174 GEOLOGY [CHAP. IX 



Letter 517 in Glen Spean, and the one in Kilfinnin), and explains the 

 coincidence of " roads " with the watersheds more simply 

 than my view, and as simply as the common lake theory. 

 But how was the Glen Roy lake drained when the water 

 stood at level of the middle "road"? It must (for there is 

 no other exit whatever) have been drained over the glacier. 

 Now this shelf is full as narrow in a vertical line and as 

 deeply worn horizontally into the mountain side and with a 

 large accumulation of shingle (I can give cases) as the other 

 shelves. We must, therefore, on the glacier theory, suppose 

 that the surface of the ice remained at exactly the same level, 

 not being worn down by the running water, or the glacier 

 moved by its own movement during the very long period 

 absolutely necessary for a quiet lake to form such a beach 

 as this shelf presents in its whole course. I do not know 

 whether I have explained myself clearly. I should like to 

 know what you think of this difficulty. I shall much like to 

 talk over the Jura case with you. I am tired, so good-bye. 



Letter 518 To L. Horner. 



Down [1846]. 



In following your suggestion in drawing out something 

 about Glen Roy for the Geological Committee, 1 I have been 

 completely puzzled how to do it. I have written down what 

 I should say if I had to meet the head of the Survey and 

 wished to persuade him to undertake the task ; but as I have 

 written it, it is too long, ill expressed, seems as if it came from 

 nobody and was going to nobody, and therefore I send it to 

 you in despair, and beg you to turn the subject in your mind. 

 I feel a conviction if it goes through the Geological part of 

 Ordnance Survey it will be swamped, and as it is a case for 



1 It was agreed at the British Association meeting held at South- 

 ampton in 1846 "That application be made to Her Majesty's Government 

 to direct that during the progress of the Ordnance Trigonometrical 

 Surveys in the North of Scotland, the so-called Parallel Roads of Glen 

 Roy and the adjoining country be accurately surveyed, with the view 

 of determining whether they are truly parallel and horizontal, the inter- 

 vening distances, and their elevations above the present sea-level" 

 (Brit. Assoc. Rep., 1846, p. xix). The survey was undertaken by the 

 Government Ordnance Survey Office under Col. Sir Henry James, who 

 published the results in 1874 (Notes on the Parallel Roads of Glen Roy} ; 

 the map on which the details are given is sheet 63 (one-inch scale). 



