Dr. Mac Culloch on the Parallel Roads of Glen Roy. 373 



they have acquired their present abrupt form, and contracted their 

 original dimensions ; while in certain cases even their very remains 

 may have disappeared, where peculiar or accidental circumstances 

 enabled the river to act on them to the last extremity. 



It will be readily asked why similar terraces are not found at the 

 two upper lines, as well as at the lower one. The answer is not 

 only easy, but tends to confirm the view of their origin here given. 

 It must be recollected that the present state of the valley is essentially 

 different from its condition under both the former cases. In both, 

 the bottom of the valley, instead of being the course of a running 

 stream, was a lake. Under such circumstances it is evident, that 

 the effect of these waters on any deltas occupying such a relative 

 position as the level which the lake last quitted, must have been to 

 undermine and remove them had they existed to any extent ; 

 whereas, on the final drainage of the whole, they have necessarily 

 been left in their original integrity, subject only to the gradual, cor- 

 rosive, and always diminishing action of the river. But it must 

 also be considered, that as the rapidity of the Roy and its conse- 

 quent corrosive power, was much less considerable when the valley 

 was full of water, and its fall was consequently less, a much less 

 quantity of alluvial matter would have accumulated at its entrance 

 during the existence of the two first lakes. 



But indeed, although no conspicuous terraces are found at the level 

 of the upper lines, there are sufficient indications of their former 

 existence in many places ; while in all they lie near the entrances of 

 the torrents, and thus confirm the view which I have here given of 

 their origin. I shall here also remark that this view explains those 

 irregular appearances of lines unconnected with the principal ones, 

 which I have, in the description, sometimes called supernumerary, 

 and which occur here and there in positions intermediate to them. 



Vol. iv. 3 b 



