52 OS THE PARALLEL ROADS 



trust will appear sufficiently evident, if the explanatory dia- 

 gram I have employed, Plate VII. fig. 5. be examined, that 

 the operation of the waves of a lake, in eating out a shelf on 

 the side of a mountain, will be much more powerful when the 

 surface of the mountain is yet entire, than it will continue to 

 be afterwards, when a certain proportion of sloping beach has 

 been formed, over which it may in some degree waste the fu- 

 ry of its waves. We see that the natural shelves now existing 

 around the borders of our Highland lakes, do not appear to 

 have been very much, if at all increased, beyond the breadth 

 of those remaining from the lakes which we suppose to have 

 been emptied at so very remote a period. The depth, there- 

 fore, of the indentation of a shelf, does not form any criterion 

 whereby we may judge of the length of time expended in its 

 formation. Shelf 4th furnishes an excellent example of this 

 observation, for it is as deeply indented, and as well defined, 

 in many parts of Glen Roy, as it is any where in Glean Spean y 

 although it is evident, that the lake which formed it, must 

 have existed much longer in the latter glen than it did in the 

 former. It does not therefore appear to be absolutely neces- 

 sary, that Loch Roy should have remained long at the level of 

 Shelf 3d, before its second subsidence took place ', but it is not 

 very material to the theory whether it did so or not. It is 

 evident, that one material change must have taken place in rt, 

 in consequence of the sudden fall of its level. The whole ex- 

 tent of Upper Glen Roy must have been laid dry, and, conse- 

 quently, it would now no longer discharge the united streams 

 of the Gluoy and Roy, by the channel of the river running 

 from the point at Loch Spey ; but leaving the stream from that 

 little residuary lake to pursue its own course, it would now, in 

 the first instance, become tributary to the great Loch Spean, 

 and would be carried by means of its river through the Pass of 



MuckuL 



