256 THEORY OF AGASSIZ. chap. xiir. 



corresponding shelf is seen at the same level passing round 

 the hill, as would have happened if it had once formed an 

 island in a lake or fiord. Another very remarkable pecu- 

 liarity in these terraces is this : each of them comes in some 

 portion of its course to a col, or parting ridge between the 

 heads of glens, the explanation of which will be considered 

 in the sequel. 



Those writers who first advocated the doctrine that the 

 roads were the ancient beaches of fresh-water lakes were 

 unable to offer any probable hypothesis respecting the form- 

 ation and subsequent removal of barriers of sufficient height 

 and solidity to dam up the water. To introduce any violent 

 convulsion for their removal was inconsistent with the unin- 

 terrujited horizontality of the roads, and with the undisturbed 

 asjDect of those parts of the glens where the shelves come 

 suddenly to an end. 



Mr. Agassiz and Dr. Buckland, desirous, like the defenders 

 of the lake theory, to account for the limitation of the shelves 

 to certain glens, and their absence in contiguous glens, where 

 the rocks are of the same composition, and the slope and in- 

 clination of the ground very similar, first started the theory 

 that these valleys were once blocked up by enormous glaciers 

 descending from Ben Nevis, giving rise to what are called, in 

 Switzerland and in the TjtoI, glacier-lakes. In corroboration 

 of this view, they contended that the alluvium of Glen Roy, 

 as well as of other parts of Scotland, agrees in character with 

 the moraines of glaciers seen in the Alpine valleys of Switzer- 

 land. It Avill readily be conceded that this hypothesis was 

 preferable to any previous lacustrine theory, by accounting 

 more easily for the temporary existence and entire disappear- 

 ance of lofty transverse barriers, although the height required 

 for the supposed dams of ice appeared ver}" enormous. 



Before the idea of glacier-lakes had been suggested by 

 Agassiz, Mr. Darwin examined Glen Eoy, and came to the 



