THE PARALLEL ROADS OF GLEN ROY. 223 



spheric 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 perplexity 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 a^)ove the bottom of Grlen Spean, have dissolved 

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

 insisted on by Professor Geikie, that barriers of detritus 

 would undoubtedly have been able to maintain them- 

 selves 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 

 frequent 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 

 above and below them, while on heather-covered hills 

 the absence of the dark shrub from the roads greatly 

 enhances their conspicuousness. 



