62 THE HISTORY OF THE NORTH ATLANTIC 



thickness. It is true we have no means of exploration in the 

 earth's interior, but the conjoined labours of physicists have 

 now proceeded sufficiently far to throw much inferential light 

 on the subject, and to enable us to make some general affirma- 

 tions with certainty ; and these it is the more necessary to 

 state distinctly, since they are often treated as mere subjects of 

 speculation and fruitless discussion. 



(1) Since the dawn of geological science, it has been evi- 

 dent that the crust on which we live must be supported on a 

 plastic or partially liquid mass of heated rock, approximately 

 uniform in quality under the whole of its area. This is a 

 legitimate conclusion from the wide distribution of volcanic 

 phenomena, and from the fact that the ejections of volcanoes, 

 while locally of various kinds, are similar in every part of the 

 world. It led to the old idea of a fluid interior of the earth, 

 but this seems now generally abandoned, and this interior 

 heated and plastic layer is regarded as merely an under-crust, 

 resting on a solid nucleus. 1 



(2) We have reason to believe, as the result of astronomical 

 investigations, 2 that, notwithstanding the plasticity or liquidity 

 of the under-crust, the mass of the earth its nucleus as we 

 may call it is practically solid and of great density and 

 hardness. Thus we have the apparent paradox of a solid yet 

 fluid earth ; solid in its astronomical relations, liquid or 



1 I do not propose to express any definite opinion as to this question, as 

 either conclusion will satisfy the demands of geology. It would seem, 

 however, that astronomers now admit a slight periodical deformation of 

 the crust. See Lord Kelvin's Anniversary Address to Royal Society, 

 1892. 



2 Hopkins, Mallet, Lord Kelvin, and Prof. G. H. Darwin maintain 

 the solidity and rigidity of the earth on astronomical grounds ; but different 

 conclusions have been reached by Fisher, Hennesey, Delaunay, and Airy. 

 In America, Hunt, Barnard and Crosby, Button, Le Conte and Wadsworth 

 have discussed these questions. Bonney has suggested that a mass may be 

 slowly mobile under long-continued pressure, while rigid with reference to 

 more sudden movements. 



