Geology of the Environs of Petersburg . 455 
That variety of the limestone which is filled with broken 
entrochites, small terebratulites, Sec. and is most common in 
Esthonia and on the Sas, is almost identical with the white limestone 
of the hills south of Moscow and of the central parts of Russia. 
In the limestone used at Cazan, quarried I believe on the right 
bank of the Volga, are small terebratulites having some likeness to 
those of Gatchina in the yellow Pleta. The limestone at the falls 
of the Msta above Borovichy, bears also some resemblance to it : 
more perhaps in geological situation than in outward appearance, 
as it lies above a series of sandy bituminous and pyritical beds 
which ate again supported by the clay. 
Before attempting to trace this formation out of the Russian 
territory, it must be remarked that its general line of bearing is 
from north-east to south-west. Proceeding therefore from the 
Livonian isles in a south westerly direction, we meet first with the 
Isle of Gothland, whose structure and fossils, especially the trilobites 
and orthoceratites found in its limestone, are merely a continuation 
of those of Esthonia. The same may be said of Oeland, and of 
the opposite coast of Sweden, behind which rise the primitive rocks, 
which thus form a western boundary of the formation. But it is 
remarkable that Sweden contains a great many detached districts* 
or outlying masses or basins, of these strata, separated by ridges of 
* There are those of Jetland, Dalecarlia, Nerimke, east and west Gothland, Scania, 
and the isles. The latter indeed is not properly speaking, a detached district, being in 
fact the western extremity of that which is the subject of this memoir. 
The sections given in Thomson’s Travels in Sweden, seem to render it probable that 
this formation rests immediately on the primitive rock ; supposing, as I do, that the blue 
clay of Petersburg answers to the lower bed of the Swedish secondary strata. The 
apparent absence of all intermediate rocks, between the most southern known termination 
of the Finnish granite and the most northerly point of appearance of the Russian secondary 
strata, concurs with the general resemblance they bear to those of Sweden, to support this 
idea. 
Vol, V. 3 H. 
