194 A. Gray—Forest Geography and Archeology. 
To what extent displaced, and how far superseded by the vege- 
tation which in our day borders the ice, or by ice itself, it is 
difficult to form more than general conjectures—so different and 
conflicting are the views of geologists upon the Glacial period. 
ut upon any, or almost any, of these views, it is safe to con- 
clude that temperate vegetation, such as preceded the refrigera- 
tion and has now again succeeded it, was either thrust out of 
Northern Europe and the Northern Atlantic States, or was 
reduced to precarious existence and diminished forms. It also. 
Aa that, on our own continent at least, a milder climate 
than the present, and a considesable submergence of land, tran- 
siently supervened at the north, to which the vegetation must 
have sensibly responded by a northward movement, from which 
it afterward receded. 
| these vicissitudes must have left their impress upon the 
actual vegetation, and particularly upon the trees. They fur- 
nish probable reason for the loss of American types sustained 
by Europe. 
I conceive that three things have conspired to this loss. 
First, Europe, hardly extending south of latitude 40°, is all 
within the limits generally assigned to severe glacial action. 
ond, its mountains trend east and west, from the Pyrenees 
to the Carpathians and the Caucasus beyond, near its southern 
border; and they had glaciers of their own, which must have 
un their operations, and poured down the northward flanks, 
while the plains were still covered with forest on the retreat from 
the great ice-wave coming from the north. Attacked both on 
front and rear, much of the forest must have perished then and 
there. Third, across the line of retreat of those which may 
have flanked the mountain-ranges, or were stationed south of 
them, stretched the Medit an impassable barri Some 
hardy trees may have eked out their existence on the northern 
shore of the Mediterranean and the Atlantic coast. But we 
very late — was apparently prevented by the prolongation 
of the M iterranean to the Caspian and then ‘ 
1. ii we accept the supposi 
anterior to the Glacial aan Haare was “bounded on the 
south by an ocean extending from the Atlantic over the present 
—_ ‘— and = Asia to the Pacific,” all chance 
of these American types having escaped from or re-entered 
Europe from the sow eux + eae 
thus be conceived to have been fora time somewhat in the 
condition in which Greenland is now, and, indeed to have been 
connected with Greenland in this or in earlier times. Such a 
