i8Si. 29 Tratis. N. V. Ac. Set. 



and it is really a type of instability. Probably throughout the globe 

 local subsidence and elevation are constantly in progress. In the 

 interior of continents we have no evidence or measure of these, but 

 along coasts the water line tells us that changes are constantly 

 and everywhere taking place, in the relative level of land and sea. 

 About New York the coast is sinking, though very slowly, while further 

 north, in places, it is rising, and Greenland is sinking again. Back 

 from the coast there is no such nilometer, and yet we have no reason 

 to suppose that the earth is more fixed. Some indication is given by 

 the reports of those who dwell in mountainous regions, of changes of 

 level, which have shut from their view that which was before visible, or 

 revealed what was before concealed ; but these observations have not 

 been made with accuracy and cannot be depended upon. 



In a recent paper before the Academy he had shown the vast 

 changes which had occurred along the coast in this vicinity, viz., that 

 the land once stood 600 feet higher than at present : that the Hudson 

 river had then flowed by the city through a channel from 300 to 500 

 feet deep, now in large part silted up : that the Palisades then stood 

 from 700 to 800 feet above the river : that the Housatonic then flowed 

 through the East river into New York Bay : that a sub-tropical climate 

 then prevailed throughout this region, with a varied and rich fauna 

 and flora, extending up even to the Arctic Sea : that then a depression 

 of the temperature and great change in the climate ensued, with a cor* 

 responding alteration of the fauna and flora; but that these changes 

 were very slow and progressive — the snows, which at first rested tem- 

 porarily upon the Catskill Mountain summits, became at last perma- 

 nent, and resulted in local glaciers. These glaciers produced extensive 

 erosion, cutting deeply the channels along which they moved. A 

 partial obliteration of their work then ensued through two agencies. 

 First, a continental glacier advanced southward, overtopping all the 

 mountains, grinding down the asperities of the surface, filling old val-> 

 leys, and banking up a great mass of debris along its margin — a part 

 of which is now Long Island. Afterward, the climate becoming 

 milder, local glaciers were again formed similar to those which pre- 

 ceded the great Glacier, and partially obliterated or modified the results 

 of the ancient erosion. It is a complex problem now to distinguish 

 between the phenomena which have been respectively produced by all 

 these glaciers in varied succession, by the erosion of streams, by flex- 

 ures of the earth's crust, etc. 



The excavating power of glaciers had been denied by some persons ; 

 but ice, hundreds of feet and sometimes miles in thickness — as it was 

 in the old glaciers — moving with irresistible force, and having sand, 

 gravel and boulders beneath it, or frozen into it, was the most potent 



