The Geology of Hingluim. 57 



lay over the land, many hundreds of feet in depth, during the 

 summer, particularly towards the close of the period, rivers flowed 

 over its surface, as they now do over the glaciers of the Alps. 

 As there, crevasses were formed in the ice, into which the water 

 poured and worked passages to the bottom of the great sheet, dis- 

 charging itself in torrents, often conveying stones and other mo- 

 raine matter to the rock surfaces below. Such passages in modern 

 glaciers become somewhat circular in form and are hence called 

 wells. They are also called moulins, the latter name from the 

 noise made by the rushing waters in the ice, being not unlike 

 that of a mill. The water, and the material conveyed by it through 

 such wells of the great glacier of our continent, must have smoothed 

 and worn rapidly away the rock surface on which they impinged, 

 often causing, by the same kind of action as is witnessed under 

 falls of water in some of our rivers, holes in the rocks like those 

 now under consideration. Of course the action of the water and 

 material conveyed by it would be immensely more rapid in form- 

 ing such holes, falling, as they undoubtedly did, from a great 

 height, and striking upon the rocks below with intense force. 

 This would lead to the abrasion of the rock, by any rotating 

 stones lodged in the hollows, so much more powerful than any 

 action we know under falling waters of the present day as to 

 render estimation of the result incalculable. 



It is doubtful, however, to the mind of the writer, if circum- 

 stances often favored the formation of pot-holes directly beneath 

 such a fall and where its full force would be felt. He is impressed 

 with the view that if this were the case they would not be found 

 ha vino; the form thev horizontallv present. 



It has, indeed, been thought strange that, as the ice moved con- 

 tinuously on, the holes were not found generally elongated in the 

 direction of the movement of the glacier rather than circular. 

 Such thought, however, is only consistent with the presumption 

 that the holes were made just where the water first fell upon 

 the rock surface below. Far more reasonable is it to suppose that 

 the holes were formed somewhat distant from this place, where 

 the masses of rocks borne by the waters found a lodging in some 

 depression, and there by rotation worked out the pot-holes. The ice 

 might move on and the waters descend through the moulin far from 

 where they first fell, yet continue their flow in the same direction 

 as at first,' and go on with the work of rotating the contents of the 

 hole through a whole season. In such case there could be, of course, 

 no reason to expect elongation. 



The fact that pot-holes have been found in near proximity, 

 and in such positions relative to each other as to show them to 

 be apparently the result of independent falls of water, leads to a 

 consideration of what has been noticed in the Alps. Observation 

 upon the glaciers there shows that as a crevasse is carried for- 

 ward by the general movement of the ice from where it received 

 the flow of waters in the summer, and winter cuts off the supply, 



