26 
FORMATIONS OF THE SILURIAN SYSTEM IN RUSSIA. 
tain ravines (particularly in all those occupied by the brooks which descend from 
the plateau on the south) to be overlaid by sands and schists, to which he gave the 
name of the “ intermediate bed,” because it lay between the clay of the valleys 
and the limestone of the upper tracts or “ pleta” of the Russians. In the lowest 
of these bands no fossils were then known, and since then they have only afforded a 
few fucoidal impressions. In the intermediate bed, however, Strangways recognised 
shells which he termed “ chamites,” and these were subsequently described, first 
undei the name of Obolus by Eichwald, and afterwards as Ungulites by Pander. 
The third formation in ascending order, or that of the “ pleta” limestone, is that in 
which the great mass of the fossils of the government of St. Petersburgh have been 
found, many of which have since been made known through the works of Pander 
and Eichwald, and on these we shall hereafter dilate. Those who wish to make a 
detailed study of the environs of St. Petersburgh cannot, therefore, do better than 
take the maps and sections of Mr. Strangways in hand, and apply to his faithful 
and beautiful sketches of the ravines and valleys, the palajontological knowledge 
since obtained, which enables us to show, that this group of strata fairly represents 
the Lower Silurian rocks of Scandinavia and the British Isles. 
This group occupies a zone varying from forty to sixty miles in width in the 
government of St. Petersburgh. To the west it is clearly exposed in the cliffs of the 
Gulf of Finland and Reval, and in the south of Esthonia and the government of 
Kovno is surmounted by sandy beds and limestone with Pentameri ; whilst the isles 
of Oesel and Dago consist of still younger limestone laden with corals, and repre- 
sent the Upper Silurian rocks. (See tabular view to the right-hand of the Map, 
Ph VI.) lo the east-north-east the continental or lower group traverses the rivers 
Slavenka, Ishoia, Tosna, Volkof, and Siass, and is lost under the northern drift 
beyond the fifty-first degree of east longitude. It is afterwards deflected to the 
east-north-east, and is next found in the altered form before alluded to, in asso- 
ciation with the intrusive rocks north of Petrozavodsk, and is, as far as we know, 
no further traceable in the low tracts forming the coasts of the White Sea. We 
shall first describe the Lower Silurian strata where we examined them in the 
ravines and hills south of St. Petersburgh, and on the banks of the rivers Tosna, 
Volkof, and Siass, afterwards following them along the cliffs of Esthonia and into 
Lithuania, the Isles of Oesel and Dago. 
Excepting some dislocations in the hills south of St. Petersburgh, to be hereafter 
spoken of, these Lower Silurian strata are generally inclined to the south-south- 
