261 
[Shaler. 
1S91.J 
worthy than any other physical evidence of the lapse of time can 
well be. In the first place we know by more extended observa- 
tions the measure of marine erosion thnn we do that which 
occurs along the shores of freshwater lakes. In the second 
place the several levels of marine benches which have been formed 
along the shore probably owe their diversified attitude to the 
movement of the land, and not to the oscillations of the sea ; and 
in the third place the changes of level within the historic period 
along the shore are better ascertained than they are about the old 
lakes from which conclusions have hitherto been derived. 
It is now evident that at the close of the glacial period, after 
the ice had retreated from the border of the Atlantic throughout 
the northern portions of that basin the surface of the land was 
very much below its present altitude. The amount of this de- 
pression is not yet well ascertained, but it seems likely that a con- 
siderable part of the British islands and the northern part of the 
continent of Europe as well as the shorelands from Greenland 
southward to near New York were deeply depressed. I have re- 
cently endeavored to show on what appears to me to be indisput- 
able evidence that this depression on Mt. Desert Island certainly 
amounted to as much as 300 feet, and may have been more than 
1500 feet. (See Memoir on the Geology of Mt. Desert. Eighth 
Annual Report of the Director, p. 993 et seq.) I am now inclined 
to believe that this depression extended inland and formed a 
broad sea over the regions about the great lakes. In no other 
way can we so well account for the existence of an extended sheet 
of evenly diffused drift fading out to the southward in the southern 
portions of Indiana, Illinois, and the region yet further to the 
west. The point which immediately concerns us, however, is as 
follows : From this condition of great depression, the surface of 
the land was gradually lifted until on the seaboard it attained an 
elevation somewhat above its present position, as is shown by the 
existence of numerous submerged freshwater swamps and forest 
beds along the Atlantic coast from New York to Newfoundland. 
This return of the surface to its present attitude was not accom- 
plished continuously but by successive rapid movements with 
long continued intervening periods of repose. In these times of 
repose the sea, where the circumstances where favorable, con- 
structed extensive marine benches ; so far as I have observed 
these beaches are best indicated on the southern face of the Mt. 
