FORMATIONS OF THE SILURIAN SYSTEM IN RUSSIA. 
25 * 
external aspect (in some instances not at all) from the tertiary and cretaceous 
rocks which are spread out around the estuaries of many parts of Europe. 
Little elevated above the Baltic sea, the Neva, and other tributary rivers of the 
northern watershed before alluded to, the calcareous or harder portion of these 
Silurian rocks constitutes, in fact, a low terrace, the strata of which, whether abso- 
lutely horizontal, occasionally undulating and even partially dislocated, or almost 
imperceptibly inclined to the south and south-east, are surmounted towards the 
interior by other masses, which represent the Old Red Sandstone or Devonian 
system of English geologists. 
Whatever may be the thickness of these deposits, they thus cover a large area, 
and offer numerous points for examination and comparison, wherever they can be 
detected beneath the superficial detritus. And although this detritus is a great 
impediment to clear observation, a large portion of it has a direct connexion with 
the true subsoil. Abstracting the erratic northern blocks, sand or gravel, often 
accumulated in patches, and which will be described in the concluding chapters, a 
practised geologist can form a tolerably correct estimate of the nature of the sub- 
strata, by the colour and aspect of the broken materials upon the surface. In 
other words, much of the drift is merely local, and the subjacent rocks have been 
so worked up, as to give a dominant colour to the outline of each geological tract. 
Thus the Silurian zone of the Baltic provinces is at once distinguished by its dull 
light grey colour, from the red (Devonian) zone of Livonia and Novogorod, which 
lies to the south of it. 
As developed in the government of St. Petersburgh the Silurian or grey zone 
consists, in an ascending order, of the following subformations:' — 1. Blue Shale or 
Clay. 2. Ungulite Grit and Bituminous Schist. 3. “ Pleta,” or Ortlioceratite 
Limestone. 
This, in truth, is the exact order long ago pointed out by an English geolo- 
gist. So early as the year 1819, and long therefore before anything like the true 
general succession of the lower or transition formations had been ascertained, the 
physical features of the environs of St. Petersburgh and the ascending order of the 
strata were correctly and even minutely laid down upon maps and described by 
Mr. Strangways 1 . That author showed, that the lowest stratum was a blue 
clay, which occupying all the flat country around the metropolis, was seen in cer- 
1 Trans. Geol. Soc., 1st Series, vol. v. pp. 382, 392 et seq. (See remarks on the successive and subse- 
quent contributions of other authors in the Preface.) 
E 2 
