W. J. McGee—Local Subsidence produced by an Ice-sheet. 369 . 
this and other instances it appears that a mass of sediment 
produces a deformation equal to its own thickness. Now since 
the specific gravity of ice to average rock is something over 
1: 8, it follows that an ice-sheet three miles in thickness ought 
to depress the subjacent strata about a mile. 
But time is an important element in the motion of all imper- 
fectly fluid bodies.) The approximate numerical equivalence 
between cause and effect in cases of subsidence with deposition 
indicates that if sufficient time be given the rigidity of the 
terrestrial crust is practically ni; though it is probable that 
the function is variable and represented by an infinite series, 
no terms of which are known. The time of continuance of 
quaternary ice to that of the deposition of the Cretaceous and 
Eocene sediments in Utah is as some unknown ratio, probably 
between 1: 100 and 1:10,000;—say 1:1,000. If, however, 
the deformation during various ee is represented by an 
Infinite series, the ratio between quaternary and Cretaceous- 
Kocene subsidence is much higher—say . The subsi- 
dence produced by an ice-sheet three miles in thickness ought 
accordingly to be only 500 or 600 feet. It will be understood 
that while it is certain that subsidence would occur, very little 
value can be attached to this estimate of its amount. 
e hydrostatical principles in accordance with which deform- 
ation beneath a thick ice-sheet must occur, equally demand 
that the crust should return to its original form after the 
melting of the ice; and it is manifest that as much time would 
be required to produce this secondary as the primary deform- 
ation. Assuming then int the periods of advance and retreat, 
or of growth and decay of the ice are of like duration, it follows 
that the earth's surface must continue below the normal level at any 
latitude, after the withdrawal of the ice, for as long a period as that 
during which the ice remained st ‘onary at that latitude. . 
Should the application of the principles sought to be eluci- 
dated in the paper on “Maximum Synchronous Glaciation” to 
any single continental area ever be attempted, the foregoing 
considerations will afford a means of testing their accuracy ; 
for late-quaternary depression, being accompanied by sub- 
Mergence in all low-lying areas, bas left unmistakable traces, 
hot only of its occurrence but of its extent, in many localities, 
Farley, Iowa, Sept. 15, 1881 : ¥ 
Am. Jour, fice Teaee Series, Vou. XXII, No. 131.—Novemser, 1881. 
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