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 
‘his recent annual address: ‘‘ We know that the descent of a glacier, even 
down the steep declivities of Mont Blane, 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. * * * 
IT 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 1s 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 
