230 FOREST LANDS OF NORTHERN RUSSIA. 
the forests must have been rich, varied, and vigorous in 
their growth up to the immediate vicinity of the ancient 
craters. Itis not then in eruptive phenomena, whatever 
violence we may attribute to them, that we must seek for 
the true cause of the disappearance of the Arctic tertiary 
flora. This was brought about exclusively by the climate, 
the fall of temperature, at first almost nothing, then, 
scarcely perceptible in the cretaceous period, gradually 
increased, and from the time that it passed a certain limit 
it brought on necessarily the retreat or the definite dis- 
appearance of a multitude of species, which, up to that 
time, had been the ornaments of the lands of the north. *In 
proportion, as this eliminatory process progressed, the 
glaciers, the extension of which in. Europe towards the 
close of the tertiary period must have increased and 
descended from the high summits, and finally overrun all 
and effaced all. This invasion of the glaciers of the north, 
an invasion not partial as in Europe but almost universal 
or general, has been doubtless the proximate and direct 
cause of the elimination of the later tertiary vegetation ; 
or rather it should be said, the reduction of temperature 
at once cause and effect, in promoting the extension of 
the glaciers into countries evidently very moist, has contri- 
buted by this very extension to intensify the cold, and 
ultimately to transform the climate, so as to render it 
incapable of promoting the vegetation of the greater part 
of plants, while by a kind of exappropriation the ground 
stole away and came under their denomination.’ 
There are embodied in these citations deliverances on 
two perfectly distinct questions, in regard to which a 
difference of opinion on either of which may exist without 
affecting the conclusion to which one may have come upon 
the other : the one relates to the course followed in the diffu- 
sion of the plants specified ; the other relates to the cause or 
occasion of changes of temperature of which this diffusion 
may be considered an indication. It is the former alone with 
which I am conversant, and which I desire more especially 
