412 ■ Notices of Memoirs — Australia, 1914 — 



gas-springs are sometimes found in areas of marked gravity defect, and some 

 of these are now also being drilled for natural gas. 



When our ideas of the state of affairs below the surface thus begin to yield 

 economic results, there is hope that they are at last steadying down, becoming 

 more settled, and indeed more ' scientific'. It may not be unprofitable, therefore, 

 to review some of the advances recently made in developing theoretical concep- 

 tions regarding the interior of the Earth that are of direct importance to 

 geologists. In undertaking this review I am conscious of the fact that I shall 

 be traversing ground that is generally familiar to all, and much of it the special 

 property of specialists whose views I hesitate to summarize and should not dare 

 to criticize. As the author of the Ingoldshy Legends said of the only story 

 that Mrs. Peters would allow her husband to finish, " The subject, I fear me, 

 is not over new, but will remind my friends — 



'Of something better they have seen before.' " 



The intensity and quantity of polemical literature on scientific problems 

 frequently varies inversely as the number of direct observations on which the 

 discussions are based : the number and variety of theories concerning a subject 

 thus often form a coefficient of our ignorance. Beyond the superficial observa- 

 tions, direct and indirect, made by geologists, not extending below about one 

 two-hundredth of the Earth's radius, we have to trust to the deductions of 

 mathematicians for our ideas regarding the interior of the Earth ; and they 

 have provided us successively with every permutation and combination possible 

 of the three physical states of matter — solid, liquid, and gaseous. 



Starting, say, two centuries back with the astronomer Halley, geologists were 

 presented with a globe whose shell rotated at a rate different from that of its core. 

 In more recent times this idea has been revived by Capt. (Sir) F. J. Evans (1878) 

 to account for the secular variations in the declination of the magnetic needle. 



Clairault's celebrated theorem (1743), on which Laplace based the most 

 long-lived among many cosmogonies, gave us a globe of molten matter sur- 

 rounded by a solid crust. Hopkins demanded a globe solid to the core, and, 

 though his arguments were considered to be unsound, his conclusions have been 

 revived on other grounds ; while the high rigidity of the Earth as a body has 

 been maintained by Lord Kelvin, Sir George Darwin, Professor Newcombe, 

 Dr. Eudski, and especially by the recent observations of Dr. 0. Hecker, supple- 

 mented by the mathematical reasoning of Professor A. E. H. Love. Hennessy 

 (1886), however, concluded that the astronomical demands could be satisfied 

 by the old-fashioned molten Earth in which the heavier substances conformed 

 to the equatorial belt. 



As long ago as 1858 Herbert Spencer suggested that, on account of its 

 temperature being probably above the critical temperature of known elements, 

 the centre of the Earth is possibly gaseous. Late in the seventies Dr. Eitter 

 revived the idea of a gaseous core surrounded by a solid crust, and this was 

 modified in 1900 by the Swedish philosopher, Svante Arrhenius, whose globe 

 with a solid crust, liquid substratum, and gaseous core is now a favourite 

 among some geologists. 



Wiechert (1897) supposed that the core of the Earth, some 5,000 kilometres 

 in radius, is composed mostly of iron with a density of 7-8, while this is sur 

 rounded by a shell of lithoidal material having a density of about 3-0 to 3-4 ; 

 and this great contrast in density is about that which distinguishes the iron 

 meteorites generally from those of the stony class. Arrhenius also assumes 

 that iron forms the main part of the central three-quarters, and he shows that 

 this distribution of substance may still be consistent with his theory of a gaseous 

 core : indeed, he not only imagines that the whole of the iron nucleus is 

 gaseous, but also most of the siliceous shell, for he leaves only 5 per cent of 

 the radius as the depth of the solid and liquid shells combined. 



But the variety of ideas does not end with theories on the present constitution 

 of the globe. Poisson required the process of solidification to begin from the 

 centre and to progress outwards, while other mathematicians had been happy 

 with the Leibnitzian consistentior status as the first external slaggy crust. 



