240 MAN AND THE GLACIAL PERIOD. 



face and so covered with boulders. They are from ten to 

 sixty feet broad, and merely differ from the side of the 

 mountain by being somewhat less steep. 



" On closer inspection, we find that these terraces are 

 stratified in the ordinary manner of alluvial or littoral de- 

 posits, as may be seen at those points where ravines have 

 been excavated by torrents. The parallel shelves, there- 

 fore, have not been caused by denudation, but by the dep- 

 osition of detritus, precisely similar to that which is dis- 

 persed in smaller quantities over the declivities of the hills 

 above. These hills consist of clay-slate, mica-schist, and 

 granite, which rocks have been worn away and laid bare 

 at a few points immediately above the parallel roads. The 

 lowest of these roads is about 850 feet above the level of 

 the sea, and the next about 212 feet higher, and the third 

 82 feet above the second. There is a fourth shelf, which 

 occurs only in a contiguous valley called Glen Gluoy, which 

 is twelve feet above the highest of all the Glen Eoy roads, 

 and consequently about 1,156 feet above the level of the 

 sea. One only, the lowest of the three roads of Glen Roy, 

 is continued through Glen Spean, a large valley with which 

 Glen Roy unites. As the shelves, having no slope towards 

 the sea like ordinary river terraces, are always at the same 

 absolute height, they become continually more elevated 

 above the river in proportion as we descend each valley ; 

 and they at length terminate very abruptly, without any 

 obvious cause, or any change either in the shape of the 

 ground or in the composition or hardness of the rocks." * 



Early in his career Charles Darwin studied these an- 

 cient beaches, and ascribed them to the action of the sea 

 during a period of continental subsidence. In this view 

 he was supported by the majority of geologists until the 

 region was visited by Agassiz, who saw at once the true 

 explanation. If these were really sea-beaches, similar de- 



* Antiquity of Man, pp. 252, 253. 



