206 THE FOREST LANDS OF NORTHERN RUSSIA, 
new sinking of the land; the formations found in brackish 
water, and formations purely marine recommenced ; the 
carbonaceous schists and the mountain limestone again 
covered the ground, previously submerged with their vege- 
table imprints. The great extension of mountain limestone 
over different points in Europe and North America, and 
the small number of deposits of continental origin which 
it contains, shows to us that this lowering of the lands 
must have been the result of a general submergence. The 
Northern hemisphere must then most certainly have pre- 
sented an entirely different aspect from what it had done 
during the Ursien stage. But then one sees renewed the 
same phenomenon as occurred at the beginning of the 
carboniferous period. We find that at the end of a subse- 
quent reclothing of the ground, effected on a vast scale, 
the continental formation of culm, and subsequently that 
of the middle carboniferous strata, which marks the time 
when these kinds of deposits attained their greatest exten- 
sion and their complete development. The flora, viewed 
as a whole, had changed but little during so long a period. 
Many of the dominant species remained such till even 
after this time, and they thus furnish a proof that in the 
mountain limestone epoch the land had never been entirely 
submerged, but that there remained always a certain 
continental emerged space sufficient to afford an asylum 
to these species of plants, so that, as soon as the culm by 
its emergence had presented to them a new space, they 
profited by this to extend themselves and propagate them- 
selves more and more. 
‘We cannot question the great length of time which 
must have passed from the commencement of the Ursien 
stage to that of the culm; and during the long series of 
ages which then succeeded each other, the vital conditions 
ot organised beings doubtless did not remain unchanging. 
It is a remarkable fact to establish, that, notwithstanding 
these changes, the species which were so numerous tra- 
versed the whole duration of this age, and penetrated 
beyond it, without experiencing any appreciable modifica- 
