BLUE CLAY OF ST. PETERSBURG!!. 
26* 
east at an angle of about 2° or 3° only ; an inclination indeed for the most part so 
slight, as scarcely to be measurable by a clinometer. With such a feeble dip 
the true direction could only be detected by observing those points on the rivers 
Tosna, Volkof, and Siass, at which the Silurian group passed successively under the 
overlying Devonian rocks, and they were thus found to range, as before said, from 
west-south-west to east-north-east, or parallel to the Gulf of Finland and the 
shores of Esthonia. 
1. Blue Clay . — The oldest beds of this zone which are visible, consist, as before 
said, of clay which occupies the low country on each side of the delta of the Neva. 
This clay or shale is often of a pale greenish or bluish grey colour, in which respect it 
is not unlike some of the soft shales or mudstones of the Silurian series, though it 
varies in being partially sandy, occasionally slightly micaceous and streaked. 
Certain yellow argillaceous veins which are peculiar to it on the sides of the 
Cocrovca ravine near St. Petersburgh are described by Mr. Strangways, and it is 
unnecessary that we should here enlarge on points of this nature, since, in common 
with that author, we have not been able to observe in this stratum any organic 
remains, except a few fucoids, which M. Pander has kindly submitted to us‘. 
Though a small thickness only of this clay is visible in any natural section, it has 
been pierced in search of water to several hundred feet in depth, both in the vicinity 
of the metropolis and at Reval, without offering any notable difference in the beds 
traversed, except a few occasional sandy courses. This fact and the absence of all 
traces of animal remains are of high geological importance ; and as a few fucoids 
only have been detected in this deposit, that underlies a zone which unquestion- 
ably contains fossils belonging to the very oldest known Silurian or protozoic 
type, we may fairly believe that this band, like the Swedish fucoid sandstone, is 
the true base of the palaeozoic series, as indicated by a gradual dwindling out of 
animal life in the lowest sedimentary deposit of a region, where no eruptions have 
taken place, and where the strata are wholly unaltered. 
If, owing to the absence of elevatory movements, the bottom of the shale is 
unknown, the relations of its upper part to the overlying strata are satisfactorily 
seen on the sides of several rivers and water-courses, as also in many of the ravines 
' This clay is not only exceedingly useful in the manufacture of bricks, coarse pottery, &c., but 
from the very fine levigation of its particles, is also an excellent material for casts in sculpture, and is 
largely used for that purpose in St. Petersburgh. When moist it has the greenish tint of many of the 
Silurian mudstones. 
