18401881] EARTH-MOVEMENTS 145 



Forbes, however, would know. What you say about the Letter 495 

 plications being steepest in the central and generally highest 

 part of the range is conclusive to my mind that there has 

 been the chief axis of disturbance. The lateral thrusting 

 has always appeared to me fearfully perplexing. I remember 

 formerly thinking that all lateral flexures probably occurred 

 deep beneath the surface, and have been brought into view 

 by an enormous superincumbent mass having been denuded. 

 If a large and deep box were filled with layers of damp 

 paper or clay, and a blunt wedge was slowly driven up from 

 beneath, would not the layers above it and on both sides 

 become greatly convoluted, whilst those towards the top 

 would be only slightly arched ? When I spoke of the Andes 

 being comparatively recent, I suppose that I referred to the 

 absence of the older formations. In looking to my volume, 

 which I have not done for many years, I came upon a 

 passage (p. 232) which would be worth your looking at, if 

 you have ever felt perplexed, as I often was, about the sources 

 of volcanic rocks in mountain chains. You have stirred up 

 old memories, and at the risk of being a bore I should like to 

 call your attention to another point which formerly perplexed 

 me much viz. the presence of basaltic dikes in most great 

 granitic areas. I cannot but think the explanation given at 

 p. 123 of my Volcanic Islands is the true one. 1 



To Victor Carus. 2 Letter 496 



Down, March 2ist, 1876. 



The very kind expressions in your letter have gratified me 

 deeply. 



I quite forget what I said about my geological works, but 

 the papers referred to in your letter are the right ones. I 

 enclose a list with those which are certainly not worth trans- 

 lating marked with a red line ; but whether those which are 



1 On p. 123 of the Geological Observations on the Volcanic Islands 

 visited during the Voyage of H. M.S. Beagle, 1844, Darwin quotes several 

 instances of greenstone and basaltic dikes intersecting granitic and allied 

 metamorphic rocks. He suggests that these dikes "have been formed 

 by fissures penetrating into partially cooled rocks of the granitic and 

 metamorphic series, and by their more fluid parts, consisting chiefly of 

 hornblende oozing out, and being sucked into such fissures." 



2 Professor Victor Carus translated several of Mr. Darwin's books 

 into German (see Life and Letters, III., p. 48). 



VOL. II. 10 



