 
RADIOACTIVITY AND MOUNTAIN STRUCTURE. 145 
 
 Hippen, can be traced to distant sources ; to the massif 
 of Belladonne, to Mont Blanc, Lugano, and the Tyrol. 
 The Prealpes are, in fact, mountains without local 
 roots. 
 
 In this last-named essential feature, the Prealpes do 
 not differ from the still greater limestone Alps which 
 succeed them to the south. These giants, e.g., the 
 Jungfrau, Wetterhorn, Eiger, &c., are also without local 
 foundations. They have been formed from the over- 
 thrown and drawn-out anticlines of great crust-folds, 
 whose synclines or roots are traceable to the south 
 side of the Khone Valley. The Bernese Oberland 
 originated in the piling-up of four great sheets or 
 recumbent folds, one of which is continued into the 
 Prealpes. With Lugeon 1 we may see in the pheno- 
 menon of the formation of the Prealpes a detail; 
 regarding it as a normal expression of that mechanism 
 which has created the Swiss Alps, For these lime- 
 stone masses of the Oberland are not indications of a 
 merely local shift of the sedimentary covering of the 
 Alps. Almost the whole covering has been pushed 
 over and piled up to the north. Lugeon 2 concludes 
 that, before denudation had done its work and eut off 
 the Prealpes from their roots, there would have been 
 found sheets to the number of eight superimposed and 
 extending between the Mont Blanc massif and the 
 massif of the Finsteraarhorn : these sheets being the 
 overthrown folds of the wrinkled sedimentary covering. 
 The general nature of the alpine structure will be 
 
 1 Lugeon, Bulletin Soc. Geol. de France^ 1901, p. 772. 
 
 2 Lugeon, loc. cit. 
 
 L 
 
146 RADIOACTIVITY AND GEOLOGY. 
 
 understood from the presentation of it diagrammatically 
 after Schmidt of Basel (Plate V.). 1 The section ex- 
 tends from north to south, and brings out the relations 
 of the several recumbent folds. We must imagine 
 almost, the whole of these superimposed folds now 
 removed from the central regions of the Alps by 
 denudation, and leaving the underlying gneisses rising 
 through the remains of Permian, Triassic, and Jurassic 
 sediments; while to the north the great limestone 
 mountains, and further north still, the Prealpes, 
 carved from the remains of the recumbent folds, now 
 stand with almost as little resemblance to the vanished 
 mountains as the memories of the past have to its 
 intense reality. 
 
 These views as to the origin of the Alps, which are 
 shared at the present day by so many distinguished 
 geologists, had their origin in the labours of many now 
 gone ; dating back to Studer ; finding their inspiration 
 in the work of Heim, Suess, and Marcel Bertrand ; and 
 their consummation in that of Lugeon, Schardt,Kothpletz, 
 Schmidt, and many others. Nor must it be forgotten 
 that nearer home, somewhat similar phenomena, neces- 
 sarily on a smaller scale, were recognized by Lapworth, 
 twenty-six years ago, in his work on the structure of 
 the Scottish Highlands. 
 
 An important tectonic principle underlies the de- 
 velopment of the phenomena we have just been 
 reviewing. The uppermost of the superimposed recum- 
 bent folds is more extended in its development than 
 those which lie beneath. Passing downwards from the 
 
 1 Schmidt, EC. Oeol. Helvetia*, vol. ix., No. 4. 
 
t> 
 
 H 
 Ex 
 
 
 
 
RADIOACTIVITY AND MOUNTAIN STRUCTURE. 147 
 
 highest of the folds, they are found to be less and less 
 extended both in the northerly and in the southerly 
 direction, speaking of the special case the Alps 
 now before us. This feature might be described some- 
 what differently. We might say that those folds which 
 had their roots further to the south were the most 
 drawn-out towards the north : or again we might say 
 that the synclinal or deep-seated part of the fold has 
 lagged behind the anticlinal or what was originally the 
 highest part of the fold, in the advance of the latter 
 to the north. The anticline has advanced relatively to 
 the syncline. To this law one exception only is 
 observed in the Swiss Alps ; the sheet of the Breche 
 (breccien decke) falls short, in its northerly extension, 
 of the underlying fold, which extends to form the 
 Prealpes. 
 
 Contemplating such a generalized section as Professor 
 Schmidt's, or, indeed, more particular sections, such as 
 those in the Mont Blanc Massif by Marcel Bertrand, 1 of 
 the Dent de Morcles, Diablerets, Wildhorn, and Massif 
 de la Breche by Lugeon, 2 or finally Termier's section of 
 the Pelvoux Massif, 3 one is reminded of the breaking 
 of waves on a sloping beach. The wave, retarded at 
 its base, is carried forward above by its inertia, and 
 finally spreads far up on the strand ; and if it could 
 there remain, the succeeding wave must necessarily 
 find itself superimposed upon the first. But no effects 
 of inertia, no kinetic effects, may be called to our 
 
 1 Marcel Bertrand, Cong. Geol. Internal, 1900, Guide Geol., xiii. a, 
 p. 44. 2 Lugeon, loc. cit., p. 773. 
 
 3 De Lapparent, Traite de Geol., p. 1773. 
 
 L2 
 
148 RADIOACTIVITY AND GEOLOGY. 
 
 aid in explaining the formation of mountains. Some 
 geologists have accordingly supposed that in order to 
 account for the recumbent folds and the peculiar 
 phenomena of increasing overlap, or deferlement, an 
 obstacle, fixed and deep-seated, must have arrested the 
 roots or synclines of the folds, and held them against 
 translational motion, while a movement of the upper 
 crust drew out and carried forward the anticlines. 
 Others have contented themselves by recording the 
 facts without advancing any explanatory hypothesis 
 beyond that embodied in the incontestable statement 
 that such phenomena must be referred to the effects of 
 tangential forces acting in the earth's crust. 
 
 It would appear that the explanation of the phe- 
 nomena of recumbent folds and their dtferlement is 
 to be obtained directly from the temperature conditions 
 prevailing throughout the stressed pile of rocks ; and 
 here the subject of mountain tectonics touches that 
 with which we are specially concerned the geological 
 influence of accumulated radioactive energy. 
 
 As already remarked (p. 110), a rise of temperature 
 due to this source of from 300 to 500 might be added 
 to such temperatures as would arise from the mere 
 blanketing of the earth, and the consequent upward 
 movement of the geotherms. The time element is here 
 the most important consideration. The whole sequence 
 of events from the first orogenic movements to the 
 final upheaval in Pliocene times must probably have 
 been comprised within a period of about six to eight 
 million years, although we cannot be sure that a longer 
 time was not involved 
 
RADIOACTIVITY AND MOUNTAIN STRUCTURE. 149 
 
 Unfortunately the full investigation of the distribu- 
 tion of temperature after any given time is beset with 
 difficulties ; the conditions being extremely complex. If 
 the radioactive heating was strictly adiabatic that is, 
 if all the heat was conserved and none entered from 
 without the time required for the attainment of the 
 equilibrium radioactive temperature of 400 would 
 be just about six million years, assuming the lower 
 estimates of the radioactivity of the Simplon rocks as 
 indicative of the radioactive state of the materials. 
 The conditions are not, indeed, adiabatic ; but, on 
 the other hand, the rocks upraised by lateral pressure 
 were by no means at 0C. to start with. They must 
 be assumed to have possessed such temperatures as 
 the prior radiothermal effects, and the conducted heat 
 from the earth's interior, may have established. 
 
 It would from this appear probable that if durations 
 of even five or six million years were involved, the 
 equilibrium radioactive temperatures must nearly have 
 been attained. The effects of heat conducted from the 
 underlying earth- crust have to be added (and here 
 again radiothermal effects must be taken into account, 
 according to the principles explained in Chapter V), 
 leading to a further rise in temperature of not less than 
 500 or 600. In such considerations the observed 
 indications of high temperatures in materials now laid 
 bare by denudation, probably find their explanation. 
 
 The first fact that we infer from the former existence 
 of such a temperature distribution is the improbability, 
 indeed the impossibility, that anything resembling a 
 rigid obstacle, or deep-seated horst, can have existed 
 
150 RADIOACTIVITY AND ftEOLOGY. 
 
 beneath the present surface-level, and opposed the 
 northerly movement of the deep-lying synclines. For 
 such a horst can only have been constituted of some 
 siliceous rock-material such as we find everywhere 
 rising through the worn- down sediments of the Alps ; 
 and the idea that this could retain rigidity under the 
 prevailing temperature conditions, must be dismissed. 
 There is no need to labour this question; the horst 
 cannot have existed. To what, then, is the retardation 
 of the lower parts of the folds, their overthrow, above, 
 to the north, and their deferlement, to be ascribed ? 
 
 A little consideration shows that the very conditions 
 of high temperature and viscosity, which render un- 
 tenable the hypothesis of a rigid obstacle, suffice to 
 afford a full explanation of the retardation of the roots 
 of the folds. For directed translatory movements 
 cannot be transmitted through a fluid, the pressure in 
 which is necessarily hydrostatic, and must be exerted 
 equally in every direction. And this applies, not only 
 to a fluid, but to a body which will yield viscously 
 to an impressed force. There will be a gradation, 
 according as viscosity gives place to rigidity, between 
 the states in which the applied force resolves itself 
 into a purely hydrostatic pressure, and in which it is 
 transmitted through the material as a directed thrust. 
 The nature of the force, in the most general case, of 
 course, has to be considered; whether it is suddenly 
 applied and of brief duration, or steady and long- 
 continued. The latter conditions alone apply to the 
 present case. 
 
 It follows from this that, although a tangential force 
 
RADIOACTIVITY AND MOUNTAIN STRUCTURE. 151 
 
 or pressure be engendered by a crustal movement occur- 
 ring to the south, and the resultant effects be transmitted 
 northwards, these stresses can only mechanically affect 
 the rigid parts of the crust into which they are carried. 
 That is to say, they may result in folding and crushing, 
 or horizontally transporting, the upper layers of the 
 earth's crust ; but in the deeper-lying viscous materials 
 they must be resolved into hydrostatic pressure which 
 may act to upheave the overlying covering, but must 
 refuse to transmit the horizontal translatory move- 
 ments affecting the rigid materials above. 
 
 Between the regions in which these two opposing 
 conditions prevail there will be no hard and fast line ; 
 but with the downward increase of viscosity there will 
 be a gradual failure of the mechanical conditions and 
 an increase of the hydrostatic. Thus while the upper- 
 most layers of the crust may be transported to the full 
 amount of the crustal displacement acting from the 
 south (speaking still of the Alps), deeper down there 
 will be a lesser horizontal movement, and still deeper 
 there is no influence to urge the viscous rock-materials 
 in a northerly direction. The consequences of these 
 conditions must be the recumbence of the folds formed 
 under the crust-stress, and their dtferlement towards 
 the north. To see this, we must follow the several 
 stages of development. 
 
 The earliest movements, we may suppose, result 
 in flexures of the Jura-Mountain type that is, in a 
 succession of undulations more or less symmetrical. 
 As the orogenic force continues and develops, these 
 undulations give place to folds, the limbs of which are 
 
152 RADIOACTIVITY AND GEOLOGY. 
 
 approximately vertical, and the synclinal parts of 
 which become ever more and more depressed into the 
 deeper, and necessarily hotter, underlying materials; 
 the anticlines being probably correspondingly elevated. 
 These events are slowly developed, and the temperature 
 beneath is steadily rising in consequence of the con- 
 ducted interior heat, and the steady accumulation of 
 radioactive energy in the sedimentary rocks and in the 
 buried radioactive layer of the earth. The work 
 expended on the crushed and sheared rock also con- 
 tributes to the developing temperature. Thus the 
 geotherms must move upwards, and the viscous con- 
 ditions extend from below ; continually diminishing 
 the downward range of the translatory movements 
 progressing in the higher parts. While above the 
 folded sediments are being carried northward, beneath 
 they are becoming anchored in the growing viscosity of 
 the medium. The anticlines will bend over, and the 
 most southerly of the folds will gradually become 
 pushed or bent over those lying to the north. Finally, 
 the whole upper part of the sheaf will become hori- 
 zontally recumbent ; and as the uppermost folds will be 
 those experiencing the greatest effects of the continued 
 displacement, the dtferlement or overlap must neces- 
 sarily arise. 
 
 We may follow these stages of mountain evolution 
 in a diagram (Plate VI) in which we eliminate inter- 
 mediate conditions, and regard the early and final 
 stages of development only. In the upper sketch we 
 suppose the lateral compression much developed and the 
 upward movement of the geotherms in progress. The 
 
RADIOACTIVITY AND MOUNTAIN STRUCTURE. 153 
 
 dotted line may be assumed to be a geotherm having a 
 temperature of viscosity. If the conditions here 
 shown persist indefinitely, there is no doubt that the 
 only further developments possible are the continued 
 crushing of the sediments and the bodily displacement 
 of the whole mass to the north. The second figure is 
 intended to show in what manner these results are 
 evaded. The geotherm of viscosity has risen. All 
 above it is affected mechanically by the continuing 
 stress, and borne northwards in varying degree 
 depending upon the rigidity. The folds have been 
 overthrown and drawn out ; those which lay originally 
 most to the south have become the uppermost ; and, 
 experiencing the maximum amount of displacement, 
 overlap those lying beneath. There has also been a 
 certain amount of upthrow owing to the hydrostatic 
 pressure. This last-mentioned element of the pheno- 
 mena is of highly indeterminate character, for we 
 know not the limits to which the hydrostatic pressure 
 may be transmitted, and where it may most readily 
 find relief. While, according to some of the published 
 sections, it would seem to have influenced the final 
 results of the orogenic movements, a discussion of its 
 consequences would not be profitable. 
 
CHAPTER VIII. 
 
 RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 
 
 DIFFICULTIES INVOLVED IN ASSUMING THE EARTH TO BE MERELY A 
 COOLING BODY KELVIN'S CALCULATION OF THE AGE OF THE EARTH 
 AT VARIANCE WITH THE INDICATIONS OF OTHER METHODS FROM 
 THIS AN ARGUMENT FOR THE RADIOACTIVE HEATING OF THE EARTH 
 THE FURTHER ARGUMENT FROM THE OBSERVED SUFFICIENCY OF THE 
 RADIOACTIVE SUBSTANCES IN THE CRUST AND FROM THE COMMON 
 DESTINY OF ALL RADIOACTIVE HEAVENLY BODIES DIFFICULTIES 
 ATTENDING THE INVESTIGATION OF THE INTERIOR URANIUM 
 
 KELVIN'S PROOF OF THE THERMAL ISOLATION OF THE INTERIOR 
 THIS REMOVES THE DIFFICULTY OF ASSUMING THE EXISTENCE OF 
 RADIOACTIVITY IN THE CENTRAL PARTS OF THE EARTH ARGUMENTS 
 
 IN FAVOUR OF SUPPOSING A WIDESPREAD DISTRIBUTION OF URANIUM 
 IN THE EARTH THE ULTIMATE CONSEQUENCES OF INTERIOR RADIO- 
 ACTIVITY POSSIBILITIES OF THE PAST AND OF THE FUTURE. 
 
 THE fact that heat is continually passing out at the 
 surface of the earth is conclusive evidence of some 
 source of heat within. Until the discovery of the 
 heat-producing properties of radium it was assumed 
 without controversy, it may be said, that the source of 
 heat was the original heat with which the world was 
 endowed at the time of its aggregation, and which was 
 probably ultimately of dynamical origin. There was, 
 indeed, strong evidence to support this view. That the 
 earth had once been at a higher temperature was to 
 be inferred directly, not only from an examination of 
 the crust, but by the mere logical process of reversing 
 the heat-losses of past time. Again, the facts of moun- 
 tain elevation to which we have referred required a 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 155 
 
 cooling and shrinking magma as the precursor of the 
 existing crust. It was but natural to draw the infer- 
 ence that the energy giving rise to such conditions, 
 still lingering in the materials of the earth, continued 
 to escape at the surface. 
 
 The theory of the former high temperature of the 
 earth appears well founded on observation ; but when 
 it is sought to enter more particularly into the thermal 
 history of the earth, difficulties present themselves. 
 These difficulties first became apparent in the work 
 which Lord Kelvin directed towards determining the 
 time which had elapsed since the earth's cooling from 
 a temperature of solidification. 1 
 
 The principles underlying this work are easily 
 grasped. Kelvin started with a world at 7000 F. 
 (3710 C.)> at which temperature he assumed the cessa- 
 tion of convective currents, with the freezing of the 
 materials of which it is composed. We have, then, a 
 sphere 8000 miles in diameter at a uniform temperature 
 of 3710, and cooling from the surface inwards due to 
 the continued loss by radiation. Assuming the actual 
 surface to stand always at a temperature approximating 
 to its present surface temperature, and so getting rid of 
 a radiation t coefficient, the problem is to calculate how 
 long a period must elapse before the temperature has 
 fallen from the surface inwards so as to afford the 
 average thermal gradient which to-day prevails in the 
 outer parts. The solution of this problem, of course, 
 involves the conductivity of the rocks or other materials 
 through which the thermal flux to the surface takes place. 
 
 1 Kelvin, Trans, Eoyal Soc. Edin., 1862, yol. xxiii. 
 
156 EADIO ACTIVITY AND GEOLOGY. 
 
 Lord Kelvin's earlier calculations led to periods of 
 from 20 to 400 millions of years; but later, as the 
 constants entering into his work became replaced by 
 more reliable data, his final conclusions pointed to 
 from 20 to 40 millions of years, and more nearly 
 20 millions than 40 millions. 1 
 
 But this conclusion is at variance with other methods 
 of determining the age of the earth : for other methods, 
 based upon the accumulated effects of certain processes 
 going on at the surface, exist. These geological methods 
 rely, in short, on surface changes which have taken 
 place since the waters of the ocean condensed and rain 
 and rivers began their work. The period of time 
 included in the "geological age" must therefore be 
 somewhat less not very much possibly than what is 
 included in the period estimated by Kelvin : that is, from 
 the consistentior status, or state of terrestrial interior 
 stability. Nevertheless, the geological age, no matter 
 how we approach its estimation, consistently yields a 
 greater interval than Kelvin's 20 to 40 millions of years. 
 
 The method of finding the geological age by the 
 accumulation of the sediments owes much to Sir 
 Archibald Geikie. In it the total column of the 
 sedimentary rocks, as derived from the integration of 
 all geological strata where measured at their maxima, 
 serves as numerator, and the denominator is sought in 
 the rate at which sediment is borne from the land and 
 collected on the areas of deposition. Geikie has deduced 
 an age of about 100 million years on this basis. 2 
 
 1 Kelvin, Phil. Mag. 47, p. 66, 1889. 
 
 2 Geikie, Presidential Address to Sect, C. y Brit. Assoc. Rep., 1899. 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 157 
 
 There is, indeed, great difficulty in arriving at the 
 required data; and it cannot be said that up to the 
 present the final verdict deducible from this method 
 has been heard. Sollas, assuming a high rate of depo- 
 sition, arrives by this method at a lesser age 26 millions 
 of years. 1 It will be seen in a later chapter that the 
 sources of error attending this method would most 
 probably lead to an underestimate of geological time. 
 
 A second method is to proceed on the facts, more 
 accessible, of solvent denudation, and to take the 
 accumulation of dissolved substances in the ocean as 
 numerator. Stated, however, in these terms the method 
 will not work ; for, with one exception, the substances 
 brought by the rivers to the ocean are again abstracted 
 from it. Eesults obtained by using the method with- 
 out attention to this point are valueless. However, 
 in the element sodium a highly soluble substance is 
 found, which is not abstracted unless by accidental 
 impounding and evaporation of ocean lagoons, &c. 
 but these losses are negligible. 2 As the ascertainment 
 of the total sodium in the sea is easily accomplished 
 without serious error, it only remains to find the 
 denominator in the annual river-supply of the element 
 sodium. Available data respecting the quantity of this 
 element in average river-water, and the amount of the 
 latter annually reaching the oceans, give a value of 
 about 90 to 100 millions of years as the result obtained 
 by this method. There is no doubt that before long more 
 reliable values of the denominator will be forthcoming. 
 
 1 Sollas, The Age of the Earth, p. 37 et seq. 
 
 2 Joly, Tratts. Royal Dublin Soc., vol. vii., ser. ii., p. 23 et sey. 
 
158 RADIOACTIVITY AND GEOLOGY. 
 
 Both these geological methods proceed on the assump- 
 tion that there has been sensible uniformity in geo- 
 logical processes throughout the period it is sought to 
 determine. The arguments in favour of this assump- 
 tion cannot be enlarged upon here. We cannot suppose 
 these methods to err threefold or fourfold on the side 
 of excess. Latterly the method of fixing the ages of 
 the geological strata by the helium accumulated in 
 radioactive minerals, as used by Rutherford, and 
 recently systematically applied by Strutt, point to 
 ages not less, but possibly even greater, than the 
 geological methods indicate. The subject is more 
 fully dealt with in Chapter XI. 
 
 We have then an evident contradiction between the 
 physical method founded on the cooling of the earth 
 and other methods. This suggests that it may be 
 erroneous to assume that the earth is merely a cooling 
 body, losing from a primitive store of heat, like an 
 ember dropped from a furnace. No explanation of the 
 discrepancy was forthcoming till the principles of 
 radioactive heating were made known, and the fact 
 was established that radium was a widely diffused, if 
 much-diluted, surface constituent of the crust. It 
 was then apparent that the difficulty could be bridged 
 by supposing that the heat-loss of the earth had 
 during a part of geological time been made good by 
 radioactive supplies. 1 The work of Strutt finally 
 showed that there was actually an embarras de richesse 
 of radium in the surface rocks. 
 
 We have then, so far, found two good reasons for 
 
 1 Joly, Nature, October 1, 1903. 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 159 
 
 accepting the view that the earth's escaping heat is 
 mainly radiothermal in origin : the fact that this 
 view reconciles the physical and geological methods 
 of determining the age of the earth ; and the fact that 
 the radium is there. 
 
 Let us amplify this last reason by a homely illustra- 
 tion. We find ourselves in a room in which we see a 
 hright and ample fire. Are we justified in ascribing 
 the temperature of the apartment to the visible 
 source of heat ? There may be other sources of heat ; 
 indeed we may argue, and argue rightly, that the fire 
 is not responsible for the more primitive heat of the 
 walls and the building ; but are we justified in assum- 
 ing that the heat which is continually escaping is 
 supplied by the fire ? The analogy is really exact, for 
 if, on the one hand, it be urged that we know not the 
 depth to which the surface radioactivity of the earth 
 extends, we may also say reverting to the illustration 
 we are without knowledge of the actual heat-flow 
 from the fire. Only in one particular is the analogy 
 unfaithful. We know the radiothermal source is all 
 but eternal in duration, and therefore we possess an 
 assurance of sufficiency which does not exist in the 
 case of the illustration. 
 
 But we find another reason for the belief that the 
 earth is now, and has for long been, losing radioactive 
 heat, and only cooling so fast as the radioactive 
 materials themselves decay, in the broad generalization 
 that in accepting this view we hardly do more than 
 anticipate an inevitable condition. 
 
 Every solar and every planetary body originating as 
 
160 RADIOACTIVITY AND GEOLOGY. 
 
 a dynamically heated aggregate, and containing above 
 certain small amount of uranium among its constituents, 
 must pass finally into the stage of the radiothermal 
 sustentation of its temperature, unless the original heat 
 of the body and its capacity of retaining it are so pre- 
 ponderating that they outlive the radioactive elements. 
 For in the beginning the heat-loss by radiation is great, 
 and so the primitive capital is run down, and with its 
 diminution and the cooling of the surface the rate of 
 loss declines. Finally, the secular production of heat 
 from the long-lived radioactive substances must come 
 to equilibrate the rate of loss. At this stage there can 
 be no further cooling, save in so far as the radioactive 
 atoms themselves become transformed and cease to 
 yield their energy. The surface richness of the earth 
 in uranium by no means suggests that the earth is now 
 in the process of living down the radioactive heat- 
 supplies by its own original stores of heat. Thus in 
 making the assumption that the earth is to-day radiat- 
 ing into space the heat derived from the atomic 
 systems being transformed in the rocks, we are but 
 claiming for the present a status which, if not now 
 prevailing, inevitably awaits the future. 
 
 The uranium which thus appears to make good the 
 heat-loss of the earth is very small relatively in 
 amount. The quantity is easily arrived at. 
 
 It has already been shown that we can calculate 
 from the gradient and conductivity the average amount 
 of heat escaping from the earth. The result arrived at 
 assigned to this quantity the value 125 x 10- 8 calories 
 per square centimetre per second. 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 161 
 
 Now, one gram of radium involves the liberation 
 of 5 '6 x 10~ 2 calories per second. Hence, if the whole 
 of the earth-heat came from radium and its radioactive 
 relatives, the maximum quantity of radium present is 
 the quotient of the heat-loss of the earth by the above 
 radio thermal constant. Dividing we get 2'2 x 10~ 8 
 grams. This is the amount of radium which, on the 
 assumptions, must be supposed to exist beneath each 
 square centimetre of the earth's surface inwards. This 
 calculated quantity of radium will, of course, vary with 
 our assumptions as to the thermal gradient. The very 
 steep gradients accepted by some geologists might make 
 the quantity as much as 3 x 10~ 5 grams per square 
 centimetre inwards. 
 
 The parent uranium bears to this quantity of radium 
 the ratio of 1 to 3*4 x 10~ 7 : the latter figure being the 
 amount of radium in equilibrium with one gram of 
 uranium. Hence, from 70 to 90 grams of uranium 
 beneath each square centimetre of the earth, inwards, 
 is the greatest quantity of uranium we could assume. 
 It will be found later that even this is an excessive 
 estimate, for the entire output of heat can in no case 
 be taken as a measure of the uranium and radium 
 within. 
 
 When, next, we seek to find the distribution of these 
 radioactive substances inwards, we are met by diffi- 
 culties. The existence of a specially rich surface 
 layer is easily demonstrated. Its probable down- 
 ward extension we will discuss later. We shall 
 also find from closer study of the questions involved 
 that there must be uranium and radium deep beneath 
 
 M 
 
162 RADIOACTIVITY AND GEOLOGY. 
 
 this layer, probably in very feeble concentration. 
 But this refers only to depths of a couple of hundred 
 miles or thereabouts, and tells us nothing about the 
 state of the great mass within. 
 
 The only method now available of approaching the 
 subject of the earth's interior thermal state appears 
 to be that of Lord Kelvin. Unfortunately the know- 
 ledge we derive from it is of a negative kind. We learn 
 that we are barred, and, perhaps, barred for ever, from 
 a certain knowledge of the state of the interior. 
 Strange to say, this fact in itself may furnish us with 
 an answer, not certain but probable, to our inquiry 
 concerning the existence of radioactive substances 
 within. 
 
 It is not difficult to imagine, even without mathe- 
 matics, that the movement inwards of the low tem- 
 peratures prevailing near the surface must be a process 
 of extreme slowness. The distances are enormous, 
 and the feeble conductivity offers a highly retardative 
 mode of thermal escape. We have to imagine columns 
 of material more than comparable with the width of the 
 Atlantic, cooled at one end and throughout charged 
 with great stores of heat. How long must be required 
 for the slowly advancing process of cooling to creep 
 along on its way, nearly four thousand miles extended, 
 the rate of escape diminishing year by year as the sur- 
 face gradient falls ! Immense periods must obviously 
 be involved. 
 
 Kelvin showed that if the interior materials of the 
 earth possess approximately the same average thermal 
 conductivity as the surface rocks, and the same average 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 163 
 
 specific heat, even after one thousand million years of 
 free surface-cooling, there would be no sensible loss of 
 heat from an interior core equal to about half the bulk 
 of the earth the calculation revealing that the entire 
 loss of heat during that long period would be made 
 good from an outer shell 568 miles in thickness; the 
 temperature below that depth not being appreciably 
 less than at the beginning. 1 
 
 It has been argued that the conductivity of the 
 interior materials of the earth may differ greatly from 
 those of which we have knowledge at the surface. 2 
 And, doubtless, so far as the materials are metallic,- 
 there would be greater conductivity within, but so far 
 as they are stony and it mast be remembered that 
 the decisive factors here are the conductivities and 
 specific heats prevailing in the outer 500 miles there 
 does not appear to be evidence forthcoming in favour 
 of increased conductivity under conditions of higher 
 temperature ; indeed, the evidence appears to be the 
 other way (p. 74). The experimental data are, it 
 must be admitted, hardly sufficient to permit any 
 final discussion of the question. When, however, the 
 fact that the probable geological age is but a fraction 
 of the thousand million years is taken into account, 
 it appears improbable that the essential teaching of 
 Kelvin's results will be set aside on the physical 
 constants involved. The extreme slowness, relatively 
 
 'See also Davison, Proc. R, S. 9 1887, xlii., p. 325, where the con- 
 clusion is reached that after 174,240,000 years the rate of cooling is 
 practically insensible at a depth of 400 miles. 
 
 2 Perry, Nature, li., 1895, p. 585. 
 
 M 2 
 
164 RADIOACTIVITY AND GEOLOGY. 
 
 to the duration of geological time, with which heat 
 would escape from the central parts, seems certainly 
 established. Not only has the mathematical correct- 
 ness of Kelvin's treatment of the subject never been 
 impugned, but the amendment of his data, whether as 
 regards thermal conductivity, specific heat, density, or 
 assumed initial temperature, seems unlikely to upset 
 his conclusions, stated generally, respecting the rela- 
 tions of the surface with the interior. This general 
 result, which in the original memoir was but a step 
 towards the investigation of the age of the earth, is of 
 much importance to our present views as to the 
 physical state of the earth's interior. 
 
 The physical conditions which thus limit our know- 
 ledge of the earth's interior may, as stated above, in 
 themselves involve a solution of the problem of the 
 inner thermal state. 
 
 Obviously if uranium exists in the thermally isolated 
 interior, these regions must now be rising in tempera- 
 ture. A little thought will show that Kelvin's result 
 really removes the only argument that might be urged 
 against such a condition of things. It may have been 
 so for many millions of years since in fact, convective 
 conditions came to an end and yet have produced no 
 exterior effect, nor interfered with the secular cooling of 
 the earth's surface which mountain-building phenomena 
 indicate as having progressed throughout geological 
 time. Let us assume for a moment that radium was 
 contained in the interior materials of the earth in 
 quantities comparable, say, with those prevailing in 
 ocean waters : say 0*02 x 10~ 12 grams per gram. Then 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 165 
 
 it is easy to show, by multiplying this value by the 
 heat out-put of radium per second and by the time in 
 seconds, that in a hundred million years only 3 J calories 
 are evolved per gram ; so that if the specific heat 
 be taken as one-fifth that of water, the rise of tem- 
 perature is only 17*9 degrees. Evidently a much more 
 considerable amount of radium might be present, and 
 the resulting temperature-rise during 10 8 years be such 
 as would probably produce no surface effects. 
 
 This of course leads us to regard the question of the 
 existence of interior uranium as an important one: 
 and we turn with some interest to the arguments for 
 or against the internal radioactivity of the earth. 
 There are many and weighty reasons (so far as argu- 
 ments on probability and analogy can be regarded as 
 weighty) for believing that there is interior uranium 
 in the earth. 
 
 (a) Its presence at the surface suggests that it is 
 an original terrestrial constituent. It is in the highest 
 degree improbable that an element can be entirely 
 restricted to one part of the earth, and not exist in 
 some degree in every part. Ultimately, indeed, it is 
 probably true that every element is everywhere, and 
 would be so perceived had we but the sufficiently 
 delicate means of appreciation. Apart from this con- 
 cession to the number and minuteness of atoms, 
 it is unlikely that so considerable a quantity of the 
 element uranium should occur in the surface rocks, 
 no matter how deep-seated their origin, and not exist 
 far into the interior, if not to the very centre. This 
 argument based on the principle of uniformity is only 
 
166 RADIOACTIVITY AND GEOLOGY. 
 
 met by assuming that uranium is not a terrestrial 
 constituent at all ; that it was not primevally upon the 
 earth, but was picked up from space during the long 
 ages of geological time. This view may involve no im- 
 possibility, but, in the first place, the internal materials, 
 if ever they circulated in meteoric form around the 
 parent star, should also have gathered the radioactive 
 element, whether cosmic in origin or emanating from 
 the sun. And in this way it should be incorporated 
 in the internal materials of the world as finally 
 evolved. Secondly, this theory is unsupported by 
 any exceptional radioactivity either in meteorites or 
 in cosmic particles gathered from the ocean floor. 
 
 (b) A physical explanation can be offered of the 
 surface concentration of uranium (to be referred to 
 later), but only on the assumption that it came from 
 the interior, or at least from deep beneath the crust. 
 Thus the presence of uranium at the surface if the 
 suggested explanation be correct is an indication of 
 the interior presence of the element, as the surface ore 
 tells the miner of the metal beneath. 
 
 (c) Meteorites are regarded with some probability 
 as representing terrestrial materials not accessible to 
 us at the surface. Such observations as exist on their 
 radioactivity have shown small quantities of radium 
 in the stony meteorites and highly nickeliferous 
 aeosiderites, but none in meteoric iron. However, it 
 is present in metallic iron derived from the earth's 
 interior. 1 
 
 (d) So long as we regard the earth as 'a derivative 
 
 1 Strutt, Proc. E. S. 77 A, p. 480. 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 167 
 
 of the solar mass, we cannot logically deny a similarity 
 of constituents. Now, there is good evidence for 
 the presence of uranium in the sun. There is evidence 
 from the spectroscope, and evidence from the abundance 
 of helium in the solar atmosphere : helium being 
 upon the earth unquestionably a product of radio- 
 active change, in part, if not entirely. There is 
 also the evidence based on the difficulty of accounting 
 for the long continuance of solar heat without the 
 help of radioactive contributions ; indeed, the evidence 
 from helium and from the necessities of solar theory 
 would lead us to suppose that uranium was a very 
 considerable constituent of the sun. True, we cannot 
 suppose such a quantity to be present as would account 
 for solar heat in its entirety, for it has been shown that 
 the amount of heat emitted by the sun every second 
 would require as much radium in each unit of solar 
 mass as exists in pitchblende. Now, the radium- 
 richness of pitchblende is dependent on the 60 per 
 cent, (or thereabouts) of uranium present. It seems 
 impossible to suppose that so great a proportion of 
 uranium can exist in the sun. Still it remains a fact 
 that the evidence is in favour of regarding uranium 
 as a solar constituent. And as such, consistently 
 with theories of the earth's genesis, we regard it as 
 an original terrestrial constituent. 
 
 This array of argument for the presence of uranium 
 within the earth might still be deemed inconclusive if 
 there was any strong case for the opposite view ; but, 
 in a word, there is no case for the other side. For we 
 really possess no criterion in our surface observations 
 
168 RADIOACTIVITY AND GEOLOGY. 
 
 as to whether or not the interior is rising in tempera- 
 ture. We know nothing of how the effects of even a 
 considerable rise might be kept under control. Every 
 event of geological history might have gone on as we 
 believe it to have occurred, and no sign be manifest 
 of accumulating energy within the central core. 
 
 Thus the conclusion that the earth's interior tem- 
 perature is rising seems the logical outcome of the 
 probable conditions taken along with Kelvin's proof 
 that the central parts of the earth are thermally 
 isolated for immeasurable periods of time from the 
 surface. Unless we take refuge in arbitrary denial of 
 the existence of the radioactive elements within, or 
 assume that in some manner the radioactive trans- 
 formation of uranium is annulled by the conditions of 
 pressure or temperature, we seem to have no option 
 but to accept this conclusion as the answer to our 
 inquiry into the state of the earth's interior. 
 
 The question as to what must ultimately result from 
 a radioactive production of heat and its storage in the 
 interior of the earth seems beyond our power to deter- 
 mine. All must, of course, depend upon the amount of 
 uranium and radioactive elements contributing to the 
 accumulating heat. If the supply is rapid, the effects 
 must ultimately be felt at the surface ; if slow enough, 
 the intervention of another factor has to be considered : 
 the decay and gradual disappearance of the radioactive 
 materials themselves. These, under certain conditions 
 of radiothermal supply, must entirely perish, and the 
 accumulation of heat come to an end, before any 
 surface disturbance is produced. The collected heat 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 169 
 
 would, in this case, slowly escape in remote future 
 times, unattended by any effects upon the crust. This 
 will be understood when the maximum amount of 
 accumulated heat is estimated for a particular case. 
 Thus, if we assume, as before, a quantity of radium 
 0*02 x 10~ 12 grams per gram for the central parts of the 
 earth, and the quantity of uranium present per gram 
 be taken as roughly a measure of the total potential 
 radium, we have : 
 
 0-02 x 10-" 
 3-4 x 1(T 
 
 grams of radium (uranium) per gram. (The denomi- 
 nator of the fraction will be recognized as the quantity 
 of radium in equilibrium with a gram of uranium.) 
 The total quantity of heat which is involved in the 
 transformation of one gram of uranium, reckoned on 
 the average life (2540 years) of the potential radium, 
 is 4' 5 x 10 9 calories. Hence the total heat during the 
 whole life of the uranium derived from materials of 
 the radium-richness assumed is 6 x 10~ 8 x 4*5 x 10 9 = 270 
 calories per gram ; and if the specific heat is one-fifth 
 that of water, the temperature attained is 1350. 
 
 This result assumes that during the whole time of 
 decay of the uranium there is no escape of heat, which 
 would hardly be the case : the greatest rise of tempera- 
 ture would probably be much less. In short, it is 
 possible that a uranium-richness of this order would 
 not result in a disturbance of the crust. 
 
 But if we assumed that there was 100 times as much 
 uranium that is, that the radioactivity was 2 then it 
 is easy to show that the stored heat accumulating in 100 
 
170 RADIOACTIVITY AND GEOLOGY. 
 
 million years would give rise to the temperature of 
 1800; that is, it would increase the initial temperature 
 by this amount. And on Kelvin's investigation we 
 see that this great quantity of energy might to a large 
 extent be retained. Where such temperatures are 
 involved it is probable that before the hundred million 
 years had run their course the world, with such a con- 
 stitution, must become once again a heated mass 
 throughout. 
 
 We possess really no assurance that such a con- 
 summation does not await the future, nor evidence 
 that such has not more than once been an event of 
 the past. 
 
 A celestial body possessing any considerable store 
 of long-lived radioactive elements will not cool as an 
 ordinary body, simply parting with its stored-up heat, 
 and falling in temperature gradually from its surface 
 inwards. Suppose it starts on its career at a tempera- 
 ture of liquefaction. The rate of cooling will then be 
 great, and will only sensibly be delayed by the dynami- 
 cal heat generated by its shrinkage upon itself. But 
 this supply, too, will gradually become exhausted, a 
 crust will form, and if Kelvin's ideas are right, a consis- 
 tentior status will inaugurate a geological age. But the 
 rate of cooling has now become relatively insignificant ; 
 probably not the ten- thousandth part of what prevailed 
 before the consistentior status. The quiet accumulation 
 of radioactive energy proceeding throughout the mass 
 will, near the surface, make good the radiation loss, 
 but in the interior, where no means of escape exists, 
 must collect during the passing geological periods. 
 
RADIOACTIVITY AND THE INTERIOR OF THE EARTH. 1*71 
 
 There can only be one result: general surface vul- 
 canicity and reversion to temperature conditions which 
 may involve the repetition of the entire sequence of 
 events. 
 
 If such has been the past history of our globe and 
 such the origin of our geological age, we find the exist- 
 ing earth-heat to be more or less completely radio- 
 active in origin. But this invalidates nothing of 
 what we have previously stated as to radium-heating 
 reconciling estimates of the earth's geological age. 
 The earth-heat might be entirely radioactive in origin, 
 and yet the quantities of radioactive materials involved 
 might be no more than would preserve the present 
 surface conditions for many millions of years. 
 
 The beforementioned possibilities must be clearly 
 distinguished from the view that the existing state 
 of equilibrium between radioactive supply and loss by 
 radiation also obtained at a period when the earth was 
 radiating many thousands of times its present amount 
 of heat. For in this latter case we necessarily assume 
 the former existence of thousands of times the present 
 amount of radioactive materials ; and, if we mean 
 uranium, we have to put back the consistentior status 
 such a time-interval as will suffice for the decay of this 
 uranium to its present small quantity. To multiply 
 the existing uranium 4000 times requires us to revert 
 to a past 60,000 million years ago. Such a lapse of 
 time since the consolidation of the earth or of its 
 surface brings us into hopeless difficulties not only 
 with reference to geological time, but with regard to 
 solar theory. 
 
172 RADIOACTIVITY AND GEOLOGY. 
 
 We have no reason to find in the advent of radio- 
 activity into cosmic and geological sciences any new 
 difficulties, but rather the clearing away of old ones. 
 The duration of solar heat finds a plausible explanation, 
 and the prolonged duration of geological time without 
 the corresponding cooling of the earth's surface becomes 
 accounted for. But it cannot be said that it enables- 
 us to foretell the destiny of our globe. Unless other 
 lines of evidence exist, we are left to the imagination 
 as to what has been the history of the past, and what 
 may result in the future. But wonderful possibilities 
 are brought before us. Peaceful cooling may await the 
 earth or catastrophic heating may lead in a new era 
 of life. Our geological age may have been preceded 
 by other ages, every trace of which has perished in 
 the regeneration which has heralded our own. 
 
 Whatever be the future or the past of our world, we 
 have the untrammelled regions of space in which such 
 varied destinies must surely find their accomplishment. 
 The planets may now be in varying phases of such 
 great events. And when a star appears in the heavens 
 where before we knew of none, may not this be a 
 manifestation of the power of the infinitely little over 
 the infinitely great the unending flow of energy from 
 the unstable atoms wrecking the stability of a world ? 
 
CHAPTEK IX. 
 
 KADIOACTIVITY IN THE EXTERIOR PARTS OP THE EARTH. 
 
 INDICATIONS OP HIGH 8UB-CRU8TAL TEMPERATURES THICKNESS OP 
 THE CRUST LIMITING DEPTH OP THE RADIOACTIVE LAYER ALLOW- 
 ANCE FOR COOLING OF THE EARTH BY DECAY OF THE URANIUM 
 NECESSITY OF A DEEPER AND LESS CONCENTRATED DISTRIBUTION OF 
 THE URANIUM APPROXIMATION TO THE THICKNESS OF THE RADIO- 
 ACTIVE LAYER SUGGESTBD ORIGIN OF THE SURFACE CONCENTRATION 
 OP THE URANIUM. 
 
 AMONG the surface materials of the earth we everywhere 
 find manifestations of deep-seated high temperature. 
 It may, indeed, be said that almost every part of the 
 earth has been at some time visited by volcanic out- 
 breaks. And where evidences of such are no longer 
 obtainable, we are sure that it only requires the dis- 
 closing effects of denudation to show the buried igneous 
 masses some solidified deep beneath the outer rocks, 
 others brought higher up on their way to the surface. 
 Again, outflows, not strictly volcanic, have flooded great 
 areas to the extent of thousands of square miles. No 
 more convincing evidence of internal heat could be 
 required, and indeed it is hard to imagine any addi- 
 tional evidence of the existence of fusion temperatures 
 beneath. 
 
 The reasons for supposing that radioactivity has 
 been and is instrumental in sustaining the tem- 
 perature of the earth have been given in the last 
 chapter. Following on this, we see that, whatever 
 
174 RADIOACTIVITY AND GEOLOGY. 
 
 conclusions we now adopt respecting the distribution 
 of uranium near the surface of the earth, we necessarily 
 conclude it must be such as will have sustained the 
 intense temperatures beneath the crust during a large 
 part of geological time. 
 
 Differences of opinion, comparatively unimportant, 
 exist as to what precise temperatures may prevail 
 beneath the crust. The only guide we appear to have 
 in the matter is the temperature of melted lava 
 flowing out at the surface. This has been estimated 
 at from 1000 to 1300, and even to 1400. There 
 are reasons for believing that 1300 is the least tem- 
 perature we can ascribe to basaltic magmas deep 
 within, or just below, the crust. 1 
 
 The thickness of the crust is an indeterminate and, 
 in any case, a somewhat indefinite quantity. If we 
 mean of the rigid crust, we first require to know the 
 temperature at which rigidity fails under the pre- 
 vailing conditions of stress. There appears to be no 
 abrupt beginning of plasticity in a siliceous material 
 when its temperature is gradually raised. The range 
 of plasticity is considerable (especially in the more 
 acid rocks), and long-continued and intense stress will 
 affect siliceous materials at temperatures considerably 
 below the melting-point. We are, however, not con- 
 cerned here with the precise distribution of temperature 
 and rigidity. It is sufficient to state that a full fusion 
 temperature is, by some writers, believed to prevail at 
 about 38 kilometres from the surface ; others would 
 refer such temperatures to greater depths. What has 
 
 1 Daly, Am. Joiirn. of Science, xxvi., 1908, p. 35. 
 
RADIOACTIVITY IN EXTERIOR PARTS OF THE EARTH. 175 
 
 already been assumed, in relation to the subject of 
 the stability of the crust, would place the true fusion- 
 point some few kilometres lower down. It is probable 
 that the thickness of the crust is variable, within 
 limits, over the earth, and, as we have seen, may 
 be considerably reduced beneath the areas of deep 
 sedimentation. 
 
 These views define, to some extent, the limitations 
 which must guide us in dealing with the question 
 of surface radioactivity. 
 
 The most immediately important conclusion arising 
 from the investigation of the radioactivity of the 
 surface-rocks is, that a layer of richly radioactive 
 materials must cover the earth, which can only extend 
 but a few miles downwards. This is easily demon- 
 strated. The proof is based on the known amount 
 of heat escaping from the crust and being continually 
 radiated into space. We have seen in the last chapter 
 that if we refer, without deductions of any kind, the 
 whole of the escaping heat to a radiothermal origin, 
 we can have no more than from 2 '2 x 10~ 5 to 3 x 10~ 5 
 grams of radium beneath each square centimetre of 
 the earth's surface. Now, if we take it that 3 x 10~ 12 
 grams per gram, or say, 8 x 10~ 12 grams per cubic 
 centimetre, represent the average radioactivity of the 
 igneous rocks, it is evident we can estimate the depth 
 to which rocks of this richness can extend by a simple 
 division of the maximum quantity of radium by the 
 amount in each cubic centimetre. This gives us a 
 limiting depth of from 27*5 to 37*5 kilometres (17'2 
 to 23*5 miles) according to the gradient we prefer. 
 
176 RADIOACTIVITY AND GEOLOGY. 
 
 Hence if we take the surf ace- richness as indicative 
 of what is beneath, the radium must entirely cease 
 at these depths. 
 
 This simple calculation, however, fails to represent 
 the actual facts. We find on examination that neither 
 can we suppose the thickness of the radium-rich layer 
 to be so great as that which we have arrived at, nor can 
 we even suppose that all the radium is collected in the 
 assumed manner at the surface. Hence we have to 
 modify the estimate of this layer as arrived at above. 
 The necessity of doing this must now be shown. 
 
 In the first place, we based our calculation on the 
 quantity of heat escaping at the surface, assuming 
 that this was a measure of the radium. But is it ? 
 The parent substance, uranium, is continually waning in 
 amount. True, the process is very slow, but still it is 
 ever diminishing according as it becomes transformed : 
 at a rate of about the ten thousand-millionth of its, 
 amount in a year. But by assumption this substance is 
 responsible for the maintenance of the temperature of 
 the earth. It may even, as we have seen, lead to a rising 
 temperature within. In the exterior parts it acts to 
 make good the surface loss. Hence, as it diminishes in 
 quantity, the temperature must fall. Thus the fall in 
 temperature of those parts of the earth which send heat 
 to the surface, and are kept hot by the uranium, 
 will just be the ten thousand-millionth part of their 
 temperature each year. This is admitting, of course, 
 that the earth is not, at least at the surface, preserved 
 at a strictly constant temperature, but is cooling. A fall 
 in temperature, however, involves the escape of some 
 
RADIOACTIVITY IN EXTERIOR PARTS OF THE EARTH. 177 
 
 of the existing store of heat. That is, if the materials 
 fall t degrees, there must pass out a quantity of heat 
 which is found by multiplying t by the specific heat, 
 and by the mass which is cooled. Plainly the im- 
 portance of the heat-loss will, in this case, depend 
 mainly on the mass which is cooling. Suppose the 
 whole earth was cooling, the conductivity being 
 sufficiently good to permit an almost uniform drain 
 of heat from every part of the interior; then the 
 number of calories annually escaping at the surface 
 becomes very great, in spite of the minuteness of the 
 drop in temperature. Thus, if we take the mean 
 temperature of the earth as, say, 1500, the annual 
 fall will be 10~ 10 of this, using here a rather low value 
 for the transformation constant of uranium. We have 
 also the mass as 6 x 10 27 grams. The average specific 
 heat may approximate to one- fifth that of water. The 
 quantity of escaping heat due to cooling is then 
 
 1500 x 10- 10 x 6 x 10 27 x 0-2 = 1-8 x 10 20 calories. 
 
 But we cannot assume the total heat escaping from 
 the earth, according to the gradients, as more than 
 2'6 x 10 20 calories per annum. So that we are left 
 with but 0'8 x 10 2 calories per annum as a measure of 
 the radium actually contributing heat to the surface : 
 i.e., less than a third part of the quantity we assume 
 if we neglect the fact of the decay of the uranium. 
 We can (partially) escape this conclusion in only one 
 way ; by recalling the fact that the whole mass of the 
 earth does not send heat to the surface, but a large 
 part of it must be accumulating what radioactive 
 
 N 
 
178 RADIOACTIVITY AND GEOLOGY. 
 
 heat is generated therein. For this interior part the 
 decay of the uranium only involves a lessened rate 
 of accumulation. From this we see that 6 x 10 27 is too 
 large a value to allow as representing the cooling 
 mass sending heat to the surface ; and hence the 
 deduction for earth-cooling must be less. 
 
 The principles assumed in the foregoing argument 
 are perhaps not easily followed by those unaccustomed 
 to calculations involving thermal measurements. An 
 analogous case may help. Let us take the familiar 
 instance of a boiler heated by a furnace within. The 
 heat of combustion we may suppose diminishes a little 
 owing to gradual consumption of the fuel, and the 
 temperature of the whole mass, boiler and its contents 
 falls by a corresponding amount. Now, it cannot do this 
 without parting with some of its stored-up heat. This 
 must escape by radiation at the surface ; it can go 
 nowhere else. The amount of this heat in calories is 
 calculated on the average specific heat (the average 
 quantity of heat which is liberated when a gram of 
 the materials falls one degree in temperature), multi- 
 plied by the fall in temperature. The quantity of heat 
 thus arrived at as passing out at the surface might, 
 by an observer without, be confounded with the con- 
 tinued heat-production of the furnace. In fact, the 
 observer of the escaping heat is liable to overestimate 
 the activity of the combustion unless he can also 
 observe the fall in temperature. But this might be 
 too small to be detected, and yet if the mass of the 
 boiler and its contents is great enough, the error 
 might be a very large one. So it is in the case of 
 
RADIOACTIVITY IN EXTERIOR PARTS OF THE EARTH. 179 
 
 the earth ; the temperature-fall is quite beyond detec- 
 tion, but, owing to the enormous mass, the heat of 
 cooling as distinct from the radioactive heating may 
 be very large. 
 
 The difficulty in determining the true correction for 
 cooling in the case of the earth is similar in nature 
 to that which we met before when discussing Kelvin's 
 results as to the interior temperature, and obviously 
 arises from the same source our ignorance of the 
 physical properties of the materials far down. If at a 
 venture we assume that one quarter of the earth's mass 
 is falling in temperature with the decay of the uranium, 
 then we have the following figures. On the 32-metre 
 gradient the total annual loss of heat from the whole 
 earth is 2 x 10 20 calories, and the heat referable to 
 falling temperature is 0*45 x 10 20 calories : hence there 
 remains T55 x 10 20 calories as the heat derived from 
 continued radioactivity. 
 
 This necessarily entails a lesser thickness for the 
 radioactive layer than that with which we started. 
 We shall next see that even the quantity of radium 
 to which we would be limited by this calculation is 
 in excess of what we can assign to the surface 
 layer. This arises from the requirements as regards 
 temperature of the deeper parts of the crust. 
 
 On the evidence there must be temperatures of 
 1200 to 1400 prevailing beneath the crust. Now, 
 if the sub-crustal temperature is indeed maintained by 
 the uranium, then the radiothermal actions must be 
 adequate to give rise to these temperatures, or we must 
 look to some other source of heat say, the original 
 
 N 2 
 
180 RADIOACTIVITY AND GEOLOGY. 
 
 heat of the earth to explain the high temperatures. 
 The arguments against having recourse to the original 
 heat of the earth have already been given. We there- 
 fore agree to the necessity of so distributing the 
 radioactive materials in the exterior parts of the 
 earth that the said temperatures may be attained. 
 As a preliminary consideration it is plain that if 
 the uranium be all placed near the surface, lesser 
 temperatures will be attained by radiothermal actions 
 than if it, or a part of it, be more deeply buried; 
 for in the first case the easy escape of the heat 
 would forbid the attainment of a high temperature. 
 And this is just what would happen if all the radium 
 in the outer layer were at a mean concentration of 
 8 x 10~ 12 grams per cubic centimetre of rock. Such 
 a layer would extend downwards a distance defined by 
 the assumption already arrived at, that I 1 55 x 10 20 
 calories per annum may be assigned to the radio- 
 thermal actions. This, however, involves the uncertain 
 correction for the heat of cooling of a portion of the 
 earth. To more effectually establish the present point, 
 let us neglect this correction entirely, and assume 
 the radioactive materials to extend to 27'5 or 37'5 
 kilometres, as first calculated. 
 
 "We are, it must be remembered, going on the hypo- 
 thesis that the earth derives its escaping heat and its 
 temperature from this radium jacket. The loss of heat 
 has, in fact, guided us in calculating the thickness of 
 the jacket, and now we seek to find if this layer can 
 give rise to a sufficient sub-crustal temperature. The 
 calculation of the maximum temperature to which it 
 
RADIO ACTIVITY IN EXTERIOR PARTS OF THE EARTH. 181 
 
 can give rise is not difficult. No heat enters it from 
 below, for the temperature beneath is supposed to 
 depend on the basal temperature of the layer, and 
 to be in equilibrium with it. Heat, therefore, only 
 flows out above as fast as it is generated in the 27'5 
 or 37'5 kilometres of radioactive materials. 
 
 The equation given in a previous chapter (p. 77) 
 enables the basal temperature to be found 1 : 
 
 qhD 2 
 
 9 = 
 
 2K' 
 
 where q is the radium content of a cubic centimetre of 
 the layer, D its thickness in centimetres, h the heat 
 generated per second by one gram of radium, and K the 
 thermal conductivity throughout the layer. We have 
 q and D as above. The value of h already given more 
 than once is 5 - 6 x 10~ 2 , and K we take as 4 x 10~ 3 . 
 Putting in these figures, we find that the basal tem- 
 perature is but 420 if D is 27'5 x 10 5 , and it is 790 if 
 D is 37-5 x 10 5 . 
 
 The reason for this failure of the radium to give 
 the necessary high temperature is simply that we 
 have placed it too near the surface, so that its heat 
 too readily escapes. By supposing a sufficiently sparse 
 distribution of it in the upper layer we may give the 
 layer a greater depth, and so raise the basal tempera- 
 ture ; but then we appear to abandon the indications 
 of the surface rocks, and admit that they do not indi- 
 cate the radioactivity throughout the jacket. 
 
 And this is what we really are driven to do. The 
 
 1 See Appendix A. 
 
182 RADIOACTIVITY AND GEOLOGY. 
 
 calculation shows that the immediate surface distri- 
 bution cannot indicate a condition continued down- 
 wards till all the allowable radium is exhausted. There 
 must be radium of very great dilution extending 
 downwards beneath the immediate surface materials to 
 such depths as will maintain the high temperatures 
 beneath the crust. Only by assigning to the lower 
 parts of the crust and the sub-crustal magmas a feeble 
 radium-content, considerably less than we perceive at 
 the surface, can we bring our views into harmony with 
 the temperature conditions which we know to prevail 
 beneath the crust. 
 
 Stated in terms so general this conclusion is probably 
 a safe one ; but when we try to make it more definite, 
 uncertainties thicken around us. The requirements of 
 our surface observations may, however, guide us to 
 some idea of the depth to which the radium-richness 
 observed in the igneous rocks probably extends. 
 
 The intruded and extruded igneous rocks appear to 
 show no diminution in radioactivity. They must 
 probably represent general conditions prevailing far 
 down. Not necessarily so deep as the zones of general 
 fusion, however, for, as we have seen (Chapter V), 
 where the phenomenon of igneous intrusion is most 
 evident, the sub-crustal geo therms have been raised 
 from their normal positions, so that re- fusion and 
 outpouring of materials, which elsewhere might be 
 located above the fusion zone, are brought about. 
 Still we here find difficulties if we assume the higher 
 radioactivity to prevail but a little way down ; and, 
 on the other hand, we must abandon the view 
 
RADIOACTIVITY IN EXTERIOR PARTS OF THE EARTH. 183 
 
 that the earth's temperature is maintained by radium 
 if we concentrate radioactive substances entirely in 
 the upper parts of the crust. 
 
 Now, taking the gradient of 32 metres as the best 
 approximation we can make to a mean gradient, we 
 have 27*5 kilometres of crust having the radioactivity 
 of 3 as a manifestly excessive estimate of the thickness. 
 We have to reduce it in consequence of some of the 
 earth's escaping heat being derived from cooling by 
 decay of the uranium, and again we have to reduce it 
 because a considerable amount of uranium must be 
 located further from the surface. We do not know 
 what to allow for either of these retrenchments. If we 
 said that the richer part of the crust must be between 
 9 and 15 kilometres deep, we cannot be far from the 
 truth. This appears to be the best we can do on our 
 present knowledge. Twelve kilometres have been 
 taken for purposes of calculation in Chapter V. 
 
 The surface concentration of a part of the uranium is 
 a fact beyond question. At first sight it appears as if 
 this was just what we would not have expected. 
 Uranium is the heaviest element known, and a central 
 concentration would rather have met our expectations. 
 However, a physical cause can be assigned which does 
 not appear to conflict with those views of earth- 
 genesis which involve the formation of the globe by 
 meteoric accretion. 
 
 The suggested explanation follows easily if we admit 
 the possibility that the uranium contained in the outer 
 
184 RADIOACTIVITY AND GEOLOGY. 
 
 shell of the earth say, to a depth of a few hundred 
 miles was not originally uniformly distributed, but 
 richer here and there in a manner that might naturally 
 arise on any meteoric or " planetesimal " theory of the 
 earth's origin. 1 Thus the constituents might have been 
 derived from different depths of the solar mass, so that 
 when finally aggregated to form our earth, they would 
 give rise to local variations of material. We might 
 say simply, without theorising, that they possessed the 
 chemical differences observed in meteoric materials. 
 The consequences arising from this state of things are 
 readily seen. 
 
 There would be a certain temperature ensuing from 
 dynamical effects which would originally prevail 
 throughout these aggregates, and which would be 
 independent of their uranium content. But, as time 
 went on, the materials richest in uranium would acquire 
 the highest temperatures; and when slow convective 
 motions were possible, such hotter masses would tend 
 to move away from the centre of the earth under the 
 force of displacement arising out of their lessened 
 density just as a region of heated water beneath the 
 ocean must slowly rise to the surface. Such convec- 
 tive actions may have taken place principally in the 
 early stages of terrestrial heating, and possibly may 
 have been restricted to limited radial displacements ; 
 but, evidently, if they involved the transport of 
 materials even for a very few hundred miles, and 
 took place near the surface, the result must be the 
 
 1 Chamberlin and Salisbury, Geology, vol. ii, 
 
RADIOACTIVITY IN EXTERIOR PARTS OF THE EARTH. 185 
 
 accumulation at the surface of the more uranium- 
 rich materials contained in the outer parts of the 
 heated earth. And once these attain the surface, 
 there are afterwards no forces to cause them to retrace 
 their path. 
 
 Again, there is no improbability that such move- 
 ments, acting with extreme slowness over geological 
 time, and requiring long periods for the accumulation 
 of radioactive heat, might affect the already formed 
 crust of later geological times. Some of the great lava 
 flows of comparatively recent date may be associated 
 with such actions. 
 
 It considerably strengthens this view of the cause of 
 the radioactive surface layer if we consider as possible 
 cases some of the great lava flows which have broken 
 through the crust in comparatively recent times. The 
 most striking instance is that of the Deccan basalts and 
 traps, which, towards the close of Cretaceous times, 
 were poured out, covering an area of 200,000 square 
 miles to an average depth of about one mile. This 
 volume of basalt would fill a sphere 115 kilometres 
 (72 miles) in diameter. According to the observations 
 quoted in Chapter III, the average radioactivity 
 appears to be about 14 x 10~ 12 per cubic centimetre. 
 Suppose such a mass, situated 100 or more miles 
 beneath the surface, originally at the temperature and 
 density of its surroundings, and now left to heat up 
 under the radioactive energy. If we assume that over 
 the surface approximately spherical heat freely 
 escapes, so that the surface does not rise in tempera- 
 ture above the surrounding magma, then the equation 
 
186 RADIOACTIVITY AND GEOLOGY. 
 
 giving the final excess of temperature at any distance r 
 from the centre is 1 : 
 
 where q is the radioactivity, h the heat constant for 
 radium, and K the conductivity. These constants have 
 previously been given. 
 
 This gives at the centre an excess temperature above 
 the surroundings of 1070. The mean temperature 
 would, of course, be less. At a distance inwards from 
 the surface of the sphere of 28 kilometres that is, 
 30 kilometres from the centre the excess temperature 
 is 784. These temperatures would be lower if the 
 shape was not spherical, but would certainly be higher 
 under the actual surface conditions ; for the escape of 
 heat would not be perfect, but the surrounding magma 
 would tend to shut in the heat, being itself a bad 
 conductor. Indeed, the viscous state of this surround- 
 ing magma might be conditioned by the heat generated 
 in the radioactive materials. If we suppose that some 
 mean excess of temperature of about 500 U can be 
 ascribed to the whole mass (and convective effects 
 would interfere with the attainment of a very high 
 temperature at the centre), then we can calculate 
 approximately what the force of buoyancy, which acts 
 to urge the whole to the surface, would be. 
 
 Barus gives the coefficient of expansion of liquid 
 diabase from 1100 to 1500 as 0*0000470 ; so that 
 heated through 500, one cubic centimetre becomes 
 
 1 See Appendix A. 
 
KADIO ACTIVITY IN EXTERIOR PARTS OF THE EARTH. 187 
 
 1*0235 cubic centimetres ; or the gain in volume per 
 cubic centimetre is - 0235 cubic centimetres. But the 
 density of diabase at its melting-point (1094) is 
 from the data given by Barus deducible as about 2 '6 2. 
 If this be taken as the density of the surrounding 
 magma, then the increase in volume of the super- 
 heated mass involves a displacement force of 0*061 
 gram, or about 60 dynes, per cubic centimetre. We 
 may put this more briefly by saying that the volume 
 increases about 2*3 per cent., due to the rise of tem- 
 perature, and that, therefore, an upward-directed force, 
 amounting to 2-3 per cent, of the weight of the 
 displaced magma, acts to bring the radioactive mass to 
 the surface. It is probable that effects of this kind 
 would be restricted to conditions of high viscosity, and 
 that convective movements would be extremely slow. 
 The calculation may, however, give some idea of the 
 magnitude of the forces involved. The general out- 
 come of such displacements of the more radioactive 
 nuclei to the surface must be the impoverishment of 
 the depths and the corresponding enrichment of the 
 upper crust. 
 
CHAPTER X. 
 
 THE EADIOACTIVITY OF THE ATMOSPHERE. 
 
 EFFECTS OF EMANATION IN THE ATMOSPHERE IONIZATION OF GASES 
 NOT SPONTANEOUS NUMBER OF IONS IN THE ATMOSPHERE RATE 
 
 OF PRODUCTION AND RECOMBINATION GEOLOGICAL EFFECTS OF THE 
 
 IONS ACTIVE DEPOSIT OBTAINED FROM THE AIR GENERAL 
 PREVALENCE OF THE ACTIVE DEPOSIT AND DEPENDENCE ON 
 METEOROLOGICAL CONDITIONS ACTIVE DEPOSIT FROM RADIUM 
 EMANATION AMOUNT OF RADIUM EMANATION IN ONE CUBIC METRE 
 OF AIR EXPERIMENTAL RESULTS ON EFFECTS OF EMANATION IN 
 THE ATMOSPHERE CALCULATED EFFECTS OF THE EMANATION 
 PERCENTAGE LEAK OF EMANATION FROM THE SOILS IONIZATION 
 OVER THE OCEAN EFFECTS OF PENETRATING RADIATIONS FROM THE 
 EARTH ESTIMATE OF THE RADIOACTIVITY OF THE SURFACE 
 ROCKS PRESENCE OF THORIUM EMANATION IN THE ATMOSPHERE 
 EFFECTS OF RADIOACTIVITY ON THE CHEMICAL COMPOSITION OF 
 THE AIR. 
 
 THE facts recorded in Chapter III show that radio- 
 active substances are to be found in small quantities 
 not only in the rocks but in the ocean and rivers. We 
 have also seen that the transformations of parent 
 radioactive elements give rise in every case to gaseous 
 elements the emanations of radium, thorium, and 
 actinium which, after a short period of existence, 
 lose a part of their mass and re -transform to solid 
 substances. The solids which result from the trans- 
 formation of the emanations are among the most 
 unstable of the radioactive elements; they possess 
 periods of existence measured by minutes or hours, 
 and are then succeeded by generations as unstable as 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 189 
 
 themselves; so that a considerable part of the total 
 atomic evolution is comprised within some minutes 
 or hours succeeding the break-up of the emanation. 
 The fact that a gaseous element arises in the radio- 
 active sequence suggests the probability of radioactive 
 energy not being confined to the solid and liquid 
 substances of the earth, but extending to the gases 
 collected around the globe. If the gaseous radioactive 
 elements escaped into the atmosphere, it is to be 
 expected that the succession of changes which succeed 
 the gaseous state must exert effects similar to those 
 arising in the electroscope when emanation is admitted : 
 that is, they must ionize the air, rendering it thereby 
 a conductor of electricity. 
 
 We have already referred to the theory by which it 
 is sought to explain the electric conductivity of gases ; 
 and, incidentally, to the fact that the air in the electro- 
 scope possesses a certain amount of conductivity even 
 when no radioactive gas has been intentionally intro- 
 duced. This persistent conductivity of air and indeed 
 of all gases was by the earlier observers referred to 
 causes which are now known to be inadequate. Within 
 recent years, and mainly in consequence of the work 
 of Sir J. J. Thomson and his pupils, the conductivity 
 of gases has found a more consistent explanation in 
 the carriage of elemental electric charges of both signs, 
 by certain ionized atoms or particles in the gas; the- 
 gas itself being, when free from these carriers, a non- 
 conductor. 
 
 This view must not be understood as in all respects 
 similar to the much earlier theory of the electrolytic 
 
190 RADIOACTIVITY AND GEOLOGY. 
 
 conductivity of liquids containing salts in solution. 
 We might refer to important points of distinction 
 between the two hypotheses : one of these claims 
 our special attention. The ionization within the salt 
 solution arises from internal conditions and the inter- 
 action of internal forces. It is an inherent property 
 of the solution, and the number of ions present 
 in unit volume is controlled mainly by the concen- 
 tration of the dissolved substance. Even when no salt 
 is present, and we have to do with the solvent only, a 
 certain amount of ionization exists arising from internal 
 conditions which we do not seem able to control. It 
 appears to be otherwise with gases. There seems to be 
 no reason to believe that any inherent specific ionization 
 exists, although internal factors influence the electric 
 state. Certain it is that we can largely control the 
 electric conductivity within a closed vessel by taking 
 precautions, as to (a) the freedom of the enclosed air 
 from any trace of radioactive emanation ; (&) the use 
 in the construction of the vessel of materials which 
 are free from radioactive elements ; and (c) the pro- 
 tection of the enclosed air from penetrating radiation 
 from without. Now, it may be said that not one of 
 these precautions can be applied with any assurance 
 of completeness. The retention in every case of a 
 certain amount of conductivity need not, therefore, 
 cause us to modify the view that the ionization of a gas 
 is something impressed upon it from without. 
 
 In addition to a host of experiments showing how 
 largely the applied external conditions control the 
 .onization of a gas in a closed vessel, the striking fact 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 191 
 
 has been observed that the ultimate residual ionization 
 is proportional to the density of the particular gas. 
 This observation has been found to apply even to gases 
 of complex molecular structure and much higher in 
 density than air. 1 This seems to show that the ioniza- 
 tion observed must be due to a radiation either from 
 the walls of the vessel or from external sources. For 
 it would be expected that effects arising from radiated 
 energy would be more manifest in gases wherein a high 
 density favours the more complete absorption of the 
 energy. 
 
 The atmosphere possesses normally a variable number 
 of ions which may be from 1000 to 3000 of each electric 
 sign per cubic centimetre, and may even vary con- 
 siderably outside those limits. Thus Eve obtained at 
 Montreal, in hazy, hot weather, so few as 370 positive 
 and 367 negative ions per cubic centimetre. The 
 numbers seem to be affected by atmospheric conditions 
 to a certain extent. Dust or haze in the air appears 
 to facilitate the combination of the ions, and so to 
 reduce their numbers. There are generally found to 
 be a larger number of positive than of negative ions, 
 the ratio being sometimes so great as 1*5 to 1. 
 
 As the recombination of the ions is continually 
 going on, it is evident that there must be a perpetual 
 supply to make good the losses. This at once follows 
 from the principles of the ionic theory. Now, it can be 
 shown that the relation between the number q, of ions 
 of one sign, brought into existence in unit volume 
 and in unit time, with the number, n, present when 
 
 1 Itutherford, liadioactivity, p. 535. 
 
192 BADIOACTIVITY AND GEOLOGY. 
 
 there is equilibrium between production and recombi- 
 nation, may be represented by the equation q = ari 2 , 
 where a is the coefficient of recombination ; a relation 
 expressing the fact that the number recombining is 
 proportional to the square of the number present. 
 
 The determination of the number present at any 
 instant may be effected by observing the rate of loss of 
 charge of an insulated electrified body immersed in a 
 current of the gas ; the conditions being those of 
 saturation : that is to say, the potential being such 
 that all the ions present are utilized to discharge the 
 electric system before appreciable recombination can 
 occur. These conditions do not seem to be, in practice, 
 perfectly accomplished. Certain slow-moving ions have 
 been shown to be present in the atmosphere which 
 will not be detected by this method. In these deter- 
 minations the charge upon the ion necessarily enters 
 into the calculation; and as Kutherford and Geiger 1 
 have recently shown that the value previously 
 ascribed to the charge 34 x 10~ 10 E.S. unit should 
 be raised to 4*65 x 10~ 10 , the estimates generally given 
 of the number of ions in the atmosphere must be 
 reduced in the ratio 4'65 : 3*4. 
 
 The value of a for dust-free air has been determined 
 by Townsend 2 as equal to 3420 x e, where e is the ionic 
 charge. Taking Kutherford and Geiger's value of e (as 
 above), we have a = 1*6 x 10~ 6 . If now we assume 2000 
 ions present per cubic centimetre, and apply the value 
 of a, we get for <? 6*4 ions per cubic centimetre per 
 
 1 Proc. R. S., 81 A, p. 170. 
 
 2 Townsend, Phil. Trans. R. 8., 81 A, p. 170. 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 193 
 
 second. However, under ordinary conditions it would 
 appear that q may have a considerably greater value : 
 the rate of recombination being higher in dusty or 
 hazy air. Thus at Manchester, Schuster found that a 
 was two or three times as great as for dust-free air, and 
 that q varied from 12 to SS'5. 1 On the other hand, in 
 the pure air over the ocean, with about 1000 ions per 
 cubic centimetre, the rate of creation of ions would 
 appear to be no more than one or two per cubic centi- 
 metre per second. In hazy weather n may fall to very 
 low values, owing to an increased rate of recombination 
 and the absence of any exceptional compensatory 
 ionizing actions. The effects of dust and haze have 
 been explained by Sir J. J. Thomson as arising from 
 the greater area offered by suspended particles when 
 they become electrified by union with an ion. 
 
 From these facts it will be seen that before we can 
 confidently assert the adequacy of any source of ioniz- 
 ation which we may discover as accounting for the rate 
 of production of ions in the atmosphere, the local, 
 simultaneous conditions must be completely deter- 
 mined. For a may vary not only from place to place, 
 but from day to day in the same locality. And, as will 
 presently be seen, the probable causes which create the 
 ions are themselves variable in character. 
 
 The continued creation of large numbers of ions in 
 the atmosphere is the fundamental fact with which we 
 are concerned, and to which we must here confine 
 our attention. Its bearing on geological processes is 
 latent in many observations made within recent years. 
 
 1 Schuster, Pros. Manchester Phil. Soe., p. 488, 1904. 
 
 
194 RADIOACTIVITY AND GEOLOGY. 
 
 Thus it is found that the ions possess the remarkable 
 property of precipitating water from the vaporous into 
 the liquid state. At the time when Aitken and others 
 first observed the fact that water- vapour mingled in 
 dust-free air refused to be precipitated under conditions 
 of supersaturation, the explanation was thought to reside 
 simply in the absence of dust-particles which might 
 act as nuclei for the initiation of condensation. It is 
 now known that something else is involved. The ions 
 present are also effective in causing condensation. When 
 all dust has been removed, the presence of any source of 
 ionization, such as X rays, or the rays from a radioactive 
 substance, is effective in bringing about precipitation 
 when a state of supersaturation exists. Again, carefully 
 made experiments reveal the fact, that in the absence 
 of any such applied sources of ionization, precipitation 
 will always ensue in dust-free air if the degree of super- 
 saturation exceeds a certain limit : a result pointing to 
 some persistent source of ionization. 1 There can hardly 
 be a doubt that we are justified in applying these 
 facts to the explanation of certain meteorological 
 phenomena. 
 
 Throughout a moisture-laden atmosphere these re- 
 markable precipitating actions must progress, in greater 
 or less degree ; and where the water- vapour is sufficiently 
 abundant, probably determine its descent in the form 
 of rain, hail, or snow. The meteorological importance 
 of the large numbers of ions normally present in the 
 atmosphere must be great. The physical principles 
 underlying these actions cannot here be enlarged upon. 
 
 a C. T. R. "Wilson, Phil. Trans., A, 189, p. 265. 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 195 
 
 It seems certain, however, that the electric charge is 
 directly involved in establishing the nuclei required for 
 the initiation of aqueous precipitation. 
 
 The last remark leads us to consider another meteo- 
 rological effect seemingly connected with the ionic 
 precipitation of water from the atmosphere. From 
 experiments made by C. T. E. Wilson 1 it has been 
 established that the precipitation of water- vapour on the 
 negative ions occurs much more readily than upon the 
 positive ions. Under conditions of increasing quantities 
 of aqueous vapour in the atmosphere, precipitation will, 
 accordingly, be first induced by the negative ions, and 
 the rain-drops so initiated will hurry the negative 
 charge to the earth, leaving the positive ionic charge 
 of the atmosphere comparatively little altered in 
 amount. Thus, consider the case of a quantity of 
 air it might be many cubic miles in volume which 
 has become charged with water- vapour near the ground, 
 and then is borne to the upper regions of the atmo- 
 sphere, by its own buoyancy, by the action of air- 
 currents, or by displacement upwards when carried 
 across a mountain range. There will ensue a rapid 
 expansion and cooling ; and the conditions obtaining in 
 Wilson's condensation vessels will be reproduced. The 
 first ionic precipitation will be brought about by the 
 ions of negative sign, and their abstraction from the 
 higher atmosphere may ensue. These effects might 
 have been anticipated on the grounds of Wilson's 
 results ; that they occur is supported by the fact 
 that the atmosphere is found to possess a prevailing 
 
 1 Phil. Trans., A, 193, p. 289. 
 02 
 
196 KADIO ACTIVITY AND GEOLOGY. 
 
 positive electrification respecting the surface of the 
 earth. It is possible that the intense electrical pheno- 
 mena which attend, and follow upon, rapid condensation 
 of water in the upper atmosphere, arise from these 
 subtle causes. 
 
 The progress of radioactive science early suggested 
 the inquiry as to whether the ionization in the atmo- 
 sphere might not be traceable to radioactive energy. 
 The pioneer observations in this direction were made 
 by Elster and Geitel. These observers made the re- 
 markable discovery that definite radioactive substances 
 could be gathered everywhere from the atmosphere, by 
 freely exposing a negatively charged surface. They 
 used a wire raised to a high potential. Upon this the 
 active substances collected, and from this they could 
 again be removed by friction, or by solution in an acid. 
 These substances, when brought into an electroscope, 
 much increased the ionization. The idea was a natural 
 one that the normal ionization of the atmosphere must 
 be, in part at least, ascribable to the presence of these 
 radioactive bodies. 
 
 It appears certain that these active substances 
 originate in radioactive emanations in the atmosphere. 
 The fact that they carry a positive charge is in keeping 
 with this view, although the origin of this charge is 
 not readily explained. The most conspicuous feature 
 attending the transformations of the products of rapid 
 change, arising from the break-up of a radioactive 
 emanation, is the loss of the a particle. This is 
 positively electrified, and we should expect that the 
 residual atom must carry a negative charge, and 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 197 
 
 would, therefore, be repelled rather than attracted by 
 a negatively charged body. The electropositive sign 
 of the residual atom is, however, abundantly proved. 
 There is some evidence that the expulsion of the a 
 particle from the transforming atoms may be attended, 
 or followed, by the escape of slowly moving electrons. 
 The intimate nature of the changes succeeding the 
 expulsion of the helium atom is as yet unknown to us. 
 There is reason to believe that even in the electric 
 field the atom remains suspended for some moments 
 after its gaseous properties have ceased to exist, during 
 which its internal changes are progressing. 1 
 
 The active deposit detected by Elster and Geitel 
 has been obtained in many parts of the world, and at 
 high altitudes on mountain tops. It has been found in 
 rain-water and in snow. Considerable variations in 
 its amount have, however, been observed. Elster and 
 Geitel found that low barometric pressures were at- 
 tended by greater quantities of the radioactive sub- 
 stances. Eutherf ord and Allen, 2 in Canada, perceived a 
 variation according as the weather was clear and bright 
 or otherwise, the clear weather being accompanied 
 by larger amounts. Dull warm weather in summer 
 was unfavourable to their presence. Eve 3 carried on 
 observations for a year in Montreal, and found that 
 temperature conditions appeared to be without effect ; 
 as much radioativity existed in the atmosphere when, 
 in March, two or three feet of snow were on the ground, 
 
 1 Rutherford, Radioactivity, p. 318. 
 
 2 Rutherford and Allen, Phil. Mag., Dec., 1902. 
 8 Eve, Phil. Mag., Oct., 1908. 
 
198 RADIOACTIVITY AND GEOLOGY. 
 
 as, in April, when the snow was gone. The approach 
 of a deep cyclone caused increase; while anticyclonic 
 conditions caused a diminution of the radioactivity. 
 
 Whence comes the active deposit ? Several observers 
 have found that the rate of decay of the activity of the 
 deposit obtained on a negatively charged wire is about 
 the same as that of the excited activity or active 
 deposit of radium emanation. This evidence is 
 strengthened by the fact that the air drawn from 
 soils or found in caves also yields an active substance 
 having the properties of this emanation; and in such 
 localities we reasonably expect the presence of the 
 emanation of radium. Again, it has been observed, 
 in many cases, that well-waters bring to the surface 
 dissolved radium emanation. At an early stage in 
 these investigations, Sir J. J. Thomson showed that 
 this was true of the Cambridge well-water. There is 
 thus a considerable d priori probability that we should 
 find this radioactive gas also in the atmosphere. 
 
 Eve 1 estimated the actual amount of radium ema- 
 nation in the atmosphere by abstracting the active 
 deposit contained in air enclosed in a large tank. 
 Comparing the ionization produced by the abstracted 
 deposit with the ionization due to a known amount of 
 the active deposit of radium emanation, he concluded 
 that in a cubic metre of air the amount of emanation 
 which would be in equilibrium with 80 x 10~ 12 grams 
 of radium must be present. 
 
 The most immediately important question respect- 
 ing the subject of the ionization of the atmosphere 
 
 1 Eve, Phil. May., July, 1906. 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 199 
 
 is as to how far the radium detected everywhere in 
 the earth's surface-materials is adequate to occasion 
 the whole ionization, or whether some other source 
 must he looked for. The evidence is conflicting. Eve 1 
 cites an experiment which would appear to show that 
 the emanation of radium is the principal source of 
 ionization. Using a very large iron tank, in which 
 the normal rate of production of ions was so small as 
 9'6 per second per cubic centimetre, he withdrew the 
 active deposit in the manner before referred to; that 
 is by exposing a negatively electrified wire in the tank. 
 The excited activity withdrawn upon the wire was 
 estimated by its ionization. This experiment was then 
 repeated in a smaller tank, using the emanation of 
 radium. The ratio of the excited activity on the wire 
 to the total ionization in the large tank was found to 
 be 7/10ths of the same ratio found for the emanation 
 of radium. From this it would appear that the larger 
 part of the ionization in the air within the tank was 
 due to the active deposit withdrawn from it. But it- 
 would seem that this experiment is not supported by 
 actual estimates of the ionization to be expected from 
 the observed amount of emanation in the air. We can 
 easily calculate from known data the number of ions 
 which would be produced per second from the quantity 
 of emanation supplying the active deposit in a cubic 
 metre of air (p. 19). Satterly, 2 who has made careful 
 determinations of the amount of active deposit in the 
 air at Cambridge, finds that the emanation is present 
 to an amount representing on an average 100 x 10" 12 
 
 1 Eve, lee. cit. 2 Satterly, Phil. Mag., Oct., 1908. 
 
200 RADIOACTIVITY AND GEOLOGY. 
 
 grams of radium per cubic metre. Kutherford and 
 Geiger have shown that the data respecting the ion- 
 ization produced by an alpha particle of radium, and 
 the numbers of such particles produced per second by 
 a known amount of radium, require revision. Thus 
 the number of ions created by a single a ray of radium 
 is 100,000 (p. 15), and 34 x 10 10 helium atoms are 
 radiated per second from one gram of radium. Boltwood 1 
 has determined the relative activities, when in mutual 
 equilibrium, of radium, emanation, and the substances 
 entering into the active deposit. Taking the ionizing 
 activity of uranium as unity, the values are : 
 
 Eadium, .... 045 
 
 Emanation, . . . 0*62 
 
 Eadium A, ... 0'54 
 C, . . . 0-91 
 
 When dealing with the effects of the emanation on 
 the atmosphere, we are concerned with the last three 
 elements only. We must conclude from the relative 
 activities (as the same number of rays is emitted per 
 second by each when they are in equilibrium) that 
 if the a ray of radium creates 100,000 ions, the last 
 three elements, for each a ray which they emit, create 
 respectively 140,000, 120,000, and 200,000 ions. Thus 
 in one second the emanation of one gram of radium in 
 equilibrium with its rapidly changing transformation 
 products, will give rise to 460,000 x 34 x 10 10 ions. 
 Taking now the emanation in the atmosphere as repre- 
 senting as much as 100 x 10" 12 grams of radium per 
 cubic metre, and multiplying by this value, we conclude 
 
 1 Boltwood, Am. Journ. Sc., Ap., 1908, p. 297. 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 201 
 
 that the creation of 1-56 ion per cubic centimetre per 
 second can be ascribed to the amount of radium emana- 
 tion present. As we have already seen, this is in general 
 insufficient to account for the whole ionization of the 
 atmosphere. 
 
 It may here be mentioned that many recent measure- 
 ments of the radioactive substances in the atmosphere 
 have been effected by the direct abstraction of the 
 emanation. Thus, Eve, 1 Ashman, 8 and Satterly 3 have 
 used either the absorptive properties of charcoal, 
 discovered by Kutherford, or the low temperatures 
 obtained by the use of liquid air, to condense it directly 
 from the air ; in this way bringing into the electroscope 
 the emanation from one hundred or two hundred litres 
 of air. The emanation so extracted exhibits the same 
 properties as the emanation of radium; and the amounts 
 obtained confirm Eve's original estimate of its average 
 quantity in the atmosphere. The variations from day 
 to day are considerable. Thus Ashman finds the 
 equivalent radium to vary from 45 x 10" 1 * to 200 x 10- 12 
 grams per cubic metre. Eve 4 finds the variation as 
 much as 1 to 7. 
 
 If it be assumed that the radium emanation found to 
 exist in the atmosphere is derived from the rocks and 
 soils, we are enabled to make some estimate of the per- 
 centage of the emanation evolved which escapes into 
 the atmosphere. We must assume a limiting depth 
 below which the emanation is completely conserved, or 
 
 ^ve, Phil Mag., Oct., 1908. 
 
 2 Ashman, Am. Journ. Sc. xxvi., 119, 1908. 
 
 3 Satterly, loc. cit. * Eve, loc. cit. 
 
202 RADIOACTIVITY AND GEOLOGY. 
 
 from which it cannot reach the surface in a sufficiently 
 short time to be effective. This limiting depth must, 
 in general, be close to the surface. Most soils extend 
 downward but a few feet before they give place to the 
 fine, compacted particles constituting the subsoil. 
 Diffusion through this close-grained and 'of ten water- 
 logged medium must be slow. Again, other soils are 
 limited by rock, from which little or no emanation can 
 escape. It would seem that an average depth of two 
 metres would be a liberal estimate. Near the surface 
 of this, the diffusion of emanation into the atmo- 
 sphere will be rapid ; but, as we go downward, the time 
 taken to reach the surface must result in the decay 
 of much of the emanation. We will assume, however, 
 that the whole of the emanation escaping from the 
 mineral particles succeeds in entering the atmosphere 
 undhninished in quantity. In calculating the amount 
 of emanation in the atmosphere, we may take following 
 Eve a height of ten kilometres, within which it is 
 permissible to apply the estimates of emanation made 
 on plains and mountains. 
 
 If now we take the radioactivity of the soil as 
 8 x 10~ 12 grams of radium per cubic centimetre or 
 8 x 10 6 per cubic metre, and that of the atmosphere 
 as 80 x 10~ 12 per cubic metre, we have 
 
 a x 2 x 8 x 10- 6 = 80 x 10- 12 x 10 4 , 
 
 from which x, the radium in the soil supplying emana- 
 tion to the atmosphere, is found to be 5 per cent. Expe- 
 riments directed to the quantitative measurement of the 
 output of emanation of various soils might enable us 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 203 
 
 to test the validity of the theory which ascribes the 
 emanation in the atmosphere to the radium in the soils. 
 That some emanation originates in the soils is certain ; 
 for, in fact, it has been observed in situ. 
 
 The ionization over the ocean has been made the 
 subject of investigation by A. Boltzmann and by Eve. 1 
 The observations in both cases were made upon the 
 North Atlantic; and the results were concordant in 
 showing that there is not much difference in the 
 number of ions in air over the ocean and over the land. 
 Boltzmann's average is 1150 + ions and 800 - ions; 
 numbers which do not differ notably from results 
 obtained in Lapland or Germany. Eve quotes from 
 G. C. Simpson, an average for 1903-1904, obtained in 
 Lapland, of 1120 + and 972 - ions. 
 
 Eve made observations from Montreal to the Isle of 
 Man. The readings at Montreal were unusually low ; 
 and in the Gulf of St. Lawrence the rise in the number 
 of ions was rapid and continuous. Some of the highest 
 readings were obtained in mid-ocean ; but they did not 
 exceed the ordinary readings observed upon the land. 
 
 Now under the conditions of Eve's observations, 
 there would be difficulty in explaining the presence of 
 emanation over the sea, if the sea was not itself a 
 source of radioactive supply. It must be remembered 
 that the perishable nature of the emanation differ- 
 entiates it from an ordinary gas. The general 
 uniformity in composition of the atmosphere is easily 
 explained in its mobility and circulation. But the 
 emanation of radium decays to half its amount in ninety 
 
 1 Eve, Phil. Mag., February, 1907. 
 
204 RADIOACTIVITY AND GEOLOGY. 
 
 hours. It cannot take long journeys unless borne by 
 strong winds. In anticyclonic conditions, its presence 
 a thousand miles from land, to the same amount as over 
 the surface from which it is derived, would be inexpli- 
 cable. "We must therefore consider the ocean a source 
 of radium emanation. It is worthy of notice that 
 the transit from the land to the central seas of the 
 emanations of thorium or actinium is an impossibility ; 
 the gaseous state in these cases practically disappearing 
 in a few minutes. 
 
 It is true that the average radioactivity of sea- 
 water is only about the -j-J-g- part of the radioactivity 
 of the rocks (p. 46). But, on the other hand, the radium 
 is in solution, and may be shown by experiment to yield 
 its emanation freely to the air. And again, there is 
 perpetual renewal of the surface supply; so that 
 if by diffusion into the atmosphere the emanation 
 falls below the equilibrium amount, water rising from 
 beneath must continually maintain the maximal con- 
 ditions at the surface. It is also well known that 
 bubbling air through radioactive liquids is a highly 
 effective although not perfect means of extracting 
 the emanation. Every bubble rising to the surface there- 
 fore brings emanation into the atmosphere. In rough 
 and stormy weather we should expect that the supply 
 of radioactive gas to the air must be very considerable. 
 Although the specific radioactivity of sea- water is so 
 inferior to that of the rocks, the dissolved state of the 
 radium, as well as the mobility and limitless amount of 
 the medium in which it is contained, may render the 
 radium content of the ocean a sufficient explanation of 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 205 
 
 the ionization observed. Estimates of the emanation 
 over the central parts of the ocean, and contained in 
 the surface waters, would be of much interest. 
 
 We shall now consider another source of ionization 
 arising from the presence of radioactive elements at the 
 surface of the earth penetrating y and )3 rays. So 
 far as these rays are referable to the known amount of 
 radium emanation contained in the air, their effects must 
 be insignificant a small fraction only of the ionization 
 due to the a rays. It is otherwise, however, when we 
 come to consider these radiations as emitted from the 
 radioactive elements contained in the rocks and soils. 
 
 That a very penetrating radiation affects gases near 
 the surface of the earth seems proved by the experi- 
 ments of M'Lennan 1 and Rutherford and Cooke 2 on 
 the screening action of thick metal walls placed around 
 closed vessels in which the rate of production of ions 
 is under observation. Thus Cooke found that in a 
 brass electroscope the rate of production of ions was 
 13*6 per cubic centimetre per second. But on encir- 
 cling the vessel with walls of lead 5 centimetres thick, 
 this number was reduced to 9 f l. It appears from 
 this, clearly, that not less than 4'5 ions per second 
 were being produced by some penetrating radiation 
 before the lead plates were in place. 
 
 An interesting calculation of Eve's appears to show 
 that this penetrating radiation may be ascribed, mainly, 
 to the radium in the rocks and soils. As the results of 
 McClelland' s investigations on the penetration of y 
 
 1 M'Lennan, Phys. Rev., 4, 1903. 
 
 2 Rutherford and Cooke, Amer. Phijs. Soc. t Dec. 1902. 
 
206 RADIOACTIVITY AND GEOLOGY. 
 
 rays, 1 Eve finds that these rays will be cut down to 
 half their amount after traversing 7'5 centimetres of 
 average rock, and to about 1 per cent, after passing 
 through 50 centimetres. There is then a strictly limited 
 depth from which the penetrating radiation must be 
 supposed to proceed. The experiment of Cooke indicates 
 that at the surface of the earth 4'5 ions per cubic centi- 
 metre per second arise from these rays in air contained 
 in a brass vessel. This number would be increased if 
 the electroscope was entirely under ground. The 
 number of ions generated should be doubled : in other 
 words, there should be 9 generated per second per cubic 
 centimetre. As it has been found that the ionization 
 produced within a vessel, by penetrating radiation, 
 varies with the nature of the material of which the 
 vessel is made, a part of the effects in Cooke's experi- 
 ment must be ascribed to a secondary radiation pro- 
 ceeding from the brass walls of the vessel. Eve, how- 
 ever, connects the ionization observed, with the amount 
 of radium giving rise to the penetrating rays, by an 
 independent experiment on the ionization produced in 
 such a vessel by a known quantity of radium. This 
 gives us the means of estimating the amount of radium 
 in the rocks which would be required to give rise to 
 9 ions per cubic centimetre per second in the brass 
 vessel when buried in the ground ; or, conversely, 
 knowing the radioactivity of the rocks, we might cal- 
 culate the ionization in the electroscope. Eve estimates 
 the radium, and finds that the radioactivity of the 
 rocks required to produce the ionization observed by 
 1 McClelland, Phil. Mag., July, 1904. 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 20*7 
 
 Cooke, must be 10 x 10' 12 grams of radium per cubic 
 centimetre of rock. This result is in satisfactory agree- 
 ment with those recorded in Chapter III for many 
 varieties of rock, but is rather high for the leached-out 
 materials of the soil ; and points, perhaps, to the pre- 
 sence of other radioactive substances in the surface- 
 materials. 
 
 Eve concludes that the ionization in the atmosphere 
 can, in fact, be accounted for by the joint effects of 
 the radium emanation actually present in it and the 
 penetrating rays coming from the rocks and soils. The 
 penetrating radiation from radioactive materials at the 
 immediate surface would be able to traverse the atmo- 
 sphere to a distance of 1000 metres before 99 per cent, 
 of it was absorbed. At 157 metres it would be absorbed 
 to the extent of one-half. 
 
 More recently C. S. Wright, 1 experimenting on the 
 ionization of air within a lead electroscope, as observed 
 when placed over ordinary soil, and again when over 
 the radium-free water of Lake Ontario, obtained results 
 which would seem to show that from 2*6 to 4*6 ions are 
 produced per second per cubic centimetre in the electro- 
 scope by penetrating radiation ; and concludes, from 
 M'Lennan's experiments on the secondary radiation in 
 lead vessels, that only one third the number of ions 
 produced in the electroscope would actually be created 
 in free air. 
 
 It would appear from the experimental work which 
 we have now reviewed that, on the whole, the balance 
 of evidence seems opposed to the view that the ioniz- 
 
 1 Wright, Phil. Mag., Feb., 1909, p. 295. 
 
208 RADIOACTIVITY AND GEOLOGY. 
 
 ation of the lower atmosphere is everywhere entirely 
 due to the effects of radium emanation in the air and of 
 radium in the rocks. This conclusion is strengthened 
 by the positive evidence, which lately has been growing 
 in force, for the existence in the atmosphere of radio- 
 active substances not of the uranium series, but referable 
 to the parent radioactive element thorium. 
 
 It has been found by several observers that the 
 active deposit condensed on a negatively charged wire 
 exposed for a sufficiently prolonged time to the atmo- 
 sphere is, for a considerable part, that arising from the 
 break-up of the emanation of thorium. Thus Bumstead 
 referred to thorium from 3 to 15 per cent, of the 
 whole amount of deposit obtained from air at New 
 Haven, Conn. 1 Underground air from the same locality 
 was found by Dadourian 2 to contain quantities of 
 thorium emanation. Similar results have been obtained 
 by G. A. Blanc at Eome, 3 who estimated that from 
 50 to 70 per cent, of the whole activity of the deposit 
 condensed from the atmosphere was due to thorium. 
 The escape of an emanation, which exists but for a few 
 minutes before its amount becomes negligibly small, 
 from any depth in the soil, seems impossible ; so that 
 the recorded observations appear to indicate a very 
 considerable content of thorium in the rocks. Blanc 4 
 believes that its active deposit moves about attached 
 to dust particles. 
 
 1 Bumstead, Am. Journ. Sc., July, 1904. 
 
 2 Dadourian, Am. Journ. Sc., Jan., 1905. 
 
 3 Blanc, Phil. Mag., March, 1907. 
 
 4 Blanc, Phys. Zeitsch., ix., 255, 1908 
 
THE RADIOACTIVITY OF THE ATMOSPHERE. 209 
 
 W. Wilson 1 examining the deposit condensed on a 
 negatively charged wire exposed at Manchester, infers 
 from the amount of the deposit of thorium collected, 
 that about seven times as much thorium as uranium 
 must exist in the soils. As the rate of transformation 
 of thorium is, according to Bragg, 2 one-fifth that of 
 uranium, and as there are six helium-producing pro- 
 ducts to eight in the case of the uranium series, the 
 energy would be approximately alike in each case. If 
 this be generally true, the geological importance of 
 thorium would be equal to that of uranium. 
 
 The problem of the continued sustenance of the 
 ionization of the atmosphere cannot be said to be 
 solved. But there can be no doubt of the important 
 part played by radioactivity ; and herein we find 
 another manifestation of the influence of the radio- 
 active elements on geological processes. 
 
 We might discuss other effects upon the atmosphere 
 not improbably connected with radioactivity. The 
 time for doing so has, however, hardly come. Some of 
 its rarer constituents may be ascribable to radioactive 
 sources. Helium is very probably so derived. We 
 have seen how its energetic flight from the atom is 
 one source of atmospheric ionization. The trace of 
 hydrogen present may in part arise from the radio- 
 active decomposition of water, which, in a minute 
 degree, must be progressing not only where moisture 
 is present in the soils, but throughout the ocean. In 
 the case of these two gases there is the additional 
 
 1 Wilson, Phil. Mag., Feb., 1909, p. 321. 
 3 Bragg, Phil. Mag., June, 1906, p. 751. 
 
210 RADIOACTIVITY AND GEOLOGY. 
 
 ground for assuming their radioactive origin that, on 
 the theory put forward by Stoney, there must be con- 
 tinual loss of these lighter gaseous molecules from the 
 earth's atmosphere ; and hence we infer that some steady 
 source of supply must exist. It might be expected that 
 traces of chlorine should reach the atmosphere through 
 the agency of the radio -chemistry of the ocean and its 
 sediments ; Cameron and Kamsay 1 having shown that 
 hydrochloric acid is decomposed by radium emanation. 
 A computation of the amount which might be produced 
 in this way seems, however, to indicate that it must be 
 very minute, notwithstanding the enormous reservoir 
 within which the processes are at work. 
 
 Sir William Eamsay's recent correction 2 on his esti- 
 mate of the helium in the atmosphere (p. 63), increasing 
 it to 4 parts in 10,000, involves the existence of 
 16 x 10 U cubic metres of atmospheric helium, taking 
 the height of the homogeneous atmosphere as 8 kilo- 
 metres. So great a quantity, if of radioactive origin, 
 cannot have been generated during geological time. 
 For it is easy to show that if all the heat leaving the 
 earth be referable to radioactive energy, the equivalent 
 evolution of helium is 36 x 10 6 cubic metres per annum ; 
 and of this only a small fraction can enter the atmosphere. 
 If the revised figures are applicable to the atmosphere 
 generally, we may, perhaps, have here an indication of 
 pre-geological time, and evidence sustaining our highest 
 estimates of radioactive matter near the surface of the 
 earth ; in other words, of the radioactive origin of 
 terrestrial heat. 
 
 1 Cameron and Ramsay, Chcm. Soe. Journ., June, 1908, p. 984. 
 
 2 Ramsay, Proc. R. S., 80 A, 599 
 
CHAPTEK XI. 
 
 URANIUM AND THE AGE OF THE EARTH. 
 
 PRINCIPLES OF THE RADIOACTIVE METHOD OF ESTIMATING THE 
 
 GEOLOGICAL AGE HELIUM AND THE ALPHA KAY RATE OF 
 
 COLLECTION OF HELIUM RUTHERFORD'S ESTIMATE OF AGE OF 
 MINERALS STRUTT'S EXPLORATIONS FOR HELIUM HIS WOKK ON 
 
 GEOLOGICAL TIME CRITICISM OF THE RADIOACTIVE METHOD 
 MIGRATIONS OF RADIOACTIVE SUBSTANCES CHANGES IN PH08PHATIC 
 SUBSTANCES HELIUM AND THORIUM ACCUMULATION OF LEAD AND 
 AGE OF MINERALS BOLTWOOD'S "WORK CONTRADICTION BETWEEN 
 RADIOACTIVE METHOD AND METHODS BASED ON DENUDATION THE 
 
 METHOD BY SEDIMENTATION SOURCF.S OF ERROR TH E METHOD BY 
 
 SOLVENT DENUDATION CONSIDERATION OF THE CORRECTIONS 
 LEGITIMACY OF UNIFORMITARIAN ASSUMPTIONS. 
 
 SUNK in depths of time which, mentally, are unfathom- 
 able, the earth's beginning has yet ever been the subject 
 of philosophic questioning or scientific exploration. 
 The deciphering by geologists of a great sequential 
 history in the piled-up debris of the mountains, while 
 impressively suggesting the vastness of the duration 
 involved, has left unsatisfied our craving for a know- 
 ledge of the interval since the records began. The 
 same overwhelming impression, the same fascination of 
 the unknown, are experienced, whether we endeavour 
 to review those activities which slowly laid the 
 foundations of mountains, uplifted and again destroyed 
 them ; or whether we contemplate the biological past 
 which stands to our brief length of days as the integral 
 
 P 2 
 
212 RADIOACTIVITY AND GEOLOGY. 
 
 life to the life of the individual. The primary question 
 of historical geology is the age which has encompassed 
 the collective events since the beginning. 
 
 The age of the earth may indeed claim an importance 
 outside of geological science. For is it not the clue to 
 the chronological scale of our solar system and for 
 want of a better even of our universe ? Distinct from 
 the age of the earth, the geological age, or period since 
 denudation began and life became possible, if of lesser 
 cosmic importance, is still a minor limit of priceless 
 value. If geological time had been as once was 
 believed almost eternal in duration, then must our 
 conception of solar chronology be enlarged accordingly. 
 If, on the other hand, Kelvin's twenty millions of years 
 could be established, then must solar events be, com- 
 paratively speaking, short-lived, and the whole cosmos 
 may have entered on its present phase of evolution 
 within a period correspondingly recent. 
 
 It is, in a word, the primary chronological fact of 
 inorganic evolution, as it is of organic evolution ; and 
 to it our fundamental ideas of duration must be re- 
 ferred, as the depths of space are referred to the 
 distance separating earth and sun. 
 
 The discoveries of radioactive science early suggested 
 the possibility of approaching the problem of the geo- 
 logical age in a new way. Uranium is constantly 
 breaking down into an element of less atomic weight ; 
 and this transformation progresses alike within the 
 hardest rocks and in the waters of the ocean. The 
 derived element changes to one of yet lower atomic 
 weight, and so this material degradation proceeds until a 
 
URANIUM AND THE AGE OF THE EARTH. 213 
 
 stable substance is reached at which the change ceases ; 
 or ceases for the present time. Now, what becomes of 
 the lost mass ? We have no reason to believe that any 
 part of it is destroyed. Indeed, we know that it is not 
 destroyed; we know that it is radiated in the alpha 
 particles, the energy of which has been so important 
 a factor in geological dynamics. The process of trans- 
 formation is thus attended with the rejection of portions 
 of the atom at every change of atomic weight. The 
 discarded particles are physically alike in every case, 
 so far as we know ; and, as Kutherf ord long ago pre- 
 dicted, are in themselves a recognized kind of matter 
 helium. 
 
 This substance, radiated in an ionic or electrified 
 state, accumulates amidst the changing matter which 
 gives rise to it, unless there be some avenue of escape. 
 In the case of the uranium family the number of alpha 
 rays or helium atoms, which the entire sequence of 
 changes involves, is fairly well known. If we know 
 the rate at which these changes progress and this 
 knowledge is also ours to a sufficient degree of 
 accuracy and in any particular case determine both 
 the amounts of radioactive substances present and the 
 quantity of accumulated helium, we are evidently in 
 possession of all the data required to determine the 
 period during which the changes have been going on. 
 
 The identity of the alpha ray with helium is a * 
 fundamental point in this procedure. The evidence 
 on which this identification is founded may be taken 
 as conclusive. Rutherford and Geiger 1 have shown 
 
 . U.S., 81 A, p. 162. 
 
214 RADIOACTIVITY AND GEOLOGY. 
 
 by a very beautiful investigation that the charge on 
 a single alpha particle is about twice the charge carried 
 by an ionized atom of hydrogen ; and hence by a know- 
 ledge of the ratio of the charge to the mass, the mass 
 is found to be 3*84 times that of the hydrogen atom ; 
 in other words, the atomic weight is 3 '84. Now, quite 
 independently it is known that the atomic weight of 
 helium is 3 P 96. The agreement is remarkable, consider- 
 ing the delicate nature of the measurements involved. 
 
 In a second investigation 1 the purified emanation of 
 radium is introduced into a glass tube so thin in the 
 walls that the alpha rays pass through, while the 
 emanation and its transformation products remain 
 enclosed. This tube is placed within a larger one 
 rendered highly vacuous. A spectroscopic examination 
 of the residual gas in this outer tube at first shows 
 no helium, but as time progresses the helium spectrum 
 appears. This shows that the alpha rays, passing 
 through the walls of the inner tube and trapped within 
 the outer one, are helium. Here we are secure from 
 the possibility that the helium known to come into 
 existence in the transformations of radium and its 
 descendants, is some resultant of the changes other 
 than the alpha ray itself. Finally, if any further 
 proof was required, Dewar 8 has measured the rate of 
 production of helium from radium, and finds that this 
 is in close agreement with that which Eutherford had 
 predicted on a count of the emitted alpha particles : that 
 is 0*37 cubic millimetres per gram of radium per day. 
 
 1 Rutherford and Royd, Proc. Manchester Lit. and Phil. Soc., 53, p. 1. 
 
 2 Sir James Dewar, Proc. E. S. t 81 A, p. 280. 
 
URANIUM AND THE AGE OF THE EARTH. 215 
 
 The diagram of the radioactive elements given in 
 Chapter I shows that in the direct line of descent, the 
 atom of uranium emits two alpha particles ; and 
 ionium, radium, emanation, radium A, radium (7, and 
 polonium, each emit one particle. When therefore 
 these substances exist together in equilibrium, as 
 they would in an old mineral, every atom of the 
 parent substance which transforms involves, on the 
 simplest assumption, the production of a total of 8 
 atoms of helium. Additional to this source of helium, 
 the actinium branch supplies a small quota. But this 
 must be a comparatively unimportant addition, the 
 activity of actinium in uranium-bearing minerals being 
 only a feeble part of the whole. 1 
 
 At the time when Eutherford 2 first applied this 
 method to determine the age of radioactive minerals, 
 the constants were not so well known as they are now. 
 The original calculations are accordingly in need of 
 correction. He cites the case of a specimen of fergu- 
 sonite which Eamsay and Travers found to contain 
 1*81 cubic centimetres of helium per gram, and about 
 7 per cent, of uranium. We now know that the ratio 
 of uranium to radium in minerals must be closely as 
 1 is to 34 x 10~ 7 . The quantity of radium present 
 must, therefore, be 23'8 x 10~ 9 grams per gram. Now, 
 3 '4 x 10 10 alpha particles are expelled per second per 
 gram of radium, 3 or if we count up the eight rays which 
 appear involved in the theory of radioactive change, 
 
 1 Boltwood, American Journ. Sc. t April, 1908, p. 291. 
 
 2 Rutherford, Phil. Mag., Oct., 1906, p. 368. 
 
 3 Rutherford and Geiger, loc. cit. 
 
216 RADIOACTIVITY AND GEOLOGY. 
 
 for each atom of radium breaking up, 27'2 x 10 l 
 alpha particles are emitted ; and this is the number 
 expelled per second per gram of radium. Taking now 
 2*72 x 10 19 molecules as the number of molecules in one 
 cubic centimetre of gas at standard pressure and tem- 
 perature, we would have 10~ 8 cubic centimetres of 
 helium evolved per second in a hypothetical mineral 
 containing one gram of radium. This is equal to 316 
 cubic millimetres per year. But in the fergusonite 
 the amount of radium is only 23*8 x 10~ 9 , and there- 
 fore the annual evolution of helium is 0*75 x 10~ 8 cubic 
 centimetres per gram. We can from this calculate 
 the age. The helium in the mineral was found to be, 
 as stated, T81 cubic centimetres per gram : dividing 
 this number by the amount of helium evolved per 
 year in one gram of this mineral, we find the duration 
 required for the accumulation of the helium was 241 
 millions of years. Nothing is said about the probable 
 geological position of this mineral. It may possibly 
 date from a time before our epoch of denudative 
 history begins. Kutherford, however, suggests that 
 the age of the geological strata might also be inves- 
 tigated in this manner. 
 
 This calculation may be taken as typical of the 
 steps involved. It will be seen that we have to 
 measure not only the helium, but also the uranium 
 or the radium. If we take the radium and of 
 necessity we must in general do so we may proceed 
 to find the age on the rate of production of helium 
 which the presence of the observed amount of radium 
 
URANIUM AND THE AGE OF THE EARTH. 217 
 
 involves. Or we may make a similar calculation, 
 working directly from a knowledge of the uranium 
 present. 
 
 According to the experimental value found by Sir 
 James Dewar for the rate of evolution of helium, 
 the quantity is 135 cubic millimetres per year per v 
 gram of radium. This, however, is only for radium in 
 equilibrium with its products of rapid change. It 
 therefore only includes four out of the eight alpha 
 particles which in an old mineral containing the 
 complete series would be emitted. Doubling his 
 figure, we find the constant is 270 cubic milli- y 
 metres per annum. This would add about seventeen 
 per cent, to the age as determined on the data cited 
 above. 
 
 Quite recently Soddy 1 has measured the helium 
 evolved by uranium itself, and obtained quantities 
 which suggest that Boltwood's view, that two alpha 
 rays are evolved from each transforming atom of 
 uranium, is incorrect. This observation, however, does 
 not, probably, point to a lessened rate of production of 
 helium, but rather to the existence of some unrecog- 
 nized element in the uranium series. The inferred 
 rate of production of helium, in the case of uranium 
 and some other of the radioactive elements, rests upon 
 measurements of the relative ionizing activity ; the 
 helium out-put in the case of radium and its products 
 of rapid change being, as we have seen, directly deter- 
 mined. 
 
 1 Soddy, Nature, Dec. 3, 1908, p. 129. 
 
218 RADIOACTIVITY AND GEOLOGY. 
 
 In a series of papers appearing in the Proceedings 
 of the Eoyal Society within the last couple of years, 
 Strutt has gone into the question of the occurrence of 
 helium as a widespread constituent of the earth's crust, 
 more completely than any of his predecessors. Sub- 
 sequent to Kamsay's discovery of helium in cleveite 
 and other minerals, Lord Eayleigh 1 in 1896 showed 
 that helium was a constituent of the hot springs 
 at Bath ; and Moureu 2 found that, in hot springs 
 occurring in France, helium was a common ingredient ; 
 and this observation has since been confirmed for 
 thermal springs in various parts of the world. 
 Natural gases obtained from the earth have also 
 been shown to contain helium as a general consti- 
 tuent. Thus Cady and McFarland, 8 in forty-one 
 sources of natural gas in America, found in all cases 
 helium present from traces up to as much as 1*84 
 per cent. 
 
 It would seem from Strutt's observations that we 
 need look no further than the radioactivity of common 
 rocks and minerals for these supplies of helium. In a 
 large number of rare and common minerals, in rocks 
 both igneous and sedimentary, helium occurs ; and 
 most generally in quantities which might be referred 
 to the uranium-radium elements present. 4 Exceptions 
 to this have, however, been observed. For instance, 
 
 L Lord Rayleigb, Proc. R. S., 59, p. 198. 
 
 2 Cotnpte Rendu, 1906. v. 152, p. 1155. 
 
 3 Annual Report of Chemical Soc. on Radioactivity, by F. Soddy, 
 1007, p. 323. 
 
 4 Strutt, Proc. R. S., 80 A, p. 572. 
 
URANIUM AND THE AGE OF THE EARTH. 219 
 
 a fluorite wherein a large quantity of helium can only 
 be ascribed to thorium, 1 and beryl, wherein the helium 
 occurs without any adequate quantity of radioactive 
 elements to account for its presence. Again, Strutt 2 
 finds reason to believe that in the Stassfurt salt 
 deposits, helium is liberated by the element potassium ; 
 the radioactive properties of which have lately been 
 shown by the researches of Campbell and Wood. 3 
 
 Throughout the results obtained by Strutt there is 
 a very wide variation in the ratio of the quantity of 
 helium to the quantity of uranium oxide as calculated 
 from the radium. This wide variation seems impos- 
 sible to account for on the score of the age of the 
 substance; indeed, as Strutt points out, we find in 
 the Cornish granites a considerably smaller ratio than 
 obtains in the minerals of the veins traversing it, and 
 which are necessarily younger in age. 4 
 
 More recently Strutt has approached the problem 
 of the geological age directly ; taking as materials 
 for examination substances which must be contem- 
 poraneous in age with the strata in which they are 
 found. More especially he makes use of phosphatic 
 nodules and phosphatized bones, which he finds to 
 contain an unusually large amount of radinm ; some- 
 times up to fifty times that found in common rocks. 
 The following table summarizes his results. 5 
 
 1 Strutt, Proc. K. S., 80 A, p. 56. 
 
 2 Ibid., 81 A, p. 278. 
 
 3 Campbell and Wood, Camb. Phil. Soc. Proc., vol. 19, p. 15. 
 
 4 Strutt, Proc. R. S., 80 A, p. 582. 
 
 5 Ibid., 81 A, p. 272. 
 

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 Geological Hoi 
 
 Pliocene, 
 
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 Upper Greensand, 
 
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 Kimmeridge Clay, 
 
 Base of Kimmerid^ 
 
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 Above Carboniferoi 
 Bala beds, . 
 
 Llandeilo limeston 
 
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 Felixstow 
 
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 Knapwell 
 
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 rri Q. 
 
 
 
URANIUM AND THE AGE OF THE EARTH. 221 
 
 The last column has been deduced on data similar 
 to those already discussed in relation to the age of a 
 specimen of fergusonite, but calculating, for convenience 
 in dealing with the data used by Strutt, the annual 
 production of helium from one gram of uranium oxide 
 (U 3 8 ). This constant is found to be 913 x 10' 8 cubic 
 centimetres. 
 
 The variations of apparent age, irrespective of the 
 order which must be assigned from the geological 
 position, are very considerable. Strutt considers this 
 is best explained as due to escape of helium, regarding 
 the deduced age as a minor limit. 
 
 We will doubtless soon have more to go upon in 
 judging of the possibilities of this method according as 
 the errors affecting it and causing the variations are 
 investigated. We cannot, however, discuss its impor- 
 tance without indicating the directions in which error 
 may lessen its value. 
 
 The assumptions involved are that at the time of 
 formation of the mineral substance a certain amount of 
 uranium was introduced, which continued in its present 
 position, only diminishing by transformation ; and that 
 all its products of change also remained, neither increased 
 nor diminished, preserved in a similar manner within 
 the substance. The gravest objection which can be 
 brought against the method is, of course, the risk that 
 these conditions are not likely all to be fulfilled in 
 any one case, and that we are in ignorance of the fact 
 whether they are fulfilled or not. Only in the event 
 of arriving at concordant results under different 
 circumstances of material and preservation would 
 
222 RADIOACTIVITY AND GEOLOGY. 
 
 there be any assurance that some one or other of the 
 conditions had not broken down during the long 
 pa^. 
 
 These doubts arise in view of the known potency of 
 underground waters in producing changes, either of sub- 
 tractive, additive, or metasomatic character, throughout 
 even hundreds of feet of rock. When along with this 
 fact the geological history of even the most undisturbed 
 strata is considered the changes of temperature and 
 pressure which must have assailed them and recol- 
 lecting that many millions of years are involved in the 
 case of even comparatively modern rocks the chances 
 of finding helium, uranium, and radium, and all the 
 chain of elements neither increased nor diminished 
 must be remote. The retentive and stable nature of 
 the material selected for investigation will be in this 
 connexion an element of importance. 
 
 On the other hand might be urged the fact that in 
 uranium ores we find a satisfactory degree of uniformity 
 in the ratio of uranium to radium. It would appear from 
 Boltwood's observations, however, that the constancy 
 of the helium ratio is less in evidence, and, indeed, 
 widely departed from. 1 Again, it should be borne 
 in mind as generally true, that great losses in 
 the past, of uranium or radium, or even gains of 
 these substances, might occur; but if the conditions 
 of change had been so far mitigated, or had so far ex- 
 hausted themselves for the last two thousand or three 
 thousand years, as to suffer equilibrium to be estab- 
 lished, we would remain in ignorance of the losses 
 
 1 Boltwood, Am. Journ. Science, Feb., 1907, p. 77. 
 
URANIUM AND THE AGE OF THE EARTH. 223 
 
 or gains of the millions of antecedent years. The 
 possibilities are indicated in the formation of secondary 
 ores such as gummite, autunite, uranophane, and secon- 
 dary pitchblende ; all fairly common in association 
 with uranium minerals, and many found deposited 
 in comparatively recent geological times. And with 
 reference to even the harder crystalline minerals, it 
 must be noted that Jaquerod and Perrot have shown 
 that helium permeates quartz at temperatures above 
 220 C, and with a velocity which increases with the 
 temperature. 1 This would seem to render untrust- 
 worthy, readings obtained from metamorphic rocks, or 
 rocks which had taken part in great earth movements. 
 The removal and redeposition of radium are not 
 merely hypothetical. Danne records a remarkable 
 case. 2 The radium was found in deposits of phosphate 
 of lead (pyromorphite), in clays containing lead, and in 
 pegmatite, but most abundantly in the phosphate. 
 No trace of uranium was present. To reach its 
 present position in the pyromorphite, the radium must 
 penetrate veins of quartz and felspar moisture due to 
 neighbouring springs being the probable carrier. The 
 quantity of radium varied considerably, and Darme 
 believes that about a hundredth part of a gram per 
 tonne must be present. This is, of course, a very large 
 amount. The source appears indicated in the occur- 
 rence of autunite in this region, and it is stated as 
 probable that other occurrences of uranium exist. The 
 storage of radium is in this case traceable, very 
 
 1 Nature, Oct. 22, 1908, p. 635. 
 
 4 See Hutherfoid's Radioactivity, p. 465. 
 
224 RADIOACTIVITY AND GEOLOGY. 
 
 probably, to carriage by water in past ages. The 
 concentration in the phosphate is remarkable. 
 
 There is some reason to fear that phosphatic sub- 
 stances, as repositories of radioactive elements and 
 their products of change, may prove unreliable. In 
 the first place, it is evident that at some time in the 
 history of the phosphatic nodules there must have 
 been an accretion or absorption of uranium or radium, 
 or of both. For, in fact, these bodies are of organic 
 origin, and probably would not originally have 
 possessed their radioactive materials. When did this 
 accretion occur? The most probable answer is that 
 it was effected upon the sea-floor. But are we justified 
 in assuming that it did not continue for a very long 
 time after ? Knowing from the facts of their genesis 
 that accretion or absorption of radioactive elements 
 occurred at one time in their career, and also knowing 
 that underground waters are not infrequently charged 
 with small quantities of radium, and often with helium, 
 why should we limit the absorption to the geological 
 period in which the nodules are f oimd ? If this reason- 
 ing is correct, there is possibility of results either 
 unduly great or unduly little, according to very com- 
 plex cr A id'tions in the long past. 
 
 The history of these phosphatic bodies shows that 
 they have had a chequered career. They originated as 
 alteration products of organic remains thrown down 
 on the floor of the ocean, where they probably 
 gathered radium or uranium, and, indeed, almost 
 certainly both, the surroundings being highly radio- 
 active. Their sojourn under these conditions may 
 
URANIUM AND THE AGE OF THE EARTH. 225 
 
 have been thousands of years : possibly in some cases 
 much longer. If during this period radium was 
 directly absorbed, a positive source of error in the 
 accumulated helium is incurred to start with. 
 
 The phosphatization of these bodies is by no means 
 completed in their original surroundings. A further 
 concentration is effected after the sediments have been 
 elevated into land surfaces ; when, by leaching with 
 meteoric or subterranean waters, the excess of calcium 
 carbonate is washed away. 1 
 
 The phosphorites of Tennessee are cited by Clarke 
 as " an excellent illustration of the several processes, 
 chemical and mechanical, which have taken part in 
 their formation." These were partly Ordovician and 
 partly Devonian, and as phosphatic limestones deriving 
 their phosphates from bones, shells, and teeth, were 
 first laid down in a shallow sea. There was then 
 leaching of the limestones, which removed carbonates, 
 leaving a mixture of phosphates, clay, and iron 
 hydroxide. The soil thus formed was again concen- 
 trated by mechanical washing, the moving waters 
 carrying away the clay and leaving the heavy phos- 
 phatic nodules. Some of the phosphates were actually 
 dissolved by percolating waters and precipitated as 
 secondary deposits in the underlying limestones. All 
 this indicates that in these substances we deal with 
 bodies specially liable to alteration. That these altera- 
 tions may take the form of the addition of substances 
 to the phosphate, appears from the following facts : 
 "In certain Cretaceous sandstones of Eussia calcium 
 
 - ] Clarke, The Data of Geochemistry, p. 448. 
 Q 
 
226 RADIOACTIVITY AND GEOLOGY. 
 
 phosphate occurs as a cement for the sand grains, and 
 also in the form of fossil-bones and fossil wood. The 
 wood has been completely replaced by the phos- 
 phate. Although bone is itself largely composed of 
 calcium phosphate, fossil bones are not identical 
 chemically with recent bones. The fossils show an 
 enrichment in calcium carbonate, iron oxide, and 
 fluorine, as A. Carnot has shown, and especially in 
 fluorine. Modern bones from various animals were 
 found by Carnot to contain a minimum proportion of 
 fluorine ; Tertiary bones were much richer ; Triassic 
 and Cretaceous bones still more so; and in bones from 
 Devonian and Silurian formations the ratio of fluoride 
 to phosphate was nearly that of apatite. This progres- 
 sive enrichment in fluorine Carnot attributes to the 
 % . agency of percolating waters, carrying small quantities 
 of fluorides in solution." 1 Bischof kne'w of the absorp- 
 tion by fossil bones of the element fluorine, and suggests 
 a chemical explanation. 2 
 
 In short, here are cumulative actions of just the kind 
 we previously referred to as possibly resulting in the 
 accretion of radioactive materials, unless we suppose 
 the fluorine generated within the phosphate. The 
 conclusion seems to be that a steady, mutual substan- 
 tiation must be required .of results which involve, to 
 so great a degree, unaltered relations having existed 
 between the mineral and its surroundings ; and the 
 further back we seek to extend our explorations, the 
 more desirable is the assurance arising from mutual 
 
 1 Clarke, loc. cit., p. 447. 
 
 2 Bischof, Chemical and Physical Geology, ii., p. 33. 
 
URANIUM AND THE AGE OF THE EARTH. 227 
 
 support. That greater security might sometimes be 
 gained by direct determination of the uranium is sug- 
 gested by the presumptive evidence in favour of 
 radium being captured by phosphates without the 
 simultaneous storage of the parent element. 
 
 These objections unfortunately apply in some degree 
 to any substance of secondary origin. Thus if we have 
 to deal with a mineral substance probably derived by 
 metasomatic change of a pre-existing rock, we have of 
 necessity to ask if it is certain that the waters con- 
 cerned in the genesis of the mineral brought in the 
 uranium and radium, and that these elements were not 
 introduced subsequently. And again, may not the 
 latter element have taken up its position in quantities 
 beyond the true equilibrium amount ? Here again we 
 seem entitled to ask for the corroboration of results, 
 before we conclude that these sources of error are 
 absent. In a word, when we examine into the claims 
 of the radioactive method, we find that, in a certain 
 sense, we postulate a degree of uniformity between 
 present and past conditions which might give pause 
 to the most advanced uniformitarian geologist. It has, 
 however, this advantage over any other known method 
 of approaching the problem of the earth's age, that 
 each experiment is, even if uncertain, an independent 
 estimate, and that we may make as many of them as 
 we please. Thus it may result that out o a large- 
 number we may haply find a degree of coincidence 
 sufficing to confer complete credibility on the evidence. 
 
 The production of helium from thorium, and the pos- 
 sibility of helium so produced making an appreciable 
 
 Q2 
 
228 RADIOACTIVITY AND GEOLOGY. 
 
 ' part of the whole amount, have been carefully considered 
 by Strutt. 1 He has brought forward much evidence to 
 show that this may indeed be the case. It may be 
 added that the fact that the alpha ray is a helium atom 
 appears to set at rest the sometime disputed question 
 whether thorium could indeed give rise to helium. 
 Direct evidence of the genesis of helium by thorium 
 has also been obtained by Soddy. 2 Obviously if this 
 source of helium is of moment, its neglect in any par- 
 ticular experiment is in the direction of producing an 
 error of excess in the age. 
 
 The idea that lead might be the ultimate disinteg- 
 ration product of uranium has been made the basis 
 of an attack on the age problem. In a paper of great 
 interest Boltwood 3 records that in unaltered primary 
 minerals from the same locality, and presumably of the 
 same age, the amount of lead was proportional to the 
 amount of uranium present ; and, further, that the 
 amount of lead relative to the uranium was greatest in 
 minerals from those localities which were of the greatest 
 geological age. This, it will be admitted, is much in 
 favour of the view that lead is indeed a disintegration 
 product of uranium. 
 
 On the foregoing basis Boltwood arrives at the age of 
 a number of uranium- bear ing minerals. His figures, 
 when corrected according to more recent data, respect- 
 ing the constants involved (the proportion of uranium 
 
 - 1 Strutt, Proc. JR. S., A 76, pp. 101, 312. 
 
 2 Soddy, Phil. Mag., October, 1908. 
 
 3 Boltwood, Am. Journ. Science, February, 1907, p. 77. 
 
URANIUM AND THE AGE OF THE EARTH. 229 
 
 to radium, and the rate of decay of radium), range 
 from 246 to 1320 millions of years. The geological 
 positions of these minerals are not cited; the oldest, 
 from Norway and Ceylon, are presumably from primi- 
 tive rocks. 
 
 It is not improbable that the radioactive method of 
 determining the age will receive a useful development 
 in the procedure adopted in this research. Although 
 the introduction of radium, etc., unattended with 
 uranium or the introduction of radioactive materials 
 at dates subsequent to the formation of the substance, 
 would give rise to larger errors than in the method 
 based on the helium ratio, on the other hand, there 
 would probably be more security against error from 
 the loss of the disintegration product chosen for 
 measurement. 
 
 The apparent tendency of Strutt's results is to assign 
 ages to the older sedimentary deposits greater than 
 would be deduced by the legitimate use of any other 
 method of estimating the age of the earth with which 
 we are acquainted. Thus Strutt selects the following 
 results as showing the bearing of his measurements, 
 regarding them as minor limits : 
 
 Years. 
 
 Phosphatic nodules of the Crag, . . 225,000 
 
 Upper Greensand, 3,080,000 
 Lower 3,950,000 
 
 Haematite overlying Carboniferous Lime- 
 stone ...... 141,000,000 
 
 If the last of these estimates is correct, there can be 
 
230 EADIOACTIVITY AND GEOLOGY. 
 
 little doubt that the Huronian must date back some 
 400 millions of years, and the " geological age " would 
 probably be somewhat greater still. It is, therefore, 
 of importance to consider whether those existing 
 methods, which have hitherto been interpreted as 
 indicating from 50 millions to 100 millions of years 
 as the period since denudation began, can be so far 
 astray. 
 
 There exist, in fact, but two methods directly deal- 
 ing with the denudation period : that which is based 
 on the records of detrital denudation, and that which 
 is based on the records of solvent denudation. 
 
 The first method seeks to find the period since 
 sedimentation began by taking as numerator the maxi- 
 mum total depth of sedimentary deposits, and as 
 denominator the rate of accumulation. The quotient 
 is the period sought. 
 
 It must be admitted that grave difficulties attend 
 the determination of both the numerator and denomi- 
 nator of this fraction : 
 
 (a) What is the maximum thickness of the sedi- 
 ments ? We are first met with the difficulty of not 
 knowing where to begin. The more ancient sediments 
 are apparently to be found in those complexes of 
 igneous and sedimentary materials wherein the dif- 
 ferentiation of the two classes is all but, or quite, 
 impossible. About the origin of such rocks we find 
 the most experienced observers differing. Again, 
 even when the evidence is adjudged decisive as to 
 an igneous origin, this may really denote no more 
 
URANIUM AND THE AGE OF THE EARTH. 231 
 
 than the complete igneous change of pre-existing 
 sediments. 
 
 (I) A fine-grained deposit will take a far longer 
 time to collect than an equal thickness of coarser 
 materials. A whole formation will include layers of 
 many different sorts. The time-values of these are 
 very various; some are of slow, some of rapid depo- 
 sition, suggesting that the area of maximum rate of 
 deposition was not fixed in position. In seeking the 
 denominator (the rate of accumulation) this must be 
 remembered or serious error will be incurred. 
 
 (c) The unconformities often represent great lapses 
 of time, which not only may be unrepresented, but may 
 even have resulted in the removal of earlier deposits. 
 As an instance, consider the almost complete removal 
 of the mountain- building sediments of the Alps. This 
 error may or may not be made good in the measurements 
 at our disposal. Certain deposits are very perishable. 
 The Chalk is an instance. This substance is not only 
 perishable, but it is, relatively, a slow-forming deposit. 
 In a few million years, if exposed so long to denudation, 
 all records of such formations might be removed. Thus 
 the time record would be falsified. 
 
 To these sources of error might be added such as 
 may arise by the conditions being homotaxial rather 
 than contemporaneous. For if Huxley's case have a 
 semblance of the truth, and it be possible that the 
 Devonian period in England was overlapped by the 
 Silurian in America, and in turn overlapped the 
 Carboniferous in Africa, we evidently are not entitled 
 
232 RADIOACTIVITY AND GEOLOGY. 
 
 to sum the several depths of deposit in these countries 
 as successive time intervals. This latter objection has 
 hardly more than academic interest at the present 
 moment. The radioactive method of investigating 
 the ages of the strata may yet put Huxley's case to 
 the test. Finally the practical difficulty of measuring 
 the thickness of strata has to be faced. The difficulty 
 of the task is shown in the diversity of opinion as to 
 many of the recorded measurements. 
 
 Supposing we have satisfactorily determined the 
 numerator, we are by no means done with our 
 troubles. "We have to find the rate at which the 
 rivers are carrying detrital sediments to the ocean. 
 This involves measurements of the silt-load of rivers, 
 at various seasons. The seasonal variation is great, 
 because the same cause which raises the river to 
 flood-level increases the supply of suspended matter. 
 The burden carried may be five-fold as much at one 
 time as at another time. 1 
 
 We have also to find the rate at which this silt 
 grows in depth when it comes to rest in the ocean. 
 And here is perhaps the most serious source of error, for 
 not only is the catchment area difficult to define, but 
 the troublesome question remains as to whether we 
 should reckon the silt as laid down over the whole 
 area, and take this mean rate of deposition, or, having 
 found this average rate of accumulation, proceed to in- 
 crease it in order to arrive at the maximum rate, having 
 in view the fact that we have taken the sedimentary 
 
 1 See table in Russell's River Development, p. 72. 
 
URANIUM AND THE AGE OF THE EARTH. 233 
 
 records at their maximum thickness. The final result 
 will vary considerably according to which procedure 
 we adopt. Thus while Geikie 1 is satisfied that the 
 whole geological column may have taken about 100 
 million years to accumulate, and De Lapparent, 2 on a 
 rather vague definition of the area of deposition, arrives 
 at seventy-five to eighty millions, Sollas, 3 adding more 
 than 100 per cent, to the mean rate of collection to 
 represent the maximum, obtains twenty-six millions. 
 But it would appear as if some moderate increase of the 
 mean rate was the more correct procedure; for, as 
 already pointed out, when reckoning up the beds we 
 include both fast- and slow-forming deposits ; admitting 
 thereby that the area in question was at one time the 
 place of rapid growth and at another time the place of 
 slow growth. In other words, the deposit is not truly 
 wedge-shaped, but rather takes the form of a shallow 
 basin. Hence, unless we reckoned coarse detrital rocks 
 exclusively, we are not entitled to assume the fastest 
 rate of deposition for the whole formation. 
 
 The foregoing method of finding the age, which has 
 given rise to so much discussion, appears to afford its 
 answer by a somewhat different mode of approach 
 from that which is generally adopted. We take first 
 the average ratio of the mass of suspended sediment to 
 the mass of the river water. 
 
 1 Geikie, Brit. Assoc. Rep., Address to Sect. C, 1899. 
 
 2 Traite de Geologic, vol. iii., p. 1860. 
 
 3 Sollas, Brit. Assoc. Rep., Address to Sect. C, 1900. 
 
234 
 
 RADIOACTIVITY AND GEOLOGY. 
 
 The following table contains estimates of this 
 factor 1 : 
 
 
 Mean annual 
 discharge in 
 cubic feet 
 per second. 
 
 fM 
 
 Total annual 
 sediment in 
 thousands of 
 tons. 
 
 Ratio of 
 sediment to 
 water by 
 weight. 
 
 Potomac, 
 
 20,160 
 
 5,557 
 
 1 3,575 
 
 Mississippi, . 
 
 610,000 
 
 406,250 
 
 1 1,500 
 
 Bio Grande, 
 
 1,700 
 
 3,830 
 
 1 291 
 
 Uruguay, . 
 
 150,000 
 
 14,782 
 
 1 10,000 
 
 Rhone, 
 
 65,850 
 
 36,000 
 
 1 1,775 
 
 Po, . . 
 
 62,200 
 
 67,000 
 
 1 900 
 
 Danube, 
 
 315,200 
 
 108,000 
 
 1 2,880 
 
 Nile, . 
 
 113,000 
 
 54,000 
 
 1 2,050 
 
 Irrawaddy, . 
 
 475,000 
 
 291,430 
 
 1 1,610 
 
 Mean, 
 
 201,468 
 
 109,650 
 
 1 2,731 
 
 It will be seen that the ratio of the weight of 
 sediment to the weight of water in the cases of all 
 the rivers cited, save two, does not differ very widely. 
 The arithmetic mean of the ratios, as given in the 
 table, fails to allow for the relative importance of 
 the rivers. When we ascribe to the ratio given above 
 for each separate river the weight proper to the dis- 
 charge, the mean ratio of sediment to water is found 
 to be 1 : 2521. 
 
 If we now take Sir J. Murray's estimate of the 
 total annual discharge of the world's rivers, 27 x 10 3 
 cubic kilometres, we find that the total carriage of 
 detrital sediments to the ocean is about 4*4 cubic 
 
 1 Russell, loc. cvt., p. 14. 
 
URANIUM AND THE AGE OF THE EARTH. 235 
 
 kilometres per annum. This is for the whole earth : 
 we have now to find an approximate measure of the 
 area of deposition. On the basis of the measure- 
 ments made in the case of the Mississippi, 1 we take the 
 area of deposition to be one-eighteenth that of denu- 
 dation. To find the latter we have first to subtract 
 from the land area of the globe the rainless areas. 
 These are estimated as making one-fifth the total 
 area (which is 14*4 x 10 7 square kilometres), leaving 
 11*5 x 10 7 square kilometres; and one-eighteenth of 
 this receives the detritus, i.e., 6*4 x 10 6 square kilo- 
 metres. Spreading the sediment over this, we get a 
 rate of growth of 6*9 centimetres in 100 years. This 
 rate we increase by 50 per cent, to allow for the fact 
 that we compute the depth of sedimentary deposits 
 where these are most developed. 
 
 According to Sollas 2 we may reckon the geological 
 column as about eighty kilometres. The geological 
 age derived would be about eighty million years. The 
 neglect of the calcareous precipitates in this computa- 
 tion tends to increase somewhat the estimated age. 
 
 The question we have to ask here is : Can we be 
 four- or five-fold in error in such an estimate ? In the 
 foregoing pages we have been at pains to set forth the 
 directions in which this method may err. But con- 
 sidering each weak point separately, we must admit 
 the great improbability of failure so complete. The 
 integral of the sediments can scarcely be 100 miles 
 instead of fifty miles thick as estimated; much less 
 
 1 Magee, Am. Journ. of Science, 1892, p. 188. 
 
 2 Sollas, The Age of the Earth, p. 14. 
 
236 RADIOACTIVITY AND GEOLOGY. 
 
 can it be really 200 miles. Can the ratio of the 
 areas of denudation and deposition be on the average 
 four or five times greater than what has been measured 
 in the case of the Mississippi? Or can the mean found 
 from the nine rivers for the ratio of silt to water be 
 equally far out? Or, finally, can the several rates 
 have been so much less in the past? The answer in 
 each case seems to be a negative, and only on the 
 chance that several of these factors have been computed 
 with error of the same sign, does it appear possible to 
 reconcile the indications of the sedimentary accumu- 
 lations with the storage of the products of radioactive 
 disintegration. 
 
 The probability that such an accumulation of errors 
 has occurred is greatly lessened by the fact that a really 
 quite independent mode of computing the geological 
 age gives a similar result. 
 
 The method of estimating the geological age by the 
 accumulated results of solvent denudation requires 
 knowledge of two quantities : the quantity of sodium 
 discharged per annum into the ocean by all the rivers 
 of the world, and the amount of this element in the 
 sea. The latter is evidently the numerator and the 
 former the denominator of the fraction, the quotient 
 of which is the sought-for geological age. 
 
 As regards the determination of the numerator 
 that is the saltness of the sea the correct value, 
 within a few per cent., is known. Two quantities 
 are involved in it: the average composition of sea- 
 water and the mass of the ocean. Our knowledge of 
 the first is favoured by the fair degree of uniformity 
 
URANIUM AND THE AGE OF THE EARTH. 237 
 
 in chemical composition prevailing in all the great 
 seas; our knowledge of the latter by the fact that 
 soundings are certainly sufficiently numerous to afford 
 the average depth of the ocean : undiscovered deeps 
 or shallows are not likely to seriously affect the 
 volume inferred from the average depths. 
 
 In the geological application of this knowledge we 
 are, however, exposed to the possibility that the early 
 ocean received salts at an accelerated rate owing to 
 the prevalence of an atmosphere possessing acid proper- 
 ties. It will be shown that error from this source is 
 calculable and its limiting amount definable. Neglect 
 of this possible correction renders our estimate of 
 geological time proportionately too long, for it is a sub- 
 tractive correction on the numerator. The reception, 
 by the ocean, of salts dissolved directly from the coasts, 
 or from the submerged sediments, gives rise to yet 
 another correction which can be shown to be probably 
 small. Its neglect again tends to an unduly great 
 estimate of the period of denudation. The rock-salt 
 deposits are obviously negligible as a source of error. 
 This will appear from the fact that the rock salt in the 
 ocean, if spread upon the dry land of the globe, would 
 cover it to a depth of 122 metres (400 feet). The 
 quantity of sodium now in the ocean is about 
 1,555 x 10 13 tonnes. The allowance to be made for 
 the sources of error referred to we consider later. 
 
 The denominator (the sodium content of the rivers) 
 is not so well known. It is indeed quite determinable, 
 but we have to go on insufficient knowledge at present. 
 We have Sir John Murray's estimate of the total 
 
238 RADIOACTIVITY AND GEOLOGY. 
 
 amount of river water, annually discharged into the 
 ocean, as 27,191 cubic kilometres ; and his average 
 analysis of 19 rivers, many of them principal rivers 
 of the world. Combining these data, we find that the 
 total amount of elemental sodium discharged yearly is 
 15,976 x 10* tonnes. As the 19 rivers do not repre- 
 sent a sufficiently large fraction of the total drainage 
 of the continents, this figure cannot be considered final. 
 
 The most important correction on the denominator 
 is concerned with the cyclical nature of part of the 
 sodium. 1 For some part of it is believed with consider- 
 able probability to be wind-borne from the sea, reaching 
 the rivers in the rain-water. The facts connected with 
 and giving rise to this belief may be briefly stated. 
 
 (a) The rain falling on islands and coastal countries 
 carries down with it a considerable proportion of 
 chlorides and other salts, which might be referred to 
 oceanic origin. The proportion of such salts in rain 
 falls off as we go inland. Thus 200 miles from the 
 coast there may be no more than -fa of what would 
 be observed in rain-water collected 30 or 40 miles 
 from the sea. In India, 300 miles from the sea, a 
 measurement showed chlorine to the extent of only 
 0'04 part in 100,000 ; whereas in British rain-water it 
 may be from O'l to 21 parts per 100,000, and even 
 more. The rate of decrease is, apparently, rapid as we 
 proceed inland. Kemote from the sea some of the 
 chlorine in rain-water must be due to dust raised from 
 the ground by winds : in arid, saline regions this might 
 affect considerably the results of rain- water analyses. 
 
 1 July, Gtoloyical Magazine, Aug., 1901, p 341. 
 
URANIUM AND THE AGE OF THE EARTH. 239 
 
 (b) Coastal rivers arid the rivers of small islands 
 show abnormal percentages of chlorine compared with 
 the sodium, indicating that but a small fraction of the 
 latter is referable to the rocks, and the greater part in 
 these cases to the supply from the rains. 
 
 (c) The chlorine in average river- water is more than 
 the observed amount of this element in the rocks 
 would warrant. Thus Clarke, as an approximate 
 estimate, gives the percentage of chlorine in the igneous 
 lithosphere as 0*07. There is apparently no measurable 
 amount in detrital rocks ; and in limestones, which con- 
 stitute only some 5 per cent, of the sediments, there 
 is very little, so that we must conclude that in 
 the process of denudation the whole of the chlorine 
 passes into solution. Now the sodium in igneous rocks 
 averages (according to Clarke) 2*43 per cent., and of this 
 it can be shown that about one-half, or a little more, 
 goes into solution. Thus we should expect that the 
 break-up of an average igneous rock would liberate 
 chlorine and sodium in the ratio of 0'07 to 1*4 (say), or 
 as 1 to 20, about. However, the rivers show a pro- 
 portion of chlorine to sodium which, on the average, is 
 1 to 2. Hence there is about ten times as much 
 chlorine as would be inferred from the mean chlorine 
 content of igneous rocks, and this fact is taken as in 
 favour of the view that much of the chlorine of rivers 
 is wind-borne from the ocean. 
 
 This last argument must not be pushed too far. 
 Clarke's estimate of the elements in average igneous 
 rocks excludes extravasated or non-magmatic sub- 
 stances. It is based on typical .rock analyses. Now 
 
240 RADIOACTIVITY AND GEOLOGY. 
 
 chlorine, as it happens, enters with difficulty into 
 the constitution of silicates. We find it accordingly 
 free (or forming hydrochloric acid) in association with 
 melted magmas in volcanoes, and rising in springs in 
 volcanic regions. And it is found in combination with 
 metallic minerals and in springs associated with ore 
 deposits. The sources of supply are in these cases 
 probably deep-seated, and would not enter into rock 
 analyses. Possibly the slow leaching of the rocks 
 within the hydrosphere is ultimately responsible ; but, 
 however this may be, it is evident that, so far as the 
 river supply is concerned, we require to include all 
 these sources. Nor must we exclude the ancient 
 relics of congested denudation which appear in old 
 salt lakes, bad lands, and probably in saline waters 
 found in deep-seated bedded rocks. Again, if we go 
 upon this argument, we may, in the case of certain 
 rivers, arrive at the absurdity that no sodium is being 
 denuded : for cases occur where (if the analyses are 
 correct) we have insufficient sodium to combine with 
 the chlorine ; so that if the chlorine and sodium reach 
 the rivers from the ocean, there can be no sodium 
 arising from solvent effects. This, of course, is an 
 untenable view, and demonstrates the unsoundness of 
 the argument which gives rise to it. 
 
 The possibility must be kept in mind that chlorine 
 is liberated from the ocean unattended by the less 
 volatile elements. The escape of free chlorine, or, 
 more probably, of chlorine combined with hydrogen, 
 seems by no means improbable. In justification of such 
 a view we have to consider the very large amount of 
 
URANIUM AND THE AGE OF THE EARTH. 241 
 
 chlorine present ; and that if the chloride of hydrogen 
 was formed by fortuitous ionic combinations near the 
 surface especially in agitated water a certain per- 
 centage of the molecules should escape. The radioactive 
 substances in the ocean will, probably, be instrumental 
 in liberating a certain amount of free chlorine ; but it 
 is not hard to show that this contribution must be 
 insignificant. On this view the chlorine of rain-water 
 might only to a subordinate degree be present as sodium 
 chloride. This, if true, would reconcile the various 
 observations in the simplest manner. 
 
 Admitting, however, that some part of the chloride 
 of sodium of rivers is derived from the ocean, conveyed 
 by the winds and falling as rain, the question arises 
 what to allow for this source of error; for evidently 
 we cannot include wind-borne sodium in the calculation 
 of the geological age. The only really satisfactory mode 
 of investigating the amount of the wind-borne sodium 
 would be by rain-water analysis. Unfortunately we 
 possess very little knowledge on this subject. Let us 
 proceed, however, on the assumption that the chlorine 
 of inland rains is a clue to the cyclical sodium. The 
 great bulk of the river- water of the world is collected 
 at distances greater than 300 miles from the sea. If 
 we take 0*04 per 100,000 as the proportion of chlorine 
 in rain-water, and, making the usual allowance for 
 concentration by evaporation before it reaches the 
 rivers, conclude that in rivers, chlorine to the extent 
 of O'l in 100,000 is wind-borne ; and, further, that this 
 chlorine enters the rivers with the same proportion of 
 combined sodium as it would show in sea-water, we 
 
 R 
 
242 RADIOACTIVITY AND GEOLOGY. 
 
 can apply the appropriate correction to Murray's 
 estimate of river sodium. 
 
 Following Murray, we have 84'7 x 10 6 tonnes of 
 chlorine annually discharged by rivers into the ocean, 
 and this chlorine is contained in 27'2 x 10 12 tonnes of 
 water. This is a proportion of about 0'3 in 100,000. 
 The correction then amounts to taking one-third of the 
 chlorine as coming from the ocean. We have to find 
 what correction this is on the sodium. Now, in the 
 ocean the chlorine is considerably in excess of the 
 sodium, so that if all the sodium in the ocean is 
 assumed to exist in the form of the chloride, only 
 82 per cent, of the chlorine is used. Hence, ocean- 
 water contains at least 18 per cent, of its chlorine 
 otherwise attached than to sodium : and, accordingly, 
 what circulates from the sea to the land can only bring 
 with it to the rivers 82 per cent, of the equivalent 
 sodium. 
 
 One-third of the chlorine annually discharged by 
 the rivers, on the foregoing basis, represents 15 x 10 6 
 tonnes of sodium. The true denudative supply of 
 sodium per annum is, therefore, 131*5 x 10 6 tonnes. 
 
 We have now to return to the numerator, and apply 
 to it such corrections as may be necessary. The 
 assumption made above, that about one-third of the 
 chlorine in the rivers is really derived from the ocean, 
 involves that the true, denudative supply of chlorine 
 to the ocean by the rivers has been 56*5 x 10 6 tonnes 
 per annum ; the cyclical chlorine (28'2 x 10 6 tonnes) 
 being subtracted from the total chlorine (84'7 x 10 s 
 tonnes). This has been the constant rate of supply 
 
URANIUM AND THE AGE OF THE EARTH. 243 
 
 since the beginning of denudative processes, so that its 
 integral over geological time represents what has heen 
 brought from the land. If we call the annual supply 
 of chlorine c and the time X, the quantity is cX. Now, 
 if we make deduction of this from the total chlorine in 
 the ocean, we are left with a quantity of chlorine which 
 must for want of any alternative hypothesis be sup- 
 posed to have existed in the primeval atmosphere of the 
 earth, and, probably combined with hydrogen, to have 
 effected a rapid denudation of the earth-crust. If C 
 is the existing amount of chlorine in the ocean, the 
 primeval chlorine is C - cX. 
 
 Such a primeval acid denudation would undoubtedly 
 be responsible for the comparatively rapid introduction 
 of chlorides into the original ocean. We can make 
 some estimate of the quantities involved. Thus, if we 
 ascribe to the original lithosphere a composition similar 
 to that of the average igneous lithosphere of to-day, we 
 have (again quoting Clarke) : 
 
 Aluminium, . . . 7 '90 per cent. 
 
 Iron, . . . 4'43 ,, 
 
 Calcium, . . . 3 "43 ,, 
 
 Magnesium, . . . 2*40 ,, 
 
 Potassium, . . . 2 '45 
 
 Sodium, . . . 2-43 ,, 
 
 We here refer only to such substances as might be 
 supposed to enter into combination with the chlorine 
 present. Assigning to these elements their combining 
 proportions of chlorine, it is found that only 6*7 per 
 cent, of the chlorine is used in bringing sodium into 
 the primitive ocean. The total quantity of sodium 
 
 R2 
 
244 RADIOACTIVITY AND GEOLOGY. 
 
 thus introduced into the ocean, at the dawn of the 
 denudative period, was 
 
 . 6-7 23 
 
 the last factor serving to convert the chlorine into its 
 equivalent of sodium. This, when simplified, becomes 
 (G - cX} 0-044. The quantity of sodium which has 
 been supplied by the rivers is found by subtracting the 
 above amount from what is now in the ocean. If this 
 last quantity be denoted by N, and n be the annual 
 sodium discharge from rivers (as previously corrected), 
 we have for the determination of geological time the 
 equation : 
 
 _ N - 0-044(ff - cX) _ N- 0-044(7 
 n n - 0"044c 
 
 In this equation N = 1,555 x 10 13 ; and from 
 Dittmar's analysis of average sea-water, we get for 
 the total chlorine in the ocean 28,769 x 10 12 = C. For 
 n we have found 131-5 x 10 6 , and for c, 56'5 x 10 6 , all 
 in tonnes. The value of X determined from these 
 figures is 111 millions of years. This is the period 
 required for the accumulation of the sodium in the 
 ocean, according to Murray's average composition of 
 river- water and his statement of its total amount ; the 
 corrections already described being applied. 
 
 It is probable that this is the best result at present 
 attainable according to the method of solvent denu- 
 dation. It may be contended by some that a longer 
 duration of denudation might be arrived at if we 
 allowed for a greater circulation of ocean salts from 
 
URANIUM AND THE AGE OF THE EARTH. 245 
 
 the sea to the land. As we cannot be sure of the 
 precise correction to apply for this source of error, 
 it is important to show that a fixed and definite limit 
 can be assigned to the effect of this correction upon 
 the duration arrived at. 
 
 We have seen that in the ocean there is an excess 
 of chlorine over sodium, so that only 82 per cent, of 
 the chlorine can be combined with sodium. Now, let 
 us suppose that all the chlorine in rivers is derived 
 from the ocean, and, taking 82 per cent, of it, subtract 
 the equivalent sodium from the average river content 
 of that element ; regarding the balance only as due 
 to denudation. This procedure evidently constitutes 
 a maximum allowance for the circulation of salt : an 
 allowance obviously excessive, for we ignore the various 
 sources of chlorine rocks, volcanoes, springs, lodes, 
 ancient salt deposits (not marine), etc., which must 
 contribute some part of the chlorine, as well as the 
 possibility that free chlorine is evolved from the ocean. 
 
 When we carry out the calculation for the geological 
 age, proceeding as before, and making c = 0, we find the 
 period of denudation to be 141 millions of years. On 
 Murray's data this is a major limit to geological time. 
 
 The question as to whether Murray's average sodium 
 content of river-water will be confirmed with increase 
 of our knowledge of the subject, is a most important 
 one. We see, also, that the ratio of the sodium to 
 the chlorine is important, if we desire to refine upon 
 the result obtained by treating the whole of the 
 sodium as derived by denudation. A knowledge of 
 average inland rain-water, so far as its sodium content 
 
246 RADIOACTIVITY AND GEOLOGY. 
 
 is concerned, would, however, make us independent of 
 the chlorine content of rivers. We shall probably have 
 these data before very many years have gone over. It 
 is of interest meanwhile to note that recent advances 
 do not appear to suggest any very wide departure from 
 the results for the age which Murray's figures have 
 afforded. I am indebted to Professor Sollas for the 
 following computation, in part founded on Clarke's 
 collection of North American river analyses. 1 
 
 Professor Sollas allows for the wind-borne sodium by 
 deducting from the several rivers the sodium equivalent 
 of all the chlorine present. The balance appears in 
 the column headed " Sodium." We have first to recall 
 that this is an excessive reduction, for even on the 
 assumption that all the chlorine came from the ocean, 
 we are only entitled to suppose that 82 per cent, of it 
 brought in sodium. And of course even this is too 
 great an allowance. 
 
 Discharge per 
 annum, cubic miles. Sodium ; tons. 
 
 St. Lawrence, . . 70'591 . 1,093,000 
 
 Androscoggin, . . 0'652 . 3,537 
 
 Merrimac, . . 0'911 . 8,187 
 
 Mohawk, . . 1-290 . 12,285 
 
 James, . . 2*144 . 16,235 
 
 Cahaba, . . 0'394 . 2,990 
 
 Mississippi, . . 132-355 . 1,322,550 
 
 Rio Grande, . . 0'298 . 33,700 
 
 Pecos, . . 0-083 . 14,640 
 
 Colorado, . . 5 '746 . 1,832,000 
 
 Snoke, . . 1-557 . 74,410 
 
 Sacramento, . . 7 '613 . 211,960 
 
 223-634 4,625,494 
 
 Clarke : The Jjuta of Geochemistry, p. 60, 6t seq. 
 
URANIUM AND THE AGE OF THE EARTH. 247 
 
 Dividing the total sodium by the total annual dis- 
 charge of these rivers in cubic miles, we get a sodium 
 content of 20,684 tons per cubic mile. Murray's data 
 after deduction of the sodium chloride afford 17,501 
 tons of sodium per cubic mile. The new data obviously 
 point to a diminished result when applied to the calcu- 
 lation of geological time. 
 
 "We see, then, clearly that, going on collective facts 
 of any authenticity, the denudation period must be 
 less than 150 millions of years, taking a round number. 
 And so far as we know, this is reached only by excessive 
 assumptions. A subtractive correction of a few millions 
 of years might be called for on the score of direct 
 marine denudation of the coasts and shore deposits. 
 It is not probably a large correction, for the land area 
 exposed to denudation is, compared to the coastal area, 
 enormous. A primeval accelerated denudation, at the 
 time when the heat conducted to the surface of the 
 earth was greater than now, might entail a further 
 subtractive correction on the " age " ; for, as in the case 
 of coastal denudation, here, too, sodium is brought into 
 the ocean otherwise than by the rivers. These are minor 
 corrections, but they tend one way. Taking all into 
 account, until we know more about the origin of river- 
 borne chlorine, the computation which gave us 111 
 millions of years seems the best estimate we can 
 make. 
 
 The method of determining the geological age by the 
 sodium content of the rivers and ocean receives con- 
 firmation in the fact that moderate estimates of the 
 
248 RADIOACTIVITY AND GEOLOGY. 
 
 mass of the detrital rocks, taken along with their 
 average deficiency in sodium as compared with the 
 parent igneous materials, will account for the sodium in 
 the sea. In a previous chapter (Chapter VI) this has 
 been referred to in another connexion. This fact brings 
 vividly before our minds the perseverance of those 
 denudative processes which, age by age, attended the 
 formation of the sediments and brought the soluble 
 substances into the ocean. 
 
 As regards the uniformitarian basis which underlies 
 this method and the method by sedimentation, it is to 
 be observed that there now is (and possibly always has 
 been) an excess of land area beyond what denudation 
 seriously affects, i.e., the riverless or rainless regions of 
 the earth. This rainless area now amounts to about 
 one-fifth of the entire land surface. Hence the eleva- 
 tion of the continents can only shift the area of 
 denudation, and their depression cannot seriously 
 lessen it, until it has proceeded so far that more than 
 one-fifth the present land surface is submerged. 
 
 As to the legitimacy of prolonging into the past 
 climatal conditions similar to those which at present 
 prevail upon the earth, the testimony of the rocks, and 
 the evidence to be derived from their fossil remains, are 
 at one. Sir Archibald Geikie brings the geological 
 testimony on this point forcibly before us in his 
 opening address to Section C at the British Association 
 meeting of 1899. Geologists, he points out, have found 
 no confirmation for the view that greater denudative 
 activity existed in the past : " On the contrary, they 
 
URANIUM AND THE AGE OF THE EARTH. 249 
 
 have been unable to discern any indication that the 
 rate of geological causation has ever, on the whole, 
 greatly varied during the time which has elapsed since 
 the deposits of the oldest stratified rocks." He instances 
 the evidence of measured tranquillity in the deposition 
 of the ancient Torridonian sediments, and the fact that 
 the same tale, with endless additional details, is told all 
 through the stratified formations down to those which 
 to-day are in course of accumulation. 
 
 The zoological aspect of the same question is considered 
 by Poulton, 1 and similar conclusions are arrived at. The 
 evidence of fossil organisms seems to be incontestable. 
 It is to be read in the proportions and structures of 
 early woody plants and trees, in the wings of birds and 
 insects, and in the existence of frail insect life in early 
 times. And this evidence is greatly strengthened when 
 the remarkable influences of severe climatal conditions, 
 as witnessed to-day, on winged animal life, and on 
 arboreal life, are considered. Thus while fossil insects 
 from the Carboniferous display great powers of flight 
 (a fossil dragon-fly showing upwards of two feet of 
 wing-expanse), we see in storm-swept countries of the 
 world (e.g., Kerguelen Land) the whole of the insect 
 fauna rendered wingless. Similarly, while forests of 
 slender Cryptogams flourished in later Palaeozoic times, 
 we find to-day that the gales on our sea-coasts practically 
 inhibit arboreal life. 
 
 We conclude that the former physical conditions 
 must have been much as now ; and the view that 
 stupendous rains, hurricanes, and tides then existed 
 
 1 Poulton, Address to Section D, Brit. Assoc. Jiep. } 1896. 
 
250 RADIOACTIVITY AND GEOLOGY. 
 
 is unsubstantiated by any records at our disposal. As 
 already remarked, however, had such conditions indeed 
 existed, an accelerated rate of denudation, and not a 
 retarded one, must have been the result. 
 
 The work of Sir George Darwin, based on the theory 
 of a genetic connexion between earth and moon, assigned 
 a minor limit of fifty-six millions of years to the time 
 since their separation. This, however, does not touch 
 the point at issue here. Similarly we have seen how 
 Kelvin's estimate of time since the consistentior status can 
 be lengthened by the heating effects of radioactivity. 
 
 With the denudation method of estimating the age 
 we close the case against the present indications of 
 the radioactive method. Can the nineteen rivers upon 
 which Murray's figures are based have given four or five 
 times the correct sodium average ? Here seems to be 
 the only possible opening for serious error. But recent 
 data suggest no possibility of so complete a failure of 
 the original estimates. And this being so, what are we 
 to conclude ? That the parentage of the stored helium 
 is not altogether to be assigned to the uranium family of 
 elements seems to be the most natural answer, if, indeed, 
 the migration of radioactive materials be proved an 
 inadequate explanation. With reference to the more 
 venerable ages arising out of the storage of lead in 
 primitive minerals (over and above the possibilities of 
 error), we must bear in mind the possibility that these 
 materials may date their radioactivity back, even 
 through many vicissitudes, to times before our denuda- 
 tive period began. 
 
 With an interest almost amounting to anxiety, 
 
URANIUM AND THE AGE OF THE EARTH. 251 
 
 geologists will watch the development of researches 
 which may result in timing the strata and the phases 
 of evolutionary advance ; and may even going still 
 further back give us reason to see in the discrepancy 
 between denudative and radioactive methods, glimpses 
 of past aeons, beyond that day of regeneration which 
 at once ushered in our era of life, and, for all that went 
 before, was " a sleep and a forgetting." 
 
CHAPTEK XII. 
 
 KADIUM MEASUREMENTS IN PRACTICE. 
 
 THE ELECTROSCOPE AND ITS CONSTRUCTION RATE OF ACCUMULATION 
 OF THE EMANATION OF RADIUM TABLE FOR REFERENCE EXPERI- 
 MENTAL STANDARDIZATION OF THE ELECTROSCOPE MODE OF USING 
 THE ELECTROSCOPE CONDITIONS OF ACCURACY IN PREPARING 
 SOLUTIONS TREATMENT OF ROCKS AND OTHKR SUBSTANCES PRE- 
 CAUTIONS AGAINST CONTAMINATION MODE OF BOILING-OFF AND 
 TRANSFERRING THE EMANATION SOURCES OF OCCASIONAL VARIA- 
 TIONS IN REPETITION EXPERIMENTS. 
 
 IN the earlier chapters sufficient has been said to 
 enable the general theory of radium measurement to 
 be understood. It is intended in what follows to give 
 an account of the details of apparatus and procedure, 
 such as will enable those having the requisite manipu- 
 lative training to pursue this line of research. 
 
 The Electroscope. The electroscope (fig. 3) is very 
 simple in construction, and may, in fact, be made from 
 a round glass flask of about 400 to 500 cubic centimetres' 
 capacity, by cutting off most of the neck. A cork 
 which has been boiled in paraffin wax is fitted to the 
 neck and is perforated in two places. One perforation 
 takes a glass tube leading to the drying-tube, through 
 which gas is removed or admitted to the electroscope. 
 The drying-tube contains phosphoric anhydride, with a 
 close-pressed plug of cotton wool inserted between the 
 powder and the flask, so as to avoid any possibility of 
 
RADIUM MEASUREMENTS IN PRACTICE. 253 
 
 the drying material being drawn into the electroscope. 
 A glass stopcock at the outer end of the drying-tube 
 controls the entry of gas ; and for greater safety a short 
 length of capillary tube is inserted between it and the 
 dry ing- tube. This guards the electroscope from too 
 rapid an inflow of gas. The second perforation in the 
 cork takes a short length of glass tube, which in turn 
 carries an inner sulphur tube of fine bore about one 
 half millimetre supporting the gold-leaf system. 
 
 FIG. 3. 
 
 The sulphur tube is prepared as follows: Into a 
 length of about 30 centimetres of glass tubing, melted 
 sulphur is drawn ; by air-suction, one end of the tube 
 dipping into the vessel of sulphur. After a moment 
 the liquid sulphur is let run back out of the tube, 
 and is again drawn up ; the process being repeated 
 till a sufficient thickness of sulphur is deposited on 
 
254 RADIOACTIVITY AND GEOLOGY. 
 
 the walls of the glass tube. This leaves a fine-bore 
 sulphur tube within the glass. Letting this stand a 
 few hours, it will be found that the sulphur has 
 shrunk away from the glass, and if now a length of 
 about 6 centimetres is cut from the glass tube, the 
 tube within may be pushed forward so as to project 
 a couple of centimetres from one end. This projecting 
 part must not be touched by the fingers at any time. 
 To make the sulphur tube fit air-tight in the glass, 
 it is requisite to put a little thick Canada balsam just 
 where it emerges from the glass. This creeps in 
 and hardens, making a perfect joint. At this stage the 
 glass tube supporting the sulphur tube is inserted in 
 the cork, in such a way that the extremity of the 
 sulphur reaches downwards to a point rather above the 
 centre of the flask. A flat piece of brass about 2J 
 centimetres by 0.3 centimetre is attached while hot to 
 the end of the sulphur tube, centrally, so that it crosses 
 the bore of the tube. With a warm penknife the 
 attachment of the brass strip is made air-tight in the 
 bore of the sulphur tube. 
 
 The gold leaf is finally attached. It is a little shorter 
 than the brass strip and about the same width, and is 
 attached by a trace of gum just where the brass leaves 
 the sulphur. 
 
 When the cork is finally inserted in the flask, the fit 
 must be made perfectly air-tight, by melting the paraffin 
 around and over the cork. It should be mentioned 
 that in order to earth the inner wall of the flask a 
 couple of tin-foil strips are brought in beside the cork 
 and gummed to the interior of the flask. These strips 
 
RADIUM MEASUREMENTS IN PRACTICE. 255 
 
 bend down outside, meeting the glass where it is free 
 from paraffin. 
 
 The electrification of the leaves is effected by 
 inserting through the bore of the sulphur tube a fine 
 platinum wire carried in a sulphur handle. This 
 makes contact with the sealed-in end of the brass 
 strip, and when a charged body is applied to it above, 
 the leaves diverge. The divergence when using the 
 instrument should not be greater than about 30 degrees. 
 When the proper divergence is secured, the platinum 
 wire is withdrawn. 
 
 The motion of the leaf is observed by a reading 
 microscope, with a scale in the eyepiece, dividing the 
 field into 100 divisions. A good stopwatch is necessary 
 when the rates of discharge are rapid. The time re- 
 quired for the extremity of the leaf to move over, say, 
 ten divisions, is noted and the rate converted to so 
 many scale divisions per hour. 
 
 The electroscope is standardized in the following 
 manner : One-fifth of a gram of Joachimsthal pitch- 
 blende is carefully weighed out and dissolved in pure 
 nitric or hydrochloric acid. The solution is made up 
 to 200 cubic centimetres with distilled water. Thus 
 one cubic centimetre contains one milligram of the 
 ore. To effect the calibration, 5 cubic centimetres are 
 measured out in a pipette into a clean flask, and about 
 600 cubic centimetres of distilled water added. This is 
 boiled vigorously for twenty minutes. The flask is then 
 closed by a good rubber stopper, and set aside for about 
 twenty-four hours. At the end of , this time the contents 
 of the flask are again boiled, and, by a procedure to be 
 
256 RADIOACTIVITY AND GEOLOGY. 
 
 described later, the boiled-off air and emanation are 
 transferred in their entirety to the electroscope. 
 During the time of closure of the flask (which must 
 be carefully noted) the radium present in the solution, 
 which was deprived of any original emanation by the 
 preliminary boiling, is generating fresh emanation, and 
 the amount of emanation generated is proportional to 
 the quantity of radium present and to the period 
 during which it collects. 
 
 The quantity of emanation formed depends also on 
 the rate at which it is derived from the radium, and 
 the rate of its own transformation. The first rate is 
 a steady one over the time interval concerned, and 
 we may consider that the radium produces continually 
 n particles per second. The rate of transformation of 
 the emanation being relatively rapid cannot be ignored. 
 We may say 1 that when N particles are accumulated 
 the number transforming per second is \N, where A is 
 the radioactive constant of the emanation or fraction 
 transforming in one second. Obviously the number 
 changing is variable, increasing till the equilibrium 
 value is reached. 
 
 At any time t, however, the net rate of increase of 
 the emanation must be n - \N\ or 
 
 - -\N 
 
 The solution of this equation is : 
 
 where a is a constant. 
 
 To find the value of the constant write t = 0, and 
 
 1 See Rutherford's JRadioactivity. 
 
RADIUM MEASUREMENTS IN PRACTICE. 257 
 
 then the first term on the right is equal to a ; and as 
 
 *Y) W 
 
 when t = 0, -2V must = 0, we have a + ^- = 0, or r- = - & ; 
 
 A A 
 
 also when t = oo, we have the first term on the right of 
 the equation equal to 0; and as then the maximum 
 
 number, N , is formed, we have ^ = N ; and the equation 
 becomes : 
 
 N-N 9 -Njru. (1) 
 
 The value of A for emanation is found by experiment 
 to be 216 x 10- 6 . 
 
 "We will now apply the equation to calculate the 
 amount of emanation accumulated during, say, twenty- 
 four hours. In this case t in seconds is 864 x 10 2 . The 
 numerical value of \t is therefore 018662. 
 
 The last member of the equation is best evaluated 
 logarithmically. Thus if we let x = N e~ xt , we have 
 logx = log^Vo - A log e. 
 
 The value of log e is 043429, and if we let N = 100, 
 the log of which is 2-00000, we find log a? = 1-91896 ; 
 from which x = 83*0. This has now to be subtracted 
 from 100, and finally we have from (1) N= 17. That 
 is to say, there is 17 per cent, of the emanation accumu- 
 lated in twenty-four hours. 
 
 The trouble of making this calculation each time 
 we calibrate the instrument, or each time we deal 
 with a substance which has been enclosed for less than 
 the time required to secure the equilibrium amount 
 of emanation, may be avoided by plotting a curve to 
 a large scale once for all, in which the abscissae are the 
 times since closing up the solutions, and the ordinates 
 the percentage collected proper to the time, derived 
 
 s 
 
258 
 
 RADIOACTIVITY AND GEOLOGY. 
 
 by solving the equation as above. The following table 
 will enable such a curve to be plotted, or may be used 
 directly for ordinary work. 
 
 Time of collection of 
 emanation. 
 
 Percentage of 
 eqnilihrium amount of 
 emanation accumulated. 
 
 6 hours. 
 
 4-55 
 
 12 
 
 8-9 
 
 18 
 
 13-1 
 
 24 
 
 17-0 
 
 30 
 
 20-8 
 
 36 
 
 24-4 
 
 42 
 
 27-8 
 
 2 days. 
 
 31-1 
 
 21 
 
 37-3 
 
 3 " 
 
 42-9 
 
 "2 " 
 
 47'9 
 
 4 
 
 52-6 
 
 4J 
 
 56-8 
 
 5 
 
 60-6 
 
 5f ,, 
 
 64-2 
 
 ^ 
 
 67-4 
 
 6| 
 
 70-3 
 
 7 
 
 72-9 
 
 8 ,, 
 
 77-6 
 
 9 
 
 81-3 
 
 10 
 
 84-5 
 
 12 
 
 89-4 
 
 14 
 
 927 
 
 16 
 
 94-9 
 
 18 
 
 96-5 
 
 20 
 
 97-6 
 
 In standardizing the electroscope according to the 
 
RADIUM MEASUREMENTS IN PRACTICE. 259 
 
 foregoing procedure, we, of course, require to know 
 the amount of radium present in the standardizing 
 solution. This we find by a chemical analysis of the 
 uraninite or pitchblende, determining the uranium 
 only. From this we can derive the radium, as the 
 proportion in which the two exist in old minerals is 
 constant. For every gram of uranium we have, ac- 
 cording to Boltwood's latest determination, 3*4 x 10~ 7 
 grams of radium. 1 
 
 The uranium is best determined by Patera's method, 
 given in Fresenius' Quantitative Analysis (vol. ii., 
 p. 310). From the results of the analysis, the per- 
 centage of metallic uranium is calculated. For instance, 
 in a particular analysis, the percentage came out as 
 594. Taking this as 60 per cent, (as there is generally 
 a slight deficiency in the amount determined by 
 analysis), and assuming that 5 milligrams of the ore 
 had been used in a standardizing experiment, we have, 
 
 for the radium : 
 fiO 
 5 x 10- 3 x x 34 x 10' 7 = 1020 x 10- 13 grams. 
 
 Suppose now the time of closure of this amount of 
 radium had been 24 hours, then the quantity of radium 
 represented in the increased rate of discharge of the 
 electroscope would be 
 
 x 1020 v 10- 1 ' - 173 x lO' 12 . 
 
 This, in fact, is the quantity of radium which would 
 produce the observed effect on the electroscope, if the 
 emanation in equilibrium with it had been collected. 
 
 1 Boltwood, Am. Journ. Science, June, 1908, p. 504. 
 S2 
 
260 RADIOACTIVITY AND GEOLOGY. 
 
 We now see that the calibration is easily completed. 
 For suppose the normal leak of the instrument is 
 15 scale-divisions per hour, and the leak, three hours 
 after the introduction of the emanation, was 335 scale- 
 divisions per hour, then the gain has been 320, and this 
 is due to the emanation in equilibrium with 173 x 10" 12 
 grams of radium. The constant of the electroscope is 
 found from this by calculating how much radium would 
 give rise to an increased rate of discharge of 1 scale- 
 division per hour : that is, we divide the quantity of 
 radium by 320 the final result being that the constant 
 is 0-54 x 10- 12 . 
 
 When now in future experiments the gold leaf 
 shows an increased rate of collapse, we have only to 
 multiply by this constant, and we get the radium 
 responsible. For example, if the increased rate was 
 only 40 divisions, then the radium present in the 
 solution was 21*6 x 10~ 13 , and if 10 grams of rock had 
 been contained in the flask, we enter as the radio- 
 activity of the rock, 2'2 x 10' 12 . 
 
 This mode of using the electroscope renders the 
 method entirely comparative; and when the analysis 
 of the standardizing substance preferably pitchblende 
 (uraninite) has been made, we are in no doubt as to 
 the indications of the instrument. It will be found that 
 successive standardizing experiments give sensibly the 
 same constant, even when the quantities of emanation 
 introduced are varied largely. This certainly holds up 
 to, and probably over, 800 scale-divisions per hour rate 
 of discharge. If the electrpscope is to keep its normal 
 rate of discharge constant, there must be careful 
 
RADIUM MEASUREMENTS IN PRACTICE. 261 
 
 attention to the exhaustion of the instrument before 
 and after each experiment: the electroscope in each 
 case being refilled with air. Occasional renewal of 
 the drying material is of course required. 
 
 Preparation of Materials and experimental Procedure. 
 The extraction of the emanation from a solution and its 
 transference to the electroscope may now be considered. 
 
 The conditions conducive to accuracy in these opera- 
 tions are : (1) The preparation of a solution free from 
 any trace of precipitate, or accidental contamination 
 with radioactive matter ; (2) Thorough boiling of the 
 solution when extracting the emanation, the ebulli- 
 tion being of the effervescent type (i.e., with very 
 numerous small bubbles) ; (3) Complete transference to 
 the electroscope of the boiled-out gases and inter- 
 mixed emanation. 
 
 To successfully attain the first condition is in the 
 case of certain rocks a matter of considerable difficulty. 
 The mode of preparation of a solution is as follows : 
 The rock is ground in the agate mortar till it passes a 
 sieve of 120 meshes to the inch. Ten to fifteen grams 
 are then weighed out, and mixed with the anhydrous 
 carbonates of sodium and potassium (taken in equal 
 proportions); the proportion of the carbonates to the 
 rock varying from twice to two and a-half times the 
 weight. The whole, being well mixed, is fused in a 
 platinum crucible having a capacity of not less than 90 
 c.cs. In this process the blowpipe flame must not play 
 into the crucible. The first part of the fusion may 
 with advantage be carried out with a strong bunsen, 
 and, after twenty minutes' treatment, the foot-bellows 
 
262 EADIO ACTIVITY AND GEOLOGY. 
 
 and blowpipe should be brought into operation. When 
 effervescence ceases, the contents of the white-hot 
 crucible are tilted into a dry platinum dish. The 
 hardened melt, collected from the crucible and the 
 dish, is then crushed to fine fragments, thrown into 
 distilled water (400 c.cs.), and left to digest on the 
 water-bath; the beaker containing the solution being 
 kept covered with a clock glass. 
 
 After some hours if the melt is not broken up in 
 the water, the hard parts are separated and crushed 
 in a biscuit-ware mortar and again returned to the 
 beaker. Finally the contents of the beaker are filtered, 
 and the filtrate, which must be quite limpid, transferred 
 to a flask and closed as the alkaline solution. The 
 residue is now stirred up in a little distilled water 
 and treated with about 40 to 60 c.cs. of pure hydro- 
 chloric acid which has been distilled in the laboratory 
 over chloride of sodium. The re-distillation of the 
 acid is necessitated by the presence of radium in most 
 " pure " HC1 on the market, and, what is a source of 
 error in the opposite direction, the presence of sulphuric 
 acid. If the rock is of the acid class (that is, has a high 
 percentage of silica), the solution with the acid will in 
 most cases be complete, and the liquid may at once be 
 closed in a flask as the acid solution. The two flasks 
 are then set aside, being numbered in correspondence 
 with the laboratory notes, and dated. It is necessary to 
 leave an air space in the flasks, such as will allow of 
 subsequent ebullition, but no more about 100 c.cs. 
 The flasks should be of about 600 c.cs. capacity. 
 
 If the action of the acid results in an undissolved 
 
RADIUM MEASUREMENTS IN PRACTICE. 263 
 
 residue, this must be carefully filtered off, dried on 
 the filter paper, which is then ignited, and the ash 
 and residue re-fused in the platinum crucible with a 
 small quantity of the mixed carbonates. It will now 
 be found, in nearly all cases, that the melt goes into 
 complete solution when leached with hot distilled water 
 and treated with a few c.cs. of HC1. The solution is 
 added to the contents of the " acid " flask. 
 
 In this process it will be seen that a much less 
 quantity of chemicals is used to effect decomposition 
 than is ordinarily recommended ; it will be found, 
 however, that the quantities stated are quite adequate. 
 It is important to keep down the amount of chemicals, 
 for not only will the carbonates certainly contain some 
 radium, but the presence of much dissolved matter 
 hinders the ebullition and the free evolution of the 
 emanation. Tests must be applied to the carbonates 
 by dissolving and closing up quantities of about 100 
 grams, and then boiling off any emanation, etc., as in 
 the ordinary experiment. The correction for radium 
 may amount to about 013 x 10~ ls grams per gram of 
 mixed carbonates. 
 
 It sometimes happens that a solution put by 
 develops a faint turbidity after some days. Often 
 this cannot be filtered out, and if the solution is 
 retained for experiment, it must be noted that the 
 resulting reading is probably only a minor value, 
 unless the precipitate is observed to go into complete 
 solution on ebullition. It is, indeed, very generally 
 the case that we are not sure of obtaining more 
 than a minor limit to the true quantity of radium 
 
264 RADIOACTIVITY AND GEOLOGY. 
 
 present. It is easy to understand the reason for 
 this. The total amount of emanation developed in 
 the solution is extremely small in volume. The 
 recent experiments of Kutherford 1 have shown that 
 the volume of the emanation in equilibrium with one 
 gram of radium is 0*585 cubic millimetre. It results 
 that in a solution of ordinary radioactivity there are 
 only a few billionths of a cubic millimetre of the 
 gas when in equilibrium amount. Now, if any cause 
 leads to the precipitation of the radium, it is certain 
 that the true amount is not boiled off, no matter 
 how prolonged the ebullition. A trace of sulphuric 
 acid appears to lead to this result, provided any 
 barium is also present in solution ; there are com- 
 paratively few rocks without it. This result can 
 be demonstrated experimentally. It will be remem- 
 bered that radium is an element of the barium group, 
 and that its sulphate is at least equally insoluble. From 
 this it will be seen how important is the attainment 
 of limpidity in the solutions. It may be mentioned in 
 this connexion that it is not uncommon for a second 
 experiment on the same solution to show a diminished 
 yield of emanation if a precipitate has developed since 
 the first experiment was made. The really astonishing 
 fact is the removal of so small an amount under any 
 circumstances. 
 
 Some of the materials dealt with in research are, of 
 course, much more easily prepared than siliceous rocks. 
 Carbonates, manganese nodules, haematite, gypsum, etc., 
 may often be brought into solution by pulverization 
 
 1 Rutherford, Proc. E. S., A, vol. 81, 1908, p. 162. 
 
RADIUM MEASUREMENTS IN PRACTICE. 265 
 
 and heating with hydrochloric acid, and may then be 
 diluted and bottled without further trouble. Eesidues, 
 in such cases, however, always involve risk of error 
 unless there is reason to believe that they are purely 
 siliceous. Sea waters and river waters are preferably, 
 but not necessarily, concentrated by evaporation. In 
 all cases they should be acidified by hydrochloric acid. 
 
 Regarding the precautions to be taken against the 
 introduction of radium contamination, the test of 
 performing a blank experiment must not be omitted ; 
 that is, the whole procedure must be gone through 
 with the chemicals only. Again, the experiments are 
 not reliable if performed in a room wherein radioactive 
 preparations have previously been dealt with at any 
 time. The investigator must not himself be concerned 
 with radium preparations while carrying out these 
 delicate measurements ; and if he has been so engaged 
 at any time in the past, he must test (by placing them 
 in close proximity to a suitable electroscope, or bringing 
 them near to a sensitive screen of phosphorescent zinc 
 sulphide) any articles likely to retain traces of adhering 
 radium, as, for instance, the handles of the laboratory 
 doors, his watch, keys, or other articles carried in his 
 pockets. Further, it is necessary to discard the use of 
 any apparatus formerly in use in connexion with radio- 
 active salts. 
 
 The attainment of the second and third conditions 
 of accuracy, i.e., the free ebullition and perfect transfer 
 to the electroscope, requires special arrangements. 
 
 In fig. 4 (p. 266) the receiver R is of about 500 cubic 
 centimetres capacity. It has two tubulures. During 
 
266 
 
 RADIOACTIVITY AND GEOLOGY. 
 
 ebullition it is placed as shown, in connexion with the 
 flask containing the solution ; a glass condenser, in 
 which cold water circulates, intervening. The attach- 
 ment of the flask to the condenser must be rapidly 
 performed. Before making up these connexions the 
 
 FIG. 4. 
 
 receiver is exhausted of air; thus when, after the 
 connexions are made, the pinch-cock B is opened, the 
 pressure falls in the flask. This condition both enables 
 ebullition to be maintained without risk in the closed 
 
RADIUM MEASUREMENTS IN PRACTICE. 267 
 
 system of vessels, so long as the water circulates in the 
 condenser, and facilitates the withdrawal of the gases 
 dissolved in the solution. The tubing forming the 
 connexions must be thick, soft rubber, and seamless. 
 
 Ebullition is carried on for half an hour, and the 
 effervescent type of boiling is secured very simply by 
 putting a small trace of powdered steatite into the 
 solution. This is best done by dropping the powder, 
 before making the connexions, into the tube of the 
 condenser, where it adheres till the condensed steam, 
 dripping back into the flask, carries it down into the 
 solution. There is very little radium to be expected in 
 this substance ; for safety it should be tested, however. 
 
 When ebullition has proceeded vigorously for about 
 thirty minutes, the burner is extinguished, and cold 
 distilled water is let flow in at the tube E which passes 
 through the rubber stopper fitting the condenser to the 
 flask; this tube being kept closed, throughout the 
 boiling, by a pinch-cock. The distilled water is let fill 
 the air space in the flask and rise into the lower tubulure 
 of the receiver above. Thus the entire quantity of 
 boiled- out gas is driven into the receiver. The pinch- 
 cock B is then closed on the rubber connexion with 
 the condenser, and the receiver removed to the electro- 
 scope. The air being pumped out of the latter, the 
 receiver is attached in such a manner that on opening 
 the connexions the gaseous contents are drawn into the 
 electroscope, while water rises and fills the receiver. 
 Thus a complete transfer is effected. An important 
 feature of this arrangement is that the emanation is 
 as short a time as possible in contact with water in 
 
268 RADIOACTIVITY AND GEOLOGY. 
 
 which it may be absorbed. Its solubility is known to 
 be as much as one-twentieth that of carbon dioxide; 
 considerably exceeding that of oxygen. 
 
 Before the transfer it is advisable to keep the gold 
 leaf for some time under observation, to make sure 
 that the leak is normal. About ten minutes after 
 the transfer, a reading of the rate of discharge 
 should be taken, and, as before stated, again in three 
 hours. A comparison of the readings assures us that 
 we are dealing with the emanation of radium, and 
 eliminates any possibility that a spurious effect may 
 obtain. The leak will be increased about 30 per cent, 
 in the three hours. 
 
 When the material under investigation has to be 
 divided between an acid and an alkaline solution, the 
 separate examination of each solution may be simul- 
 taneously carried out by duplicating the apparatus 
 shown in figure 4. Two electroscopes should be kept 
 in readiness for dealing with the emanation withdrawn. 
 In this way two solutions may be examined with hardly 
 more expenditure of time and labour than would be 
 required for one. 
 
 It sometimes will be found that successive results 
 with certain rocks, even from the same hand specimen 
 are not consistent. This may arise from failure to 
 effect complete extraction of the radium in one of the 
 experiments, or and if the solutions are well prepared, 
 this is the most probable explanation it may be due 
 to a real want of homogeneity in the radium distribu- 
 tion in the rock. In an exaggerated way we experience 
 in such cases all the difficulties which beset the assay 
 
RADIUM MEASUREMENTS IN PRACTICE. 269 
 
 of precious metal in a rock in which the ore is spora- 
 dically distributed ; exaggerated because an invisibly 
 minute aggregate of a uranium ore can exert a con- 
 siderable influence on the experimental result. It is 
 suggestive that cases of inconsistency are seldom met 
 with in sedimentary rocks which are not highly meta- 
 morphosed, but retain their original state of subdivision. 
 It is probable that in sediments derived by precipitation 
 there is very perfect distribution of the uranium. 
 This is borne out by the consistency of the results on 
 deep-sea sediments, even in the case of materials of the 
 same character from widely separated parts of the ocean. 
 In recent lavas, again, a close consistency in successive 
 experiments is generally observed. 
 
 The pleochroic haloes, which have been described in 
 Chapter III, provide us with ocular demonstration 
 of the sporadic distribution and segregation of the 
 uranium in certain ancient rocks. 
 
APPENDIX A. 
 
 I. Rise of temperature in a radioactive layer of thickness 
 Z>, losing heat only at the surface, and not receiving heat 
 from helow. The surface is supposed to he maintained at a 
 constant temperature from which the temperatures in the 
 layer are reckoned. The layer has existed so long that 
 there is equilibrium "between the radioactive supply of heat 
 and the surface loss. 
 
 Let Q + -=- dx units of heat enter at lower surface of 
 dx 
 
 an elementary volume having the thickness dx, and at the 
 distance x from the surface, and let Q units emerge at its 
 upper surface in unit time ; then 
 
 dQ 
 
 - dx = qhAdx, 
 ctx 
 
 where q is the radium- content of unit volume, and h the 
 heat developed per second per gram of radium; A is the 
 area. Hence 
 
 Also Q = AK -, where -= is the gradient at the 
 ax dx 
 
 distance x from the surface, and K the conductivity ; and 
 
 ,, . dQ #0 
 
 therefore ~^~ = A E ,, , 
 
 dx d x 
 
 and K-j = -qh. 
 
 dx z 
 
 Integrating, JT = - qhx + C^ 
 
272 APPENDIX A. 
 
 flfi 
 Now, - = when x = D, i.e. at base of layer, therefore 
 
 Hence K = - qhx + qhD = qh (D - x). 
 Integrating again, 
 
 but when as = 0, = 0, therefore <7 2 = 0. 
 
 Therefore, ^.^/)-|V (i) 
 
 and when x = D, 
 
 '&. (2) 
 
 II. Kise of temperature in a sphere of radioactive materials, 
 of radius R, which has existed so long that there is equi- 
 librium between the heat generated within the sphere and 
 the heat escaping at the surface. The surface is supposed 
 to be maintained at a constant temperature from which 
 temperatures within are reckoned. 
 
 Let Q units of heat enter in unit time a concentric 
 spherical layer of radius r, and of thickness dr ; and let 
 
 dQ 
 
 Q + -7- dr units emerge. 
 dr 
 
 dQ . 
 
 dr = 4frr z dr .q.h, 
 
 where q and h have the same significance as in (1). 
 Also, 
 
 Q = -^f = -^T.4^.f, 
 dr dr 
 
 nfi 
 
 where is the gradient at the distance r from the centre. 
 dr 
 
APPENDIX A. 273 
 
 Integrating, 
 
 but when r = 0, = ; .-. Ci = 0, 
 
 and -5T ~T~ = ~" 2 "5 * 
 
 Integrating again, 
 
 but = when r = R : 
 
 and ^ = (^-^). (3) 
 
APPENDIX B. 
 GEOPHYSICAL AND RADIOACTIVE DATA. 
 
 The Earth. 
 
 Mean radius, .... 6-37096 x 10 8 cms. 
 
 Volume, . . . . . 1-083 x 10 27 cub. cms. 
 
 Surface, . . . . . 51 x 10 17 sq. cms. 
 
 Mass, -A . 5-987 x 10 27 grams. 
 
 Mean density, .... 5-527. 
 
 The Ocean, Land and Rivers. 
 
 Area of land on the globe, . . 14,456 x 10* sq. kiloms. 
 
 Ratio of land to water, . . 1 : 2-54 (Wagner 1 ). 
 
 Area of the ocean, . . . 36,718 x 10 4 sq. kiloms. 
 
 Mean depth, .... 3- 851 kiloms. (Murray 2 ). 
 
 Volume, ..... 1,414 x 10 6 cub. kiloms. 
 
 Mass, 1,452 x 10 15 tonnes. 
 
 Density of average sea-water, . 1'027. 
 
 Mean radium content of sea-water, 0-017 x 10~ 12 grams per 
 
 cub. cm. (J.). 
 
 Total salts in the ocean (if 3 -5 / ), 508 x 10 14 tonnes. 
 
 Mass of sodium, . . . . 1,555 x 10 13 tonnes. 
 
 Mass of chlorine, . . . 2,877 x 10 13 tonnes. 
 Annual river discharge, 27,191 cub. kiloms. (Murray 8 ), or 
 27 x 10 12 tonnes, nearly. 
 
 1 Wagner, Scottish Geographical Mag., 1895, p. 185. 
 
 2 Sir J. Murray, ibid., 1888, p. 1. 
 
 3 Ibid., 1887, p. 76. 
 
APPENDIX B. 275 
 
 Sodium carried annually by rivers to ocean, 15,976 x 10* 
 
 tonnes (Murray 1 ). 
 Chlorine carried annually by rivers to ocean, 847 x 10 5 
 
 tonnes (Murray 1 ). 
 
 The Rocks and Sediments. 
 Mass of igneous rock, parent to sodium in the ocean and to 
 
 sediments, 833 x 10 15 tonnes (J.). 
 Mass of detrital sediments derived from parent rock, 558 x 10 16 
 
 tonnes (J.). 
 Mass of precipitated sediments on ocean floor, 195 x 10 16 
 
 tonnes (J.). 
 Estimated mass of calcareous rocks on the land, 29 x 10 15 
 
 tonnes (p. 58). 
 Mean radium content of igneous rocks, 5*5 x 10" 18 grams per 
 
 gram (J.). 
 Mean radium content of sedimentary rocks, 4*3 x 10~ 12 grams 
 
 per gram (J.). 
 
 Average sodium content of igneous rocks, 2'43 % (Clarke). 
 Average sodium content of sedimentary rocks, 0'9 to 1/ 
 
 (Clarke). 
 
 Thermal and Radioactive Constants. 
 Heat escaping from the earth per second per sq. cm. on 
 
 gradient of 32 metres to 1 C., 125 x 1Q- 8 gram 
 
 degrees. 
 
 Ditto, per annum for whole earth, 2 x 10 20 gram degrees. 
 Atomic weight of uranium, 238-8. 
 Atomic weight of radium, 226*4. 
 Half period of uranium, 5 x 10 9 years. 
 
 Fraction of uranium transforming each year (X), 1*4 x 10~ 10 , 
 Half period of radium, 1760 years. 
 Life of radium, 2540 years. 
 
 1 Sir J. Murray, Scottish Geographical Mag., 1887, p. 76. 
 
276 APPENDIX B. 
 
 Fraction of radium transforming each year (A.), 3*94 x 10~*. 
 
 Fraction of radium emanation transforming each second (X), 
 2-16 x 10- 6 . 
 
 Quantity of radium in equilibrium with 1 gram of uranium, 
 3-4 x 10" 1 (Boltwood). 
 
 Helium generated per year per gram of radium in equilibrium 
 with the related elements, 316 cub. mms. (Ruther- 
 ford). 
 
 Number of a-particles emitted per second per gram of radium 
 itself, 3-4 x 10 10 (Rutherford). 
 
 Volume of emanation in equilibrium with 1 gram of radium, 
 0'585 cub. mms. (Rutherford). 
 
 Number of ions created by an a-particle of radium, about 
 10 5 . 
 
 Charge carried by an a-particle, 9-3 x 10~ 10 E. S. Unit. (Ruther- 
 ford). 
 
 Mass of an a-particle, 6-18 x 10" 24 grams (Rutherford). 
 
 Average velocity of an a-particle from radium in equilibrium 
 with its products of rapid change, 1-77 x 10 9 cms. per 
 sec. (Rutherford). 
 
 Heat given out per second per gram of radium in equili- 
 brium with the related elements, 5-6 x 10~ 2 gram 
 degrees. 
 
 Ditto, in one year, 176*4 x 10 4 gram degrees. 
 
 Total heat given out during radioactive transformation of 
 1 gram of uranium (about), 4-48 x 10 9 gram degrees. 
 
 The radio-thermal constants are deduced as follows : 
 
 1. A standard preparation of radium showed by direct 
 experiment 110 gram degrees per gram per hour = 0-0306 
 gram degrees per second. This involves the activities of 
 radium, emanation, radium A, radium B, and radium C. 
 
 Now, Boltwood (Am. Jonrn. Science, April, 1908) found 
 
APPENDIX B. 277 
 
 the relative ionizing activities of the several members of the 
 uranium family to be as follows : 
 
 Uranium, .... 
 
 Ionium, .... 
 
 Radium, .... 
 
 Emanation, .... 
 
 Radium A, .... 
 
 B, . . . . 
 
 C, . . . . 
 
 ,,;-, - 
 Actinium products, . 
 
 4-64 x uranium. 
 
 Direct experiments on uranium minerals afforded the 
 nearly identical result, 4-69 x uranium. 
 
 The sum of the activities involved in the heat measure- 
 ment is 2*56. Hence the total heat will be found from the 
 proportion 
 
 x : -0306 : : 4-64 : 2-56, 
 from which 
 
 x = 5-5 x ID' 2 . 
 
 The value 4-69 x uranium gives x= 5*6 x 10~ 2 gram degrees 
 per second per gram of radium. 
 
 2. By measurement of the electrostatic deflexion, and of the 
 magnetic deflexion of the a-particles from radium C, Ruther- 
 ford (Phil. Mag., Oct., 1906), determined their initial velocity 
 to be 2*06 x 10 9 cms. per second. The mass of an a particle 
 is 3'84 times that of an atom of hydrogen, or 6-18 x lO" 24 gram 
 (Rutherford, Proc. R. 8. 9 vol. 81A, 1908, p. 172). Hence, 
 
 iJ/T 2 =13-l x 10- 6 ergs; 
 
 or, as 3*4 x 10 10 particles are expelled per second by the 
 radium C in equilibrium with 1 gram of radium, the energy 
 developed per second is 44*54 x 10 4 ergs. 
 
278 APPENDIX B. 
 
 Referring now to Boltwood's table given above, we find 
 that the activity of radium C is to the total activity of the 
 uranium series as 0*91 is to 4'64. Hence the total energy 
 would be 227-1 x 10 4 ergs. Dividing by 4-184 x 10 7 (the 
 mechanical equivalent of I gram degree), we have the total 
 heat developed per second per gram of radium = 5'4 x 10~ 2 
 gram degrees. 
 
 This last method takes no account of the energy of recoil 
 of the atoms emitting a particles. This adds about 2 per 
 cent. The ft and y radiation are also neglected. 
 
 3. According to Von Schweidler and Hess (Le Radium, 
 Feb., 1909) the evolution of heat per hour is 1 1 8 gram degrees 
 per gram of radium. This is 5'95 x 10~ 2 calories per second, 
 or 187 '5 x lo 4 per annum, per gram of radium in equilibrium. 
 
 4. The direct determination, by H. H. Poole, of the heat 
 evolved by Joachimsthal Pitchblende, will give a higher 
 value than that which is deduced above. 
 
INDEX. 
 
 a rays, heat due to, 13. 
 nature of, 13. 
 
 ionization produced by, 
 15, 200. 
 
 number of, from radium, 18. 
 penetration of, 68. 
 Actinium, relation of, with 
 uranium, 9. 
 
 activity of, 215. 
 
 Active deposit of radium, nature 
 of, 24. 
 
 effects of, in electroscope, 24. 
 Agassiz, A., on coral islands, 120. 
 Age of tbe earth, 156, 157, 211 
 et seq. ** 
 
 by helium accumulation, 229, 
 230. 
 
 by sedimentation method, 230 
 et seq. 
 
 by solvent denudation, 236 
 et seq. 
 Air, radioactivity of air from soils, 
 
 35, 208. 
 Alpine tectonics, development of, 
 
 151-153 and Plate VI. 
 Alps, time required for genesis of, 
 141. 
 
 volcanic phenomena and 
 genesis of, 141. 
 
 temperatures in alpine rocks, 
 143. 
 
 tectonics of, 144-147. 
 Andes, syncline of tbe, 101. 
 Animikean sedimentation, 96. 
 Appalachian Mountains, 97. 
 Ashman, radium emanation in the 
 
 atmosphere, 201. 
 
 Atmosphere, normal ionization of, 
 191. 
 
 radioactive substances in, 196 
 et seq. 
 
 effects of weather on radio- 
 activity of, 197. 
 
 Atmosphere, ionization and radium 
 emanation in, 200, 201. 
 
 percentage of emanation 
 entering, 202. 
 
 ionization of, by penetrating 
 radiation, 205 et seq. 
 
 active deposit of thorium in 
 the, 208, 209. 
 
 composition of, and radio- 
 activity, 209. 
 
 )8 rays, nature of, 13. 
 
 Babbage and Herschel, theory of, 
 
 101. 
 
 Balfour Bore, radium and tempera- 
 ture gradients of, 80. 
 Barbados, subsidence and re- 
 elevation of, 137. 
 
 oceanic beds of, 132. 
 Barus, thermal conductivity and 
 
 change of state, 75. 
 Basalt, radium in, 42. 
 Bertrand, Marcel, alpine structure, 
 
 148. 
 
 Blanc, thorium and the atmo- 
 sphere, 208. 
 
 Blue Mud, radioactivity of, 49. 
 Bischof, calculation of lime 
 secreted by shell-fish, 52. 
 
 fluorine in phosphatic sub- 
 stances, 226. 
 Boltwood, period of radium, 3. 
 
 relation between radium and 
 uranium, 4. 
 
 discovery of ionium, 9. 
 relation between actinium and 
 uranium, 9. 
 
 method of standardizing 
 electroscopes, 22. 
 
 activity of actinium, 215. 
 relative activity of active 
 elements, 200. 
 
 helium in uranium ores, 222. 
 
280 
 
 INDEX. 
 
 Boltwopd, genetic connexion of 
 
 uranium and lead, 28. 
 Boltzmann, ionization over the 
 
 ocean, 203. 
 Bragg and Bolt wood, rays from 
 
 transformation products of 
 
 radium, 18. 
 Bragg, rate of transformation of 
 
 thorium and uranium, 209. 
 Brecciendecke, 147. 
 Bronson, effects of temperature on 
 
 radioactivity, 32. 
 Bumstead, active deposit of 
 
 thorium in the air, 908. 
 
 Cady and McFarland, helium in 
 
 natural gases, 218. 
 Cameron and Ramsay, decomposi- 
 tion of hydrochloric acid, 210. 
 Campbell and Wood, radioactivity 
 
 of potassium, 219. 
 Carnot, fluorine in phosphatic sub- 
 stances, 226. 
 
 Chalk, radioactivity of, 60. 
 low conductivity of, 126. 
 rate of deposition of, 134, 
 depth of water required for 
 deposition of, 138. 
 Chamberlin and Salisbury, planet - 
 
 esimal theory, 184. 
 Chlorine in rain, 238. 
 in rivers, 239 et seq. 
 possible sources of, 240. 
 Clarke, soda in igneous and sedi- 
 mentary rocks, 57. 
 soda in sediments, 129, 
 soda in igneous rocks, 130. 
 changes in phosphatic sub- 
 stances, 225. 
 
 percentage of chlorine in 
 rocks, 239. 
 
 composition of lithosphere, 
 243. 
 
 American river analyses, 246. 
 Clays, radioactive substance in 
 
 clays and soils, 35. 
 Conductivity (electric) of gases, 
 
 189, 190. 
 
 Continents, permanency of, 126 
 et seq. 
 
 Coral islands and instability of 
 ocean floor, 118, 119. 
 
 subsidence of, 120, 121, 137. 
 depth of water around, 121. 
 sedimentation around, 121. 
 Coral sea, extent of, 119. 
 Coral rock, radioactivity of, 60. 
 Crust of the earth, thickness of, 
 174. 
 
 thickness of radioactive layer, 
 175 179 et seq. 
 
 4*' temperature beneath, 174. 
 Cooke, penetrating radiation, 205. 
 Cosmic particles, radioactivity of, 
 
 53. 
 Curie, Mme. and M., discovery of 
 
 radium and polonium, 1. 
 Curie, Paul, and Laborde, heat pro- 
 duced by radium, 2, 10. 
 
 Dadourian, thorium emanation in 
 
 underground air, 298. 
 Daly, temperature of lavas, 174. 
 Dana, J. D., sedimentation, 97. 
 estimate of sediments, 128. 
 Danne, occlusion of radium in 
 
 pyromorphite, 223. 
 Darwin, instability of ocean floor, 
 118. 
 
 subsidence of coral islands, 
 137. 
 
 Darwin, Sir George, radium and 
 age of the sun, 36. 
 
 age of the moon, 250. 
 David, E., boring of Funafuti, 
 . 120. 
 /Davison, interior cooling of the 
 
 earth, 163. 
 Dawson, compression of Laramide 
 
 range, 100. 
 Deccan, radium in igneous rocks 
 
 of the Deccan, 42. 
 Deccan basalt, radioactive tempera- 
 ture of, 185. 
 
 Deep-sea deposits, formation of, 
 119, 120. 
 
 slow, accumulation of, 121. 
 Deferlement of recumbent folds, 
 148. 
 
 theory of, 151. 
 age of the earth, 233. 
 
INDEX. 
 
 281 
 
 Denudation in mountain ranges, 
 115. 
 
 Deposition, rate of, from Missis- 
 sippi, 235. 
 
 Detrital rocks, radioactivity of, 59. 
 
 Dewar, production of helium by 
 radium, 214, 217. 
 
 Dolomite, radioactivity of, 60. 
 
 Dittmar, amount of chlorine in 
 the ocean, 244. 
 
 Dust, and the recombination of 
 ions, 193. 
 
 Earth, former high temperature of, 
 154. 
 
 rate of cooling of, 155. 
 
 age of, 156, 157, 211 et seq. 
 
 interior cooling of, 162. 
 
 interior thermal conductivity 
 of, 163. 
 
 interior radioactivity of, 1 65- 
 169. 
 Earthquakes and the geosynclines, 
 
 113. 
 
 Electroscope, use of, in measuring 
 radioactivity, 16. 
 
 discharge of, by ionization, 
 16, 17. 
 
 ionization in, attending radium 
 measurements, 19. 
 
 rate of discharge due to emana- 
 tion, 23. 
 
 standardization of, 22, 255. 
 
 construction of, 262 et seq. 
 Elster and Geitel, radioactivity of 
 air in soils, 35. 
 
 active deposits, from the 
 atmosphere, 63. 
 
 radioactive substances in the 
 atmosphere, 196. 
 
 Emanation of radium, use of, in 
 measurements, 18-22. 
 
 number of a rays from, 19. 
 
 properties of, 22, 23. 
 
 extraction of, from solutions, 
 25, 265 et seq. 
 
 calculation of amount col- 
 lected, 256, 258. 
 
 Eve and Adams, effects of pres- 
 sure on radioactivity, 33. 
 
 Eve, measurements of radium in 
 rocks, 39. 
 
 ionization of the atmosphere, 
 191. 
 
 weather and the radioactivity 
 of atmosphere, 197. 
 
 emanation in the atmosphere, 
 198, 201. 
 
 variation of emanation in the 
 atmosphere, 201. 
 
 ionization over the ocean, 
 48, 203. 
 
 ionization due to penetrating 
 radiation, 206, 207. 
 
 Fergusonite, helium in, 215. 
 
 age of, calculated by helium, 
 215, 216. 
 
 Finsteraar granite, high tempera- 
 ture of, 89, 90. 
 
 Fisher, Osmond, gradients in 
 mountains, 83. 
 
 Fluorine in phosphatic substances, 
 226. 
 
 Funafuti, boring of, 120, 127. 
 
 7 rays, nature of, 13. 
 
 Gases, ionization and density of, 
 191. 
 
 Geikie, Sir A., age of the earth, 
 156, 233. 
 on uniformitarianism, 248. 
 
 Geosynclines and mountain build- 
 ing, 112, 118. 
 
 Geothenns, rise of attending sedi- 
 mentary deposition, 108, 109. 
 
 Globigerina Ooze, radioactivity of, 
 49. 
 
 rate of accumulation of, 135. 
 
 Granites, radium in, 42. 
 
 Green Sand, radioactivity of, 49. 
 
 Green Sandstone, radioactivity of, 
 60. 
 
 Gypsum, radioactivity of, 60. 
 
 Heat given out by radium, 10, 11, 
 and Appendix B. 
 
 escaping from the earth, 
 calculation of, 75. 
 radioactive origin of , 171. 
 
282 
 
 INDEX. 
 
 Heim, temperature in Simplon 
 tunnel, 82. 
 
 alpine structure, 146. 
 /Helium and geological time, 215 
 et seq. 
 
 in springs and natural gases, 
 63, 218. 
 
 in the atmosphere, 63, 210. 
 produced from radium in 
 common minerals, 218. 
 permeation of quartz by, 223. 
 genesis of, by thorium, 228. 
 Herschel and Babbage, theory of, 
 
 101. 
 
 Himalayas, sedimentation of, 99. 
 Hobbs, earthquakes, volcanoes, 
 
 and sedimentation, 114, 115. 
 Horst, theory of the, untenable, 
 
 U9, 150. 
 
 Huronian sedimentation, 96. 
 Huxley, rate of deposition of 
 chalk, 134. 
 
 homotaxial relations of strata, 
 231. 
 
 Igneous rocks, radium in, 42, 44. 
 
 mean radioactivity of, 44, 45. 
 Ionic charge, 192. 
 Ionium, period of transformation 
 of, 9. 
 
 discovery of, 9. 
 
 lonization in a gas, theory of, 14. 
 amount of, attending radium 
 measurements, 19. 
 in closed vessels, 19, 190, 205. 
 over the ocean, explanation 
 of, 204. 
 
 Ions, and precipitation of rain, 194, 
 195. 
 
 in gases, production and re- 
 combination of, 191, 192. 
 Iron, native, radium in, 166. 
 
 Jaquerod and Perrot, permeation 
 of quartz by helium, 223. 
 
 Jeffreys, depths of Chalk sea, 138. 
 
 Jukes -Browne and Harrison, 
 oceanic beds of Barbados, 132. 
 
 Kelvin, Lord, cooling of the earth, 
 155. 
 
 Kelvin, Lord, temperature of 
 
 earth's interior, 162. 
 Keweenawan sedimentation, 96. 
 
 Lapparent, A. de, tectonics of the 
 
 Prealpes, 144. 
 Laramide range, sedimentation of, 
 
 98. 
 
 compression of, 100. 
 Lavas, temperature of melted, 
 
 174. 
 Lead, genetic connexion with 
 
 uranium, 228. 
 
 use of, in determining age of 
 
 minerals, 228, 229. 
 Lees, change of conductivity with 
 
 temperature, 74. 
 
 change of conductivity attend- 
 ing change of state, 75. 
 Lewisian rocks, radinm in, 43. 
 Limestone, radioactivity of, 60. 
 Linder and Picton, precipitation 
 
 of metallic ions, 124. 
 Lithosphere, soluble constituents 
 
 of, 243. 
 Love, secular movements of earth's 
 
 surface, 138. 
 Lugeon, origin of the Prealpes, 
 
 145. 
 
 Magee, deposition of Mississippi 
 
 sediments, 235. 
 Maldive Archipelago, extent of, 
 
 119. 
 Makower, effects of temperature 
 
 on radioactivity, 33. 
 Manganese Nodules, radioactivity 
 
 of, 50. 
 McClelland, penetration of 7 and 
 
 rays, 205. 
 McCoy, relation between radium 
 
 and uranium, 4. 
 Mesozoic sediments of St. Gothard, 
 
 radioactivity of , 61. 
 Meteorites, radium in, 166. 
 Mica, colour of, and radioactivity, 
 
 69. 
 
 Mississippi, deposition of sedi- 
 ments, 235. 
 M'Lennan, penetrating radiation 
 
 from the earth, 205. 
 
INDEX. 
 
 283 
 
 Mont Cenis, temperature gradient 
 
 in, 82. 
 
 Montessus de Ballore, F. de, Earth- 
 quakes and the geosynclines, 
 113,114. 
 
 Mountain - building phenomena, 
 95, 96. 
 
 source of the energy, 99. 
 rhythmic character of, 99, 
 112. 
 
 stresses confined to upper 
 crust, 100. 
 
 depression attending upheaval, 
 101. 
 
 influence of radium in, 103 et 
 seq. 
 
 association of, with the 
 geosynclines, 112, 113. 
 Mountains formed on continental 
 
 borders, 111. 
 
 Mountains and sedimentation, 96. 
 Moureu, helium in springs, 218. 
 Mud from R. Amazon, radio- 
 activity of, 60. 
 Miigge, 0., origin of pleochroie 
 
 haloes, 67. 
 
 Murray, Sir J., discovery of 
 cosmic particles, 52. 
 
 Theory of coral islands, 120. 
 total river discharge, 237. 
 Murray and Peake, rate of deposi- 
 tion of ooze, 135. 
 
 Murray andRenard, sedimentation 
 around coral islands, 121. 
 
 Nile, radioactivity of water of, 54. 
 Korth American sedimentation, 96, 
 97. 
 
 Ocean, quantity of radium in, 48. 
 ionization over the, 58, 203. 
 Ocean floor, vertical movements 
 
 of, 137. 
 
 Oceans, permanency of, 126 et seq. 
 Secular transgressions of, 136, 
 138. 
 
 Oceanic' sediments, radioactivity 
 of, 49. 
 
 extension of, 51. 
 radioactivity and content of 
 calcium carbonate, 52. 
 
 Oceanic sediments, calculation of 
 quantity of, 56, 57, 58. 
 
 total quantity of radium in, 
 59. 
 
 Parent igneons rock, mass of, 57. 
 
 Patera, analysis of pitchblende, 
 259. 
 
 Penetrating radiation and ioniza- 
 tion of atmosphere, 205 et seq. 
 
 Perry, interior conductivity of the 
 earth, 163. 
 
 Phosphatic substances, helium in, 
 219. 
 
 changes of, 224 et seq. 
 
 Pitchblende, heat given out by. 
 11. 
 
 Pleochroie haloes, properties of, 
 64 and Plate III. 
 
 radioactive origin of, 67. 
 
 Poole, heat given out by pitch- 
 blende, 11. 
 
 Poulton on uniformitarianism, 
 249. 
 
 Prealpes, origin of, 144, 145. 
 
 Prestwich, gradient in Mont Cenis, 
 82. 
 
 gradients of the St. Gothard, 
 90. 
 
 Pyromorphite, occlusion of radium 
 in, 223. 
 
 Quartz Grit, radioactivity of, 60. 
 
 Radioactive elements, families 
 of, 3. 
 
 theory, 4 et seq. 
 
 surface layer of the earth, 179 
 et seq. 
 
 Radioactivity not affected by con- 
 centration, 30. 
 
 in rocks independent of con- 
 centration, 31, 32. 
 
 independent of conditions of 
 pressure and temperature, 32, 34. 
 
 convention regarding the use 
 of the term, 42. 
 
 of Cambridge tap-water, 35. 
 
 of deep-seated rocks, 62. 
 
 of the atmosphere, cause of, 
 62, 63, 197. 
 
284 
 
 INDEX. 
 
 Radiolarian Ooze, radioactivity of, 
 
 50. 
 
 Radium, A and B^and C, genera- 
 tion of, in electroscope, 22. 
 
 discovery of, 1. 
 
 half period of, 3. 
 
 genetic connexion with ura- 
 nium, 3, 31, 32. 
 
 amount of, in one gram of 
 uranium, 6. 
 
 heat given out by, 10, 11. 
 
 theory of measurement of, 
 13. 
 
 accuracy of measurements 
 of, 20. 
 
 measurement of, 25, 29, 252 
 et seq. 
 
 in rocks, measurements by 
 Hon. R. J. Strutt, 37, 38. 
 
 in rocks, measurements by 
 Eve, 39. 
 
 in rocks, conditions of accu- 
 rate measurements, 41. 
 
 in igneous rocks, 42 et seq. 
 
 in sedimentary rocks, 45, 59 
 
 suggested extra - terrestrial 
 origin of, 52, 53. 
 and solar heat, 36. 
 quantity supplying earth- 
 heat, 161. 
 
 occlusion of, in pyromorphite, 
 223. 
 
 emanation in spring - water 
 and in air, 62, 63. 
 Ramsay, Sir W., helium in the 
 atmosphere, 63, 210. 
 
 stability of radium emanation, 
 23. 
 Ramsay and T ravers, helium in 
 
 fergusonite, 215. 
 Rayleigh, helium in Bath springs, 
 
 218. 
 Rays from radioactive matter, 
 
 nature of, 13. 
 
 Reade, Mellard, estimate of sedi- 
 ments, 129. 
 Recumbent, folds, theory of, 150 
 
 et seq. 
 
 Red Clay, radioactivity of, 50. 
 uranium in, 55. 
 
 Rio de la Plata, radioactivity of 
 
 water of, 54. 
 
 River- water, radioactivity of, 54. 
 Rocks, classification of, according 
 to radioactivity* 40. 
 
 preparation of, for determina- 
 tion of radium, 261 et seq. 
 Russell, sediment in rivers, 234. 
 Rutherford, diagram of radioactive 
 elements, Plate II., 3. 
 
 hydrostatic analogy of radio- 
 active change, 6 et seq. 
 
 relation between actinium 
 and uranium, 9. 
 
 heat-constant of radium, 11. 
 stability of radium emanation, 
 23. 
 
 radioactivity unaffected by 
 dilution, 30. 
 
 radium and heat of the earth, 
 36. 
 
 calculation of age of fergu- 
 sonite, 215, 216. 
 
 Rutherford and Allen, weather 
 and the radioactivity of the 
 atmosphere, 197. 
 Rutherford and Barnes, heat of 
 
 radium, 10. 
 
 Rutherford and Cooke, penetrat- 
 ing radiation, 205. 
 Rutherford and Geiger, period of 
 radium, 3. 
 
 ionization by a rays, 15. 
 number of a rays from 
 radium, 18. 
 
 value of ionic charge, 192. 
 nature of the a particle, 
 213, 214. 
 Rutherford and McClung, heat of 
 
 ionization, 2. 
 
 Rutherford and Petavel, effects of 
 temperature and* pressure on 
 radioactivity, 33. 
 
 Rutherford and Soddy, radium 
 and solar heat, 36. 
 
 Sand of Arabian Desert, radio- 
 activity of, 61. 
 
 Sandstone, radioactivity of, 59, 60. 
 
 Satterly, active substances in the 
 air, 199, 201. 
 
INDEX. 
 
 285 
 
 Saturation current, nature of, 17. 
 
 potential, nature of, 17. 
 Schardt, classification of Simplon 
 rocks, 86. 
 
 alpine structure, 148. 
 Schmidt, H. W., effects of tem- 
 perature on radioactivity, 33. 
 Schmidt, C., former reek-covering 
 of the Alps, 110, 142. 
 
 structure of the Alps, 146 and 
 Plate V. 
 
 Schuster, effects of pressure on 
 radioactivity, 33. 
 
 ionization of air at Man- 
 chester, 193. 
 
 Sea-water, examination of, for 
 radium, 46. 
 
 radioactivity of, 46, 47. 
 volume of emanation in, 47. 
 variation of radioactivity of, 
 47. 
 
 Sediment in rivers, 234. 
 Sedimentary rocks, origin of 
 radium in, 45. 
 
 mean radioactivity of, 61. 
 Sedimentation, depths of, 97, 99. 
 and convection of energy, 111. 
 laws of, 122 et seq. 
 Sediments, oceanic, estimate of, 
 56, 57, 58. 
 
 of the St. Gothard, radio- 
 activity of, 61. 
 
 of the Simplon, radioactivity 
 of, 61. 
 
 estimates of total, 128. 
 rate of accumulation of, 131, 
 135. 
 Spencer, oceanic beds of Barbados, 
 
 133. 
 
 Shales, radioactivity of, 59, 60. 
 Simplon, sporadic distribution of 
 uranium in rocks of, 44. 
 radium in igneous rocks of, 44. 
 tunnel, distribution of tem- 
 perature in, 81. 
 
 explanations suggested for 
 gradients, 83. 
 
 thermal conductivity of rocks, 
 85. 
 
 rocks, radium content of, 86. 
 temperature developed in, 110. 
 
 Simplon and St. Gotbard rocks, 
 
 downward extension of, 88. 
 Simpson, ionization of atmosphere, 
 
 203. 
 
 Slates, radioactivity of, 59. 
 Soda, in igneous and sedimentary 
 
 rocks, 57. 
 
 Sodium in rivers of N. America, 
 246. 
 
 in the ocean, 237. 
 in rivers, 238 et seq. 
 Soddy, helium evolved by ura- 
 nium, 217. 
 
 helium generated by thorium, 
 228. 
 Soils, radioactive substance in clays 
 
 and soil, 35. 
 
 Sollas, molten matter beneath the 
 crust, 71. .-.cu,n 
 
 boring of Funafuti, 120. 
 estimate of Eocene sediments, 
 133. 
 
 rate of accumulation of Chalk, 
 134. 
 
 Tertiary sedimentation, 141. 
 age of the earth, 157, 233. 
 depth of geological column, 
 235. 
 
 sodium in N. American rivers, 
 246. 
 
 Stapff, temperature gradients in 
 the St. Gothard, 86 et seq. 
 
 temperature in Simplon tun- 
 nel, 82. 
 Stassfurt salt deposits, helium in, 
 
 219. 
 
 St. Gothard, "radium in igneous 
 rocks of, 44. 
 
 temperature gradients in, 84, 
 
 geological structure of, 89 and 
 Plate I. . fl=& 
 
 radioactivity of, 91 and Plate 
 IV. 
 
 downward extension of, 93. 
 Stockalper, temperature in Sim- 
 plon tunnel, 83. 
 
 Stoney, escape of gaseous mole- 
 cules from the earth, 210. 
 Strutt, relation between radium 
 and uranium, 4. 
 
286 
 
 INDEX. 
 
 Strutt, measurements o radium in 
 rocks, 37, 38. 
 
 radium in native iron and in 
 meteorites, 166. 
 
 helium in common minerals, 
 218. 
 
 helium and geological time, 
 219 et seq, 
 
 age of geological strata, 220, 
 229. 
 
 helium derived from thorium, 
 64, 227. 
 
 Studer, structure of the Alps, 146. 
 Suess, movements of earthquake 
 epicentres, 114. 
 
 alpine structure, 148. 
 Sun, uranium in the, 167. 
 Syenite, radium in, 43. 
 
 
 
 Iemperature gradient at earth's 
 surface, 71, 73. 
 
 due to radioactivity in rocks, 
 76 et seq. 
 Terrigenous Mud, radioactivity of, 
 
 Termier, section of the Pelvoux 
 
 Massif, 147. 
 Tertiary sedimentation, 141. 
 
 earth movements and vulcani- 
 city, 141, 142. 
 Tethys, the, andcrustal instability, 
 
 114. 
 
 Thermal conductivity and gradient, 
 72. 
 
 of rocks, 74. 
 
 change of, with temperature, 
 74. 
 
 and'change of state, 75. 
 Thomson, Sir J. J., radioactivity of 
 Cambridge water, 35. 
 
 electric conductivity of gases, 
 189. 
 
 dust and the recombination of 
 ions, 193. 
 
 Thorium emanationin underground 
 water and in the atmosphere, 63. 
 active deposit in the atmo- 
 sphere, 208. 
 Townsend, charge on ions, 14. 
 
 coefficient of recombination of 
 ions, 192. 
 
 Trias- Jura sediments of the Sim- 
 plon, radioactivity of, 61. 
 
 Uranium and radium, constant 
 proportions in ores, &c., 3, 31, 
 32. 
 
 Uranium in the ocean and its sedi- 
 ments, 54. 
 
 rejected by lime -secreting or- 
 ganisms, 51. 
 
 in deep-sea deposits, 55. 
 
 in Red Clay, 55. 
 
 precipitation of, in oceanic 
 sediments, 125. 
 
 and transgressions of the ocean, 
 136, 138. 
 
 quantity supplying earth-heat, 
 
 in the earth's interior, 164 
 et seq. 
 
 in the sun, 167. 
 cooling of a celestial body, 
 170. 
 
 effect of decay of, on earth - 
 heat, 176 et seq. 
 
 at earth's surface, 183 et seq. 
 Underground temperature gradi- 
 ents, 71. 
 
 and radioactivity, 76 et seq. 
 Uniformitarianism in geology, 248, 
 249. 
 
 Van Hise, bulk of the calcareous 
 rocks, 57. 
 
 estimate of sediments, 128. 
 Volcanic rocks, radium in, 43. 
 
 temperatures and radium, 79. 
 Volcanoes and the geosynclines, 
 113. 
 
 Wahsatch Mountains, 97. 
 
 Weathering and loss of radioacti- 
 vity, 61. 
 
 Weber, R., change of conductivity 
 with temperature, 74. 
 
 Weinschenk, thermal metamor- 
 phism, 111, 143. 
 
 Werner, E., uranium in rocks, 32. 
 uranium in Red Clay, 55. 
 
INDEX. 
 
 287 
 
 Whetham, ions and coagulation of 
 coloids, 124. 
 
 Wilson, W. E., radium and solar 
 heat, 36. 
 
 Wilson, C. T. R., precipitation of 
 water-vapour by ions, 194, 195. 
 
 Wilson, W., thorium in the atmo- 
 sphere, 209. 
 
 Wright, C. S., ionization by pene- 
 trating radiation, 207. 
 
 Zircons of Rhenish basalt, 69. 
 Zirkel, observations on zircons in 
 
 basalt, 69. 
 Zollinger, thermal conductivity of 
 
 Simplon rocks, 85. 
 
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