36o 



NA TURE 



[August 8, 1907 



lo Oldham's calculations, the light central core occupies 

 two-fifths of the diameter of the earth. 



I ho evidence of density alone, therefore, gives no con- 

 vincing evidence of the nature of the earth's interior; and 

 geologists have been left with no conclusive reason for 

 choosing between the condensation and segregation 

 theories. Radio-activity has, however, unexpectedly come 

 to our aid, and has disclosed a further striking reseiiiblance 

 between the internal mass of the earth and the iron 

 meteorites. It has supplied direct evidence about the con- 

 i^tituents of the earth at depths which have hitherto been 

 far beyond the range of observation. Mr. Strutt has 

 shown that radium is probably limited within the earth 

 to the depth of 45 miles ; that the deeper-lying material 

 is free from radium ; and that this substance is not found 

 in iron meteorites. 



The agreement in radio-active properties between the 

 iron meteorites and the interior of the earth is an addi- 

 tional and weighty argument in favour of the view that 

 the earth is largely composed of niclcel iron. 



(3) Physical Conditions and Temperatures. — The physical 

 condition in which the material exists is now of secondary 

 interest. The old controversy as to whether the earth has 

 a molten interior inclosed within a solid shell has lost 

 its importance, because it has become a mere matter of 

 definition of terms. The facts which led geologists to 

 believe that the interior of the earth is fluid are consistent 

 with those which prove that the earth is more rigid than 

 a globe of steel. For under the immense pressure within 

 the earth the materials can transmit vibrations and resist 

 compression like a solid ; but they can change their shape 

 as easily as a fluid. They are fluid just as lead is when 

 it is forced to flow from a hydraulic press. Not onlv are 

 geologists now justified in their belief that the deeper 

 layers of the earth's crust are in a state of fluxion, but, 

 ;M-cording to Arrhenius (1000), the earth is solid onlv to 

 the depth of 25 miles, below which is a liquid zone extend- 

 ing to the depth of igo miles ; and below that level, he 

 tells us, " the temperature must, without doubt, exceed 

 the critical temperature of all known substances, and at 

 this depth the liquid magma passes gradually to a gaseous 

 magma." This distinguished physicist gives a descrip- 

 tion of the earth's interior which reminds us of the views 

 of the early geologists. Arrhenius's theory rests, how- 

 ever, on the existence within the earth of exalted tempera- 

 tures ; and this assumption a geologist mav now hesitate 

 to accept with less risk of getting into disgrace than he 

 would have run a few years ago. It is improbable that 

 the rapid increase of heat with depth which is observed 

 near^ the surface should continue below the lithosphere ; 

 for, if the earth consists in the main of iron, even although 

 it be arranged as a mesh containing silicates in the inter- 

 spaces, the heat conductivity might be sufficient to keep 

 the whole metallic sphere at a nearly equal temperature. 

 Here, again, Mr. .Strutt's work on radio-activitv is in full 

 agreement with the requirements of geologists, for he 

 estimates that below a crust 45 miles thick the earth 

 has a uniform temperature of only 1500° C. Whether the 

 further conclusion, that this heat isdue to the action of 

 the radium in the crust, be established or not, it is gratify- 

 mg to hear a physicist arguing in favour of a moderate 

 and uniform internal temperature. 



All that the actual observations prove and that geological 

 theories require is that the material vi-ithin the'earth be 

 intensely hot, and that it lie under such overwhelming 

 pressure that it would as readily change its form and as 

 ouicklv fill up an accessible cavity as any liquid would 

 do. Whether such a condition is to be described as solid, 

 liquid, or gaseous is of little concern to geologists. 



III. The Deep-seated Control over the Earth's Surface. 

 The modern view of the structure of the earth adds 

 greatly to the interest of its study, for it recognises the 

 world as an individual entity of which both the "geological 

 structure and the history have to be considered as a 

 whole. Once the earth was regarded as a mere lifeless, 

 inert mass which has been spun by the force of gravity] 

 that hurls it on its course into the shape of a simple 

 oblate spheroid. Corresponding with this astronomical 

 teaching as to the shape of the world was the geological 

 doctrine, that all its topography is the work of local 

 NO. I 97 I, VOL. 76] 



geographical agents, the control of which over the surface 

 of the earth is as absolute as that of the sculptor's chisel 

 over a block of marble. 



Both these conceptions are now only of historic interest. 

 The irregular individual shape of the earth is expressed 

 by its description as a geoid. The processes which have 

 produced its varying shape have also controlled its geo- 

 logical history and evolution, for they cause disturbances 

 of the crust, which affect the whole earth simultaneously ; 

 and so the geographical agents are given similar work 

 and powers at the same time in different places. 



Hence there is a remarkable world-wide uniformity in 

 the general characters of the sedimentary deposits of each 

 of the geological systems. The last pr.-e-Cambrian system 

 includes thick masses of felspathic sandstones alike in the 

 Torridonian of .Scotland, the sparagmite of Scandinavia, 

 the Kcwecnawan Sandstones of the United States, and 

 perhaps also the quartziles of the Rand. The Cambrian 

 has its greywackes and coarse slates and its numerous 

 phosphatic limestones ; the Ordovician its prevalent shales 

 and slates; the Silurian its episodal limestones and shales. 

 The Devonian has its wide areas of Old Red Sandstones 

 as a continental type, while its marine representatives 

 show the prevalence of coarse grits and sandstones in the 

 lower series, of limestones and slates in the middle series, 

 and the recurrence of sandstones in the upper series ; and 

 this sequence occurs alike in North-Western Europe, in 

 America, and .Australia. The Carboniferous contains the 

 first regional beds of thick limestone and the first important 

 Coal Measures. The Trias is as characterised by rocks 

 indicating arid continental conditions in America and 

 Australia and -South Africa, as I'rof. Watts has shown 

 then prevailed in the neighbourhood of Leicester. In the 

 Mesozoic era we owe to Suess the demonstration of the 

 world-wide influence of those marine encroachments or 

 " transgressions " whereby the great continents of the 

 Trias were gradually submerged by the rising sea. 



.Speaking generally, there is a remarkable lithological 

 resemblance between contemporary formations in all parts 

 of the world. This fact had been often remarked, but was 

 usually dismissed as due to a number of local isolated 

 coincidences of no special significance. But the coinci- 

 dences are too numerous and too striking to be thus lightly 

 dismissed. They are among the indications that the main 

 earth-changes have been due to world-wide causes, which 

 led to the predominance of the same types of sediinentary 

 rocks during the same period in manv regions of the 

 world. 



The' conditions that govern the geological evolution and 

 general geography of the earth are probablv due to the 

 interaction between the earth's crust and the contracting 

 interior; they may take place as slow changes in the form 

 of the earth, causing the slow rising or lowering of the 

 sea surface, or the slow uplift or depression of regions 

 of the earth's crust; or they may give rise to periods of 

 violent volcanic action in many parts of the earth, between 

 which may be long periods of quiescence. The geo- 

 graphical effects of changes in the earth's quivering mass 

 affect distant regions at the same time. Therefore the 

 landmarks of physical geology will probably be found to 

 give more precise evidence as to geological synchronism 

 than those of Pal.ncontology, on which we have hitherto 

 had to rely. 



I\'. Philouisis and Ore-formation. 

 Belief in the e.irth 's internal fires was most faithfully 

 held amongst geologists by the Plutonists of the eighteenth 

 century, and repudiated with equal thoroughness by the 

 Neptunists, who refused to concede that volcanic action 

 was due to deep-seated cosmic causes. Thus Jameson in 

 1S07 stoutly maintained that volcanoes were superficial 

 phenomena due to the combustion of beds of coal beneath 

 fusible rocks, such as basalt, and that the explosions were 

 due to the sudden expansion of .sea-water into steam by 

 contact with the burning coal. Volcanoes, according to 

 this view, were correctly described as burning mountains, 

 giving forth fire, flame, and smoke. The extreme 

 Neptunist and Plutonist schools have long since been 

 extinct, but the controversy is not quite closed. The 

 battlefield is now practically restricted to economic geology, 

 and the issue is the origin of some important ores. 



