ADDRESS. 7 



dotormined tho occuirenco of elevations along j^reat circles of the earth's 

 surface tfltignnt to the polar circles. 



We are invited by the preceding general glance at the surface of the 

 earth to ask certain questions respecting the Atlantic. (1) "What has at 

 first determined its position and form ? (2) What changes has it experi- 

 encod in the lapse of geological time ? (S) What relations have these 

 changes borne to the ievelo'iment of life on the land and in the watc 

 (4) What is its probable future ? 



Before attempting to answer these questions, which I shall not i -Ke 

 up formally in succession, but rather in connection with each other, it is 

 necessary to state as briefly as possible certain general conclusions re- 

 specting the interior of the earth. It is popularly supposed that we 

 know nothing of this beyond a superficial crust perhaps averaging 50,000 

 to 100,000 feet in thickness. It is true we have no means of exploration 

 in the earth's interior, but the conjoined labours of physicists and geo- 

 logists have now proceeded sufficiently far to throw much inferential 

 light on the subject, and to enable us to make some general affirmafciounj 

 with certainty ; and these it is the more necessary to state distinctly, 

 since they are often treated as mere subjects of speculation and fmitless 

 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, approximately uniform in quality under the whole 

 of its area. This is a legitimate conclusion from the wide distrihution 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 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 astronomical investiga- 

 tions,' 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 superficial 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 



' 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 Barnard and . 

 Crosby, Button, Le Conte, and Wadsworth have discussed these questions. 



* An objection has been taken to the effect that the supposed ellipsoidal form of 

 tlie 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 



