450 GEOLOGY OF OHIO. 



feet high, which is not as high as the highest lands of the Ohio water- 

 shed, as reported by Dr. Newberry) between the lakes and Hudson's Bay, 

 across a general depression, in which lie the lakes, and up over the water- 

 shed dividing the waters of the lakes and the Ohio River. The Duke of 

 Argyll, President of the Geological Society of London, has well said in 

 tiis recent annual address : "We know that the descent of a glacier, even 

 •down the steep declivities of Mont Blanc, is retarded by such an enor- 

 mous amount of friction that the coherence of its substance is overcome ; 

 :the base of it is, as it were, torn from its superincumbent mass, and the 

 progress of the base is reduced to one-half of the rate at which the sur- 

 face moves. We know that this is the result in a case where the force 

 of gravity is at its maximum, and none of its momentum has been lost. 

 We know, also, that in no part of the existing world is the phenome- 

 non presented of ice streams moving for great distances even over level 

 ground, still less ascending steep gradients, and this, too, at a great dis- 

 tance from the declivities which give impetus to forward motion. * * * 

 I can not help thinking that there is a fundamental fallacy in compar- 

 ing the movement of ice masses down the slopes of a mountain with any 

 movement of such masses which is possible on level ground or against 

 opposing slopes. In the one case gravity is an assisting, in the other 

 case it is a resisting, force. In the one case, the heavier the mass of ice 

 the easier and faster will be its motion ; in the other case, every addi- 

 tional ton must add to the difficulty of effecting movement. In the one 

 case, thrust and gravity act together ; in the other case, thrust must act 

 alone, with gravity and friction to counteract it. * * * * It would 

 be altogether illogical to suppose that because these molecular changes 

 (by variations of temperature acting on the molecular structure of the 

 ice) are able to overcome friction when they are powerfully assisted by 

 the gravity of the mass lying on a steep slope, therefore they are equally 

 able to overcome friction with no such assistance from gravity, but, on 

 the contrary, with gravity exerting all its force in favor of rest and 

 against motion of any kind." 



In Canada and along the old sea margin, as at Portland, Maine, the 

 Drift clays contain marine shells of existing arctic forms. Principal 

 Dawson, of Montreal, who has studied the Drift phenomena of Canada 

 with rare ability and painstaking, has enumerated from the Drift, of 

 plants, 10 species; radiata, 24; mollusca, 140; articulata, 26; and verte- 

 brata, chiefly fishes, 5. In the West no marine forms, so far as I know, 

 have been found in our Drift clays, but I doubt whether the search has 

 been sufficiently extensive and minute. Plants, however, are not un- 

 common. The living marine forms dredged from the depths of Lake 



