2G8 Mr. GeiUe [April 6, 



WEEKLY EVENING MEETING, 



Friday, April 6, 1883. 



William Bowman, Esq. LL.D. F.E.S. Honorary Secretary and 



Vice-President, in tlie Chair. 



Archibald Geikie, Esq. F.R.S. 

 Director-General of the Geological Survey of the United Kingdom. 



The Canons of the Far West. 



The drainage lines of a country are among the most permanent 

 features in its topography. Where a river has once fastened its grasp 

 upon the surface, there it for the most part remains, and nothing short 

 of some colossal upheaval can turn it into a new course. The rivers 

 of Europe have served as types of fluviatile action in geology. Yet 

 an examination of the conditions under which they work shows that 

 they have been impeded by influences of various kinds. Except in 

 their mountain tributaries, they flow over comparatively low land. 

 Their gentle declivity prevents them from attaining any great erosive 

 power, and, as one result of this characteristic, they have cut com- 

 paratively few deep narrow winding gorges. The geological structure 

 of this continent is moreover so complicated, that hard and soft rocks 

 are thrown together in rapid alternation, and little scope is afforded 

 for the excavation of continuous ravines. The climate, too, being 

 comparatively moist, much general disintegration of the surface takes 

 place, and the detritus washed off by rain loads the rivers nearly to 

 the maximum of their transporting power. Whore, therefore, as is 

 usually the case, the rivers flow in open valleys with gently sloping 

 sides, the rate of atmospheric denudation has been at least as great as, 

 or greater than, that of river erosion. Where, on the other hand, 

 they flow in narrow precipitous ravines, they have been able to 

 erode faster than the atmospheric agents have worn down the sur- 

 rounding surface. The influence of vegetation has likewise affected 

 the general disintegration of the surface. But perhaps the most 

 important factor has been the glaciation of the Ice-Age. A large 

 part of the area was under ice at that period. The minor pre-glacial 

 contours were then in great measure obliterated, either by being 

 ground down by the movement of the ice-sheets, or by being buried 

 under the masses of clay, earth, and stones spread out over the lower 

 grounds and valleys on the retreat of the ice. The river-channels 

 would especially suffer. The valleys that existed before the advent 

 of the ice generally remained valleys after the ice had gone, but the 

 actual channels of the post-glacial drainage would only occasionally 



