1881, 29 Trans. N. Y. Ac. Sct. 
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 giaciers 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 
