THE PARALLEL ROADS OF GLEN ROY. 223 



mospheric conditions or to the character of the dam 

 itself, or through local softness in the drift, small 

 pseudo-terraces would be formed, which, to the per- 

 plexity of some observers, are seen upon the flanks of 

 the glens to-day. 



In presence then of the fact that the barriers which 

 stopped these glens to a height, it may be, of 1,500 

 feet above the bottom of Glen Spean, have dissolved 

 and left not a wreck behind; in presence of the fact, 

 insisted on by Professor Geikie, that barriers of detri- 

 tus would undoubtedly have been able to maintain 

 themselves had they ever been there; in presence of 

 the fact that great glaciers once most certainly filled 

 these valleys that the whole region, as proved by Mr. 

 Jamieson, is filled with the traces of their action; the 

 theory which ascribes the parallel roads to lakes 

 dammed by barriers of ice has, in my opinion, a degree 

 of probability on its side which amounts to a practical 

 demonstration of its truth. 



Into the details of the terrace formation I do not 

 enter. Mr. Darwin and Mr. Jamieson on the one side, 

 and Sir John Lubbock on the other, deal with true 

 causes. The terraces, no doubt, are due in part to the 

 descending drift arrested by the water, and in part to 

 the fretting of the wavelets, and the rearrangement of 

 the stirred detritus, along the belts of contact of lake 

 and hill. The descent of matter must have been fre- 

 quent when the drift was unbound by the rootlets 

 which hold it together now. In some cases, it may be 

 remarked, the visibility of the roads is materially aug- 

 mented by differences of vegetation. The grass upon 

 the terraces is not always of the same character as that 

 nbove and below them, while on heather-covered hills 



