210 Canadian Record of Science. 



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 

 evident that the crust on which we live must be supported 

 on a plastic or partially liquid mass of heated rock, approxi- 

 mately uniform in quality under the whole of its area. This 

 is a legitimate conclusion from the wide distribution of vol- 

 canic 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 is now generally abandoned, 

 and this interior heated and plastic layer is regarded as 

 merely an under-crust. 



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

 mical investigations, a 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 plastic for the purposes of volcanic action and su- 

 perficial movements. - 



(3) The plastic sub-crust is not in a state of dry igneous 

 fusion, but in that condition of aqueo-igneous or hydro- 

 thermic fusion which arises from the action of heat on moist 

 substances, and which may either be regarded as a fusion 

 or as a species of solution at a very high temperature. This 

 we learn from the phenomena of volcanic action, and from 



1 Hopkins, Mallet, Sir William Thomson, and Prof. G. H. Darwin 

 maintain the solidity and rigidity of the earth on astronomical 

 grounds ; but different conclusions have been reached by Hennesey, 

 Delaunay, and Airy. In America, it was taught from 1858 by 

 Sterry Hunt, and later by Shaler and Le Conte. 



2 An objection has been taken to the effect that the supposed 

 ellipsoidal form of the equator is inconsistent with a plastic sub- 

 crust. But this ellipsoidal form is not absolutely certain, or, if it 

 exists, is very minute. Bonney has, in a recent lecture, suggested 

 the important consideration that a mass may be slowly mobile 

 under long-continued pressure, while yet rigid with reference to 

 more sudden movements. 



