Ch. XXVn.] SILURIAN EQUIVALENTS IN EUROPE. 



569 



even under such favorable circumstances, for the analogy of tertiary 

 volcanic regions lends no countenance to the notion that sedimentary 

 and igneous rocks 25,000, much less 45,000 feet thick, like those of 

 Wales, could originate while one and the same fauna should continue 

 to people the earth. If, then, we allow that about 25,000 feet of mat- 

 ter may be ascribed to one system, such as the Silurian, as above de- 

 scribed, we may be prepared to discover in the next series of subjacent 

 rocks a distinct assemblage of species, or even in great part of genera, 

 of organic remains. Such appears to be the fact, and I shall therefore 

 conclude, with the Lower Llandeilo or Arenig beds, my enumeration 

 of the Silurian formations in Great Britain, and proceed to say some- 

 thing of their foreign equivalents, before treating of rocks older than 

 the Silurian. 



SILURIAN STRATA OF THE CONTINENT OP EUROPE. 



When we turn to the Continent of Europe, we discover the same 

 ancient series occupying a wide area, but in no region as yet has it 

 been observed to attain great thickness. Thus, in Norway and 

 Sweden, the total thickness of strata of Silurian age is scarcely equal to 

 1000 feet,'" although the representatives both of the Upper and Lower 

 Silurian of England are not wanting there, and even some beds of 

 schist have been included, which, as we shall hereafter see, lie below 

 the Llandeilo group. In Russia the Silurian strata, so far as they are 

 yet known, seem to be even of smaller vertical dimensions than in 

 Scandinavia, and they appear to consist chiefly of Middle and Lower 

 Silurian, or of a limestone containing Pentamerus oblongus, below 

 which are strata with fossils corresponding to those of the Llandeilo 

 beds of England. The lowest rock with organic remains yet dis- 

 covered is " the Ungulite or Obolus grit " of St. Petersburg, probably 

 coeval with the Llandeilo flags of Wales. 



The shales and grits near St. Petersburg, above alluded to, contain 

 green grains in their sandy layers, and are in a singularly unaltered 

 state, taking into account their high antiquity. The prevailing 



Shells of the loivest known Fossiliferous Beds in Russia. 

 Fig. 65T. Fig. 658. 



Siphonolreta unguiculata, Eichwald. 



From the Lowest Silurian Sandstone " Obolus 



grits," of Petersburg. 



a. Outside of perforated valve. 



b. Interior of same, showing the termination 



of the foramen within. (Davidson.) 



Obolus Apollinis, Eichwald. 

 From the same locality. 

 Interior of the larger or ventral valve. 

 Exterior of the upper (dorsal) valve. 

 (Davidson, " Palacontograph. Monog.") 



* Murchison's Siluria, p. 321. 



