SILURIAN EQUIVALENTS IN EUROPE. 
477 
lively moderate lapse of time. The outpouring of repeated 
streams of lava—the showering down upon land and sea of 
volcanic ashes—the sweeping seaward of loose sand and cin¬ 
ders, or of rocks ground down to pebbles and sand, by rivers 
and torrents descending steeply inclined channels—the un¬ 
dermining and eating away of long lines of sea-cliff exposed 
to the swell of a deep and open ocean — these operations 
combine to produce a considerable volume of superimposed 
matter, without there being time for any extensive change 
of species. ^Nevertheless, there would seem to be a limit to 
the thickness of stony masses formed even under such favor¬ 
able circumstances, for the analogy of tertiary volcanic re¬ 
gions 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 fau¬ 
na should continue to people the earth. If, then, we allow 
that about 25,000 feet of matter may be ascribed to one 
system, such as the Silurian, as above described, we may be 
prepared to discover in'the next series of subjacent rocks a 
distinct assemblage of species, or even in great part of gen¬ 
era, of organic remains. Such appears to be the fact, and. I 
shall therefore conclude with the Arenig beds my enumera¬ 
tion of the Silurian formations in Great Britain, and proceed 
to say something of their foreign equivalents, before treating 
of rocks older than the Silurian. 
Silurian Strata of the Continent of Europe. —When we 
turn to the continent of Europe, we discover the same an¬ 
cient 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 Silu¬ 
rian age is considerably less than lopo feet, although the 
representatives both of the Upper and Lower Silurian of 
England are not wanting, there. 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 the Llandovery group, or of a 
limestone containing Pentamerus ohlongus^ below which are 
strata with fossils corresponding to those of the Llandeilo 
beds of England. The lowest rock with organic remains 
yet discovered 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 brachiopods consist of the Obolus 
