Ch. VIL] OF GLEN EOT. 87 



tains subject to disintegration by frost or tlie action of torrents, some 



loose matter is washed down annually, especially during tlie melting of 



snow, and a check is o'iven to the descent of 

 ri?. i03. , . ' . , ^ . , 



this detritus at the point where it reaches 



the waters of the lake. The waves then 

 spread out the materials along the shore, and 

 throw some of them upon the beach ; theii 

 dispersing power being aided by the ice, 

 which often adheres to pebbles during the 

 winter months, and gives buoyancy to them. 

 The annexed diagram illustrates the manner 

 in which Dr. MacCulloch and Mr. Darwin 

 ^^Sck.''^^'""'^''""'"''^'"^'"''^ suppose "the roads" to constitute mere in- 

 CDEoads or shelves in the outer dentations in a Superficial alluvial coating' 



alluvial covering of the hill. -'■ . ^ 



which rests upon the hill-side, and consists 

 chiefly of clay and sharj) unrounded stones. 



Among other proofs that the parallel roads have really been formed 

 along the margin of a sheet of water, it may be mentioned, that wher- 

 ever an isolated hill rises in the middle of the glen above the level of 

 any particular shelf, a corresponding shelf is seen at the tame level 

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

 island in a lake or fiord. Another very remarkable peculiarity in these 

 terraces is this ; each of them comes in some portion of its course to a 

 col, or passage 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 freshwater lakes, were unable to offer any probable 

 hypothesis respecting the formation 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- 

 terrupted horizontality of the roads, and with the undisturbed aspect 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 com- 

 position, and the slope and inclination of the ground very similar, started 

 the conjecture that these valleys were once blocked up by enormous gla- 

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

 zerland and in the Tyrol, glacier-lakes. After a time the icy barrier 

 was broken down, or melted, first, to the level of the second, and after- 

 wards to that of the third road or shelf. 



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 Switzerland. Al- 

 lusion will be made in the eleventh chapter to the former existence of 

 glaciers in the Grampians : in the mean time it will readily be conceded 

 that this hypothesis is preferable to any previous lacustrine theory, by 



