1891.] 
259 
[Shaler. 
serve as good measures of duration. Thus the evidence from 
our salt lake basins where the arguments are drawn from the time 
required for the construction of ancient benches or for the evapo- 
ration of their waters with advancing dessication is liable to qual- 
ification from the fact that the rainfall may have varied since 
the ice sheet passed from the region in which they lie. ft is indeed 
rather unlikely that the series of climatal changes which have led 
from the extreme humidity of the glacial period to the relatively 
dessipated conditions of the present day has been perfectly contin- 
uous. It is in fact probable that the rainfall has been subject to 
various alternations in which the water in these basins has repeat- 
edly risen and sunk. The argument from the recession of water- 
falls such as that of Niagara gorge is also liable to grave criticism 
from the fact that the attitude of the continent for a considerable 
time after the close of the last ice period evidently differed much 
from what it has at the present day. On the Atlantic coast, in the 
same parallel with Niagara and the other waterfalls which have 
been brought into the argument, there was a great depression 
which manifestly .continued for some time after the ice disap- 
peared from the region. The elevated beaches and the marine 
scarfs on Mt. Desert and elsewhere show the long continued resi- 
dence of the sea at a height which, if extended inland, would 
bring the ocean level either above the summit of these falls, or so 
far diminish their depth, as greatly to retard their cutting. Thus 
this time required for the recession of these falls may represent 
only a small part of the period which has lapsed since the ice 
sheet left the surface of North America. 
The argument from the amount of change which atmospheric 
agents have brought upon drift materials affecting either their 
physical or chemical conditions is also subject to such qualifica- 
tions as destroys its value. It is evident that for a considerable, 
though undetermined time, after the close of the glacial period 
the surface of all the regions covered by the glacier was subject 
to a climate certainly as moist as that which now prevails in 
the region. It is also' likely that all this region even what is 
now the prairie district was thickly clad with vegetation. Under 
a dense mantle of forest growth the most delicate kame to- 
pography and the finest scorings of the rocks would remain m 
most cases essentially protected from atmospheric agents. 
