164 GEOLOGY [CHAP. IX 



Letter 510 views a very long period before the Cambrian formation. If 

 it would not trouble you, I should like to hear what you think 

 of Lyell's remark on the magnetic force which comes from 

 the sun to the earth : might not this penetrate the crust of the 

 earth and then be converted into heat ? This would give a 

 somewhat longer time during which the crust might have been 

 solid ; and this is the argument on which Sir W. Thomson 

 seems chiefly to rest. You seem to argue chiefly on the 

 expenditure of energy of all kinds by the sun, and in this 

 respect Lyell's remark would have no bearing. 



My new edition of the Origin^ will be published, I suppose, 

 in about two months, and for the chance of your liking to have 

 a copy I will send one. 



P.3.--I wish that you would turn your astronomical know- 

 ledge to the consideration whether the form of the globe does 

 not become periodically slightly changed, so as to account for 

 the many repeated ups and downs of the surface in all parts 

 of the world. I have always thought that some cosmical 

 cause would some day be discovered. 



Letter 51 1 To C - 



Down, July I2th [1872], 



I have been glad to see the enclosed and return it. It 

 seems to me very cool in Agassiz to doubt the recent up- 

 heaval of Patagonia, without having visited any part; and he 

 entirely misrepresents me in saying that I infer upheaval 

 from the form of the land, as I trusted entirely to shells 

 embedded and on the surface. It is simply monstrous to 

 suppose that the terraces stretching on a dead level for 

 leagues along the coast, and miles in breadth, and covered 

 with beds of stratified gravel, 10 to 30 feet in thickness, are 

 due to subaerial denudation. 



As for the pond of salt-water twice or thrice the density 

 of sea-water, and nearly dry, containing sea-shells in the same 

 relative proportions as on the adjoining coast, it almost passes 

 my belief. Could there have been a lively midshipman on 

 board, who in the morning stocked the pool from the adjoin- 



ing coast ? 



As for glaciation, I will not venture to express any 

 opinion, for when in S. America I knew nothing about 



1 Fifth edition, May, 1869. 



