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NATURE 



[June 19, 1913 



; x 10" '•* grams per gram 



Radium and the Evolution of the Earth's Crust. 



1 1 is now well known that if the proportion of 

 radium in the interior of the earth is the same as 

 that in the surface rocks, the earth ought to be grow- 

 ing hotter, and the temperature gradient of the crust 

 ought to be much higher than we find it. A simpie 

 calculation shows that a distribution of radium 

 averaging 2xio -14 grams in each gram of rock 

 throughout the earth would suffice to preserve thermal 

 equilibrium. The amounts of radium actually found 

 in the surface rocks are approximately as follows : — 



Acid 



Intermediate 



Basic ... ... 1 ,, ,, 



Ultrabasic ... o'5 „ „ ,, 



In addition to the elements of the uranium family, 

 those of the thorium family must also be considered, 

 for they afford an equally important supply of heat. 



Prof. Strutt was the tirst to indicate the way in 

 which the obvious dilemma might be escaped. In 

 order that the earth should be nearly in thermal 

 equilibrium (i.e. not growing hotter, but cooling at 

 the very slow rate allowed by the radio-active elements 

 as they decrease in quantity in accordance with their 

 progressive disintegration), it is necessary to assume 

 that the earth's store of radium is concentrated near 

 the surface. As the following arguments indicate, 

 this conception is less arbitrary than would appear 

 at first sight. The radio-active elements are found 

 most abundantly in the acid rocks, their more basic 

 associates being less embarrassingly rich. The more 

 acid rocks are characteristic of only the outermost 

 zones of the crust, and there are many reasons for 

 believing that with depth the more basic rocks largely 

 predominate. Seismic and other terrestrial pheno- 

 mena have now provided us with data from which 

 the condition of the earth's interior may be deduced 

 with some confidence. First, there is the crustal 

 zone, rapidly becoming less silicic with depth, having 

 a mean density of 2-8, and an approximate thickness 

 of thirty miles. Within a fairly sharp surface of 

 discontinuity comes what may be called the stony 

 zone. The density is 3-4, and judging from the close 

 analogy presented by meteorites, the material would 

 be of ultra-basic composition. This zone dies out at 

 a depth variously estimated at from 600 to 900 miles. 

 The internal core of the earth is probably largely 

 composed of iron, its density being about 8. 



In a number of meteorites, the radium content has 

 been determined by Prof. Strutt and the present 

 writer, and if it may be assumed that they afford a 

 clue to the problem, the heavy metallic core should 

 be completely destitute of radium, and the stony zone 

 should contain only a small proportion, very much 

 less than that of the ultra-basic rocks of the crust. 



On the planetesimal hypothesis, the two internal 

 zones find a ready explanation. It is supposed that 

 the earth began as a nebulous knot, and that it has 

 grown up to its present mass by the capture of asso- 

 ciated planetesimals. It is very unlikely that as a 

 whole it was ever in a molten condition. Internal 

 heat probably arose largely from the condensation 

 of the mass during the period of its growth. The 

 temperature would slowly rise until the fusion point 

 of certain of the constituents was reached, and the 

 liquid tongues and pockets thus formed would tend 

 to move away from the centre, the lighter and less vis- 

 cous stony material being squeezed outwards relatively 

 to a network of the heavier and more rigid metallic 

 materials. Once vulcanism had been initiated in this 

 way, the process would continue until a highly 

 metallic nucleus had collected. Surrounding it there 

 would gradually form a thick zone of silicate rocks, 

 NO. 2277, VOL. 91] 



the differentiation from the original heterogeneous 

 mixture of stony and metallic constituents being due 

 to the selective fusion of the former. There seems to 

 be little doubt that the radio-active elements would be 

 concentrated in the stony zone. With the establish- 

 ment of ocean and atmosphere, a new factor in 

 surface differentiation arose, and sedimentary rocks 

 were deposited for the first time. In some way 

 which, as yet, we understand but vaguely, both 

 igneous and denudational differentiation then com- 

 bined in developing the earth's crust. We now find 

 in the latter all those rocks which hold a maximum 

 content both of silica and of the radio-active elements. 

 The relative concentration of these constituents having 

 taken place at the expense of the zone below, the 

 conjectural paucity of the latter in radium finds a 

 suggestive explanation. 



Before the advent of radium, geologists had not 

 recognised the difficulties presented by the peculiar 

 chemical constitution of the earth's crust. Radium 

 did not create this difficulty, but has merely directed 

 attention towards it. Any explanation of the high 

 percentage of silica in the surface rocks will explain 

 equally well their richness in radium. 



It can scarcely be said now that radium has given 

 us "a blank cheque on the bank of time." Not onlv 

 did the discovery of radium destroy the validity of 

 the older thermal arguments, but also it led directly 

 to the elaboration of a new and more refined method. 

 Every radio-active mineral may be regarded as a self- 

 contained hourglass, the radio-active end-products, 

 helium and lead, slowly accumulating at the expense 

 of their ultimate parent, uranium. In the few cases 

 which up to the present have been investigated, 

 periods of enormous duration have been revealed, and 

 the geologist who ten years ago was embarrassed 

 by the shortness of the time allowed to him for the 

 evolution of the earth's crust is now still more em- 

 barrassed by the superabundance with which he is 

 confronted. The time scale up to date, as determined 

 by the lead ratio, is as follows : — 



Carboniferous ... ... 340 million years 



Devonian ... ... ... 370 „ 



Ordovician ... ... ... 430 „ 



Algonkian ... ... ... 1000 „ 



Archaean | '3°° 



/ 1600 ,, 



We must not moan over the apparent difficulties 

 with which the geologist has been faced since the 

 advent of radium. Rather should they be welcomed 

 in that they open the way for further advances. If 

 at present some of our ideas are mutually incom- 

 patible, the discrepancies do not demand a wholesale 

 rejection of the facts, but simply a re-interpretation 

 of the fundamental hypotheses on which so many 

 of our doctrines seem to hang. 



Arthur KoLNres. 



Imperial College, South Kensington, S.W. 



An Amphipod Invasion. 



Many specimens of a small amphipod crustacean, 

 Eitlln'misto cotnpressa, Goes, have been forwarded to 

 me by Mr. T. H. Nelson, of Redcar. On May 23 and 

 24 these were washed ashore in incredible numbers on 

 the coast of Yorkshire, where they lay from Saltburn 

 to Teesmouth — a distance of ten miles — in drifts 

 several inches deep. The pools were alive with the 

 crustaceans, and to the east of Redcar a fisherman 

 was seen raking them into heaps, and wheeling away 

 barrow-loads to put on his garden as manure. In the 

 sea hundreds at a time could be scooped up in one's 

 hands. 



Euthemisto compressa is an uncommon British 



