296 GLACIERS OF CHILIAN ANDES. CHAP. xv. 



In 1841, I advanced, jointly with Mr. Darwin,* the theory 

 that the erratics may have been transferred by floating ice to 

 the Jura, at the time when the greater part of that chain, and 

 the whole of the Swiss valley to the south, was under the sea. 

 We pointed out, that if at that period the Alps had attained 

 only half their present altitude, they would yet have con 

 stituted a chain as lofty as the Chilian Andes, which, in a 

 latitude corresponding to Switzerland, now send down glaciers 

 to the head of every sound, from which icebergs, covered with 

 blocks of granite, are floated seaward. Opposite that part of 

 Chili where the glaciers abound, is situated the island of 

 Chiloe, one hundred miles in length, with a breadth of thirty 

 miles, running parallel to the continent. The channel which 

 separates it from the main land is of considerable depth, and 

 twenty-five miles broad. Parts of its surface, like the adja 

 cent coast of Chili, are overspread with recent marine shells, 

 showing an upheaval of the land during a very modern period ; 

 and beneath these shells is a boulder deposit, in which 

 Mr. Darwin found large blocks of granite and syenite, which 

 had evidently come from the Andes. 



A continuance in future of the elevatory movement, now 

 observed to be going on in this region of the Andes and of 

 Chiloe, might cause the former chain to rival the Alps in 

 altitude, and give to Chiloe a height equal to that of the Jura. 

 The same rise might dry up the channel between Chiloe and 

 the main land, so that it would then represent the great 

 valley of Switzerland. 



Sir Koderick I. Murchison, after making several impor 

 tant geological surveys of the Alps, proposed, in 1849, a 

 theory agreeing essentially with that suggested by Mr. Dar 

 win and myself, viz. that the erratics were transported to the 

 Jura, at a time when the great strath of Switzerland, and 



* See Elements of Geology, 2nd ed. 1841. 



