North American Interglacial Deposits. — Upham. 277 
all the areas which at the culmination of the uplifts became 
ice-covered. In the high plateau climate of these lands, their 
previously temperate species would be driven out or survive 
only in sheltered valleys, while the greater part of the coun- 
try would bear forests of the boreal conifers or probably be- 
come even too cold for any timber growth, like the "Barren 
Grounds" of our far northern and Arctic regions and the tun- 
dras of Alaska and Siberia. It is therefore probable that the 
species of interglacial deposits formed during the times of 
general but fluctuating growth of the ice-sheet must be pre- 
dominantly boreal and Arctic. 
But if any deposit during the stages of general ice accumu- 
lation shall be found to contain chiefly or solely species now 
restricted to temperate regions, they may be regarded as evi- 
dence of a geologicall}^ sudden uplift and change from a mild 
climate to deep snow accumulation, yet with some variability 
of its limits; or, less probably, they may represent some ex- 
ceptionally sheltered spot where a remnant of the former 
plant and animal life, surviving the general climatic change, 
w^as finally overwhelmed. 
Temperate Species of Interglacial Deposits during the 
Epoca OF Ice Departure. 
With the subsidence of the ice-burdened lands to their 
present altitude or lower, which is well ascertained to have 
been true of the time of departure of the ice-sheets, very dif- 
ferent climatic conditions ensued. On the high expanse of 
the ice there still reigned an Arctic severity of cold. For 
some time, as shown by Le Conte, the snow and ice accumu- 
lation went on faster than the subsidence, causing the maxima 
of the land depression and the thickness and extension of the 
ice during its second or lowan stage of general growth to be 
nearly contemporaneous. While the central parts of the ice- 
covered areas had sunk probabl}^ four or five thousand feet 
from their preglacial altitude, the borders of the ice in the 
northern United States were lowered apparentl}^ in general 
about half as much, thereby sinking closely to their present 
levels; and this sufficed to turn the balance from glacial 
growth to a beginning of the final retreat. The summer heat 
and rains on the glacial boundaiy, when reduced from its for- 
mer liight, melted away the ice margin faster than it could be 
