86 PAEALLEL EOADS [Ch. VII. 



operation being afterwards repeated at lower levels, there will be several 

 successive cliffs and terraces. 



Parallel roads. — Tlie parallel shelves, or roads, as they have been 

 called, of Lochaber or Glen Roy and other contiguous valleys in Scot- 

 land, are distinct both in character and origin from the terraces above 

 described ; for they have no slope towards the sea like the channel of a 

 river, nor are they the effect of denudation. Glen Roy is situated in 

 the western Highlands, about ten miles north of Fort William, near the 

 western end of the great glen of Scotland, or Caleoonian Canal, and near 

 the foot of the highest of the Grampians, Ben Nevis. Throughout its 

 whole length, a distance of more than ten miles, two, and in its lower 

 part three, parallel roads or shelves are traced along the steep sides of 

 the mountains, as represented in the annexed figure (fig. 102), each 

 maintaining a perfect horizontality, and continuing at exactly the same 

 level on the opposite sides of the glen. Seen at a distance, they appear 

 like ledges or roads, cut artificially out of the sides of the hills ; but 

 when w^e are upon them we can scarcely recognize their existence, so 

 uneven is their surface, and so covered with boulders. They are from 

 10 to 60 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 deposits, as may be seen at those 

 points where ravines have been excavated by torrents. The parallel 

 shelves, therefore, have not been caused by denudation, but by the depo- 

 sition of detritus, precisely similar to that which is dispersed 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 only, in a line just above the parallel roads. 

 The highest of these roads is about 1250 feet above the level of the sea, 

 the next about 200 feet lower than the uppermost, and the third still 

 lower by about 50 feet. It is only this last, or the lowest of the three, 

 which is continued throughout Glen Spean, a large valley with which 

 Glen Roy unites. As the shelves are always at the same height above 

 the sea, they become continually more elevated above the river in pro- 

 portion 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. I should 

 exceed the limits of this work, were I to attempt to give a full descrip- 

 tion of all the geographical circumstances attending these singular ter- 

 races, or to discuss the ingenious theories which have been severally 

 proposed to account for them by Dr. MacCuiloch, Sir T. D. Lauder, and 

 Messrs. Darwin, Agassiz, Milne, and Chambers. There is one point, 

 however, on which all are agreed, namely, that these shelves are ancient 

 beaches, or littoral formations accumulated round the edges of one or 

 more sheets of water which once stood at the level, first of the highest 

 shelf, and successively at the height of the two others. It is well known, 

 that wherever a lake or marine fiord exists surrounded by steep moun- 



