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 

 of 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- 



