Ch. XXVIL] SILUEIAN EQUIVALENTS IN ECKOPE. 443 



Nevertheless, there would seem to be a limit to the thickness of stony 

 masses formed even under such favorable circumstances, for the analogy 

 of tertiary volcanic regions lends no countenance to the notion that sed- 

 imentary and igneous rocks 25,000, much less 45,000 feet thick, hke 

 those of Wales, could originate while one and the same fauna should 

 continue to people the earth. If, then, we allow that 25,000 feet of 

 matter may be ascribed to one system, such as the Silurian, from the top 

 of " the Ludlow " to the base of " the Llandeilo " inclusive, we may be 

 prepared to find in the next series of subjacent rocks, the commencement 

 of another assemblage of species, or even in part of genera, of organic 

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

 with the Llandeilo beds, the original base-line of Sir R. Murchison, my 

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

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

 the Silurian. 



It would lead me into too long a digression to attempt to follow the 

 Upper, Middle, and Lower Silurian into Scotland, the lake country, 

 Cornwall, and other parts of the British Isles. For an account of these 

 rocks in Ireland, the reader is referred to Col. Portlock's Report on Ty- 

 rone, to the writings of Mr. Griffith and Prof. M'Coy, and those of the 

 officers of the Government Survey, as well as to the sketch recently given 

 by Sir R. I. Murchison. 



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 

 comprehended which, as we shall hereafter see, he 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 lime- 

 stone contajning Fentamer us 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. Petersbui-g, probably coeval with the Llandeilo, and not ex- 

 hibiting any of those peculiar forms which distinguish "the Li ngula flags" 

 of Wales, or the Bohemian "primordial fauna" of Barrande. 



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 or Ungulite of Pander, and a Si2:>honotetra (see 

 figs. 604, 605). As bearing on the antiquity of this formation, it is in- 

 teresting to notice that both genera have recently been found in our own 

 Dudley hmestone. 



* Alurchison's Siluria, p. 32L 



