266 Dr. J. W. Spencer — Subsidence versus Glacial Dams. 



elevations of a thousand feet above the Iroquois plain ; therefore 

 their equivalents in the northern Adirondacks should be looked for 

 at about 2000 feet above tide. The beaches reported in Vermont 

 by Prof. Hitchcock,^ at or belovs^ 2300 feet, doubtless correspond to 

 some high shore-lines of the Ontario peninsula. Upon the same 

 basis, these high beaches should be looked for at 3000 feet in the 

 White Mountains, and at greater elevation on Mount Katahdin, in 

 Maine. 



If vfe regard the gravels of the highlands of Pennsylvania as 

 having been formed at sea-level, then it would be reasonable to look 

 for their counterpart at elevations of over 4000 feet on Mount 

 Washington, in New Hampshire, and to the summit of the drift 

 (4400 feet) on Mount Katahdin. These conjectural estimates, based 

 upon a possible uniformity, may aid in the correlation of the 

 topographic features of the mountain region of the east and of the 

 lake region. 



As far as relates to the north-eastern portion of the continent, 

 our observations on Neptunian phenomena have now been epito- 

 mized. An explanation is necessary. That the pebbles of the 

 beaches and the shore-lines were the results of wave or current 

 action no one questions, but there are differences of opinion as to 

 the conditions under which the waters moulded their coast-lines. 

 Were these deserted shores constructed at sea-level, or were they 

 moulded in glacial lakes ? These are the theoretical questions 

 before us. 



The difficulties which the sea-level theory present to some minds 

 may be stated to be — (a) a great regional depression of the continent; 

 (b) the absence of absolute continuity of the beaches ; (c) the 

 absence of marine organisms in the beaches ; and [d) the personal 

 equation of theoretical views. On the other hand, the theory of 

 glacial dams presents such obstacles that their value will be con- 

 sidered at length. 



The idea of the hydrostatic stability of the continent must not be 

 too strongly relied upon, for the evidence adduced, showing that the 

 continent lately stood three, or even temporarily at six thousand feet 

 higher than now, appears conclusive. Such mobility of the earth's 

 crust being established, there appears no reason why the terrestrial 

 pendulum could not have moved equally in the opposite direction, 

 and carried down the highlands of Pennsylvania to nearly three 

 thousand feet, or those of New England to twice this depth. The 

 objections to such subsidence could only be based upon its magni- 

 tude, which observations must settle. 



The absence of the continuity of the shore-markings is an 

 objection oxAy to a limited extent. Part of the reported absence 

 arises from the imperfections in the explorations, owing to their 

 changing character; the local non-formation of beaches, as described 

 in a previous paper ;^ the failure of identification of separated 



1 Geology of Vermont. 



^ "Ancient Shores, Boulder Pavements, and High-level Gravel Deposits in the 

 Eegion of the Great Lakes," by J. W. Spencer, Bull. Geol. Soc. Amer. vol. li. 1889,. 

 pp. 77. 



