1838.] GLEX ROY. 291 



by all his characteristic acutcncss of observation and determina- 

 tion to consider all possible objections. It is a curious example, 

 however, of the danger of reasoning by a method of exclusion 

 in Natural Science. Finding that the waters which formed 

 the terraces in the Glen Roy region could not possibly have 

 been dammed back by barriers of rock or of detritus, he saw 

 no alternative but to regard them as the work of the sea. 

 Had the idea of transient barriers of glacier-ice occurred to 

 him, he would have found the difficulties vanish from the 

 lake-theory which he opposed, and he would not have been 

 unconsciously led to minimise the altogether overwhelming 

 objections to the supposition that the terraces are of marine 

 origin." 



It may be added that the idea of the barriers being formed 

 by glaciers could hardly have occurred to him, considering 

 what was the state of knowledge at the time, and bearing in 

 mind his want of opportunities of observing glacial action 

 on a large scale. 



The latter half of July was passed at Shrewsbury and Maer. 

 The only entry of any interest is one of being "very idle" at 

 Shrewsbury, and of opening "a note-book connected with 

 metaphysical inquiries." In August he records that he read 

 " a good deal of various amusing books, and paid some atten- 

 tion to metaphysical subjects." 



The work done during the remainder of the year comprises 

 the book on coral reefs (begun in October), and some work 

 on the phenomena of elevation in S. America.] 



C. Darwin to C. LyelL 



36 Great Marlborough Street, 



August 9th [1838], 



MY DEAR LYELL, 



I did not write to you at Norwich, for I thought 

 I should have more to say, if I waited a few more days. Very 

 many thanks for the present of your 'Elements,' which I 

 VOL. I. X 





