the Parallel Roads of Glen Roy. 435 



rative width and the steepness of the sides of the glens, and 

 the width of their openings into the glen of the canal, must 

 all be known before my suggestion can be either confirmed 

 or effectively objected to. If the Government could be pre- 

 vailed upon to order the Ordnance Survey of this district to 

 be completed, for the sake of assisting to clear up a very in- 

 teresting geological question, it would do itself honour. 



Keeping the topographical position of the valleys in view, 

 it will be perceived that the first effect of the debacle would 

 be to fill them with water. 



While the torrent swept over the tops of the mountains, no 

 such effect as the formation of the terraces could take place. 

 But as soon as the waters subsided below the summit level, 

 they would cease to flow towards Strathspey, and their cur- 

 rent would be changed into two, one directed towards each 

 end of the great glen. 



As long as a supply came from the N.W. all the glens in 

 question would be lakes, with surfaces comparatively still. 

 They would be to the waters of the debacle, what the sea 

 lochs are to the sea. 



Thus, then, my conjecture is, that the waters themselves 

 formed the barrier to these lakes. 



It will be easily conceived that as soon as the waters had 

 subsided to a certain extent, powerful currents and waves 

 would be produced by the opposition of the unequally elevated 

 surface of the land. 



Great waves would be produced at both ends of the great 

 valley where it expands, and these, by their action and reaction 

 along the line of the great glen, would agitate the lakes in 

 the other glens, and cause the water in them to subside, some- 

 times slowly, sometimes quickly, and thus to fulfill every con- 

 dition necessary for the deposition of the terraces. We must 

 keep in mind that amidst all the agitation, the waters were in 

 the mass gradually subsiding. 



Thus the agitation necessary to produce the terraces is 

 easily imagined ; and the gradual subsidence of the mass of 

 waters, interrupted occasionally by back waves caused by the 

 meeting of currents and the ocean tides, and agitation at the 

 outlets, will probably account for the appearance of successive 

 terraces to every mind accustomed to contemplate the vast 

 operations of natural causes that have turned and overturned 

 this world of ours. 



One difficulty will, however, immediately present itself when 

 the mass is inspected, that while there are three terraces 

 in Glen Roy, there is but one in the other giens, and that at 

 the lowest level. This, however, may hereafter be accounted 



3K2 



