170 



The following Communication was then read : — 



On the Theory of the Parallel Roads of Lochaber. By James 

 Thomson, Esq. jun., Glasgow. Communicated by Professor 

 Forbes. 



The author, after briefly stating the views of Mr Mibie, and the 

 remarks of Sir G. S. Mackenzie, gave his reasons for agreeing with 

 the former, that the terraces were the beaches of lakes, formed by 

 barriers across the valleys ; and, with the latter, in holding that these 

 barriers could not have been formed of earthy detritus. He then 

 proceeded to shew tliat the theory of Agassiz, according to which the 

 barriers were formed of glaciers, was the most probable yet advanced, 

 and while it required some modification in the details to render it 

 consistent with recently observed facts, was strongly supported by the 

 researches of Professor Forbes, both in regard to the former exis- 

 tence of glaciers in our latitudes, as demonstrated in the case of the 

 CuchuUin Hills, and in regard to the laws of the motion of glaciers, 

 as developed in Professor Forbes's papers on the Glaciers of the Alps. 

 He pointed out that all the difficulties of the theory of earthy bar- 

 riers were connected with the notion of their being composed of 

 earthy detritus, and that both Sir T, D. Lauder and Mr Milne ad- 

 mitted the great difficulty of accounting for their disappearance. 



He then explained the modifications which were i-equired to ren- 

 der the theory of Agassiz capable of explaining all the facts hitherto 

 observed. 



The highest shelf in Glen Roy stops short just above the opening 

 into Glen Glastcr, and this would have been the result had the bar- 

 rier which formed that shelf blocked up Glen Roy above the latter 

 (rlen, and thus forced the water to be discharged by the water-shed 

 at the head of the valley of the Spey, It must also have blocked 

 up Glen Collarig nearly to the Gap. To form the middle shelf, this 

 barrier had only to retire a little, so as to open up Glen Glaster, 

 when the water would discharge itself by the ancient river-course 

 leadino- from the water-shed in Glen Glaster, first pointed out by 

 Mr Milne. 



The lowest shelf would be formed when the glacier retired to near 

 the mouth of Glen Spean. 



