449 
Geology of the Environs of Petersburg* 
been describing bear to those of other parts of Europe, as well 
within the limits of Russia as beyond them. 
To the north, the blue clay disappears under the overwhelming 
mass of granitic boulders and sand, no precise boundary being 
assignable to it, or to the granite of Finland, the first rock in situ 
which appears beyond it. The intermediate bed, consisting entirely 
of loose sand, becomes so mixed with that derived from the same 
origin with the primitive boulders, increasing greatly in size and 
number as they approach their original rock in Finland, that they 
are no longer distinguishable, and all trace of a decided outcrop is 
lost. The limestone has not yet been discovered north of the Neva. 
Thus, the district described in the preceding pages may be considered 
as comprising the most northerly border of the formation which 
consists of the three most marked strata of secondary rocks which 
occur in this part of the north of Europe ; the blue clay, the 
intermediate bed, and the Pleta limestone. 
Their progress, however, is traced very satisfactorily towards the 
east, beginning at Nicolskoy on the Tosna, the last point on this 
side to which I have followed the escarpment in the detailed survey. 
Although the blue clay is seen in great strength on that river, yet 
the general rise of the bed and valley of the Neva towards the 
Ladoga lake, and consequently of the valley of its tributary streams, 
soon cause the sand of the intermediate bed to become the lowest 
visible stratum. It probably crosses under the Neva a little above 
Pella on the left bank and below Astrafki on the right, forming, as 
is supposed, together with an accumulation of primitive boulders 
which appear to have been collected on a bar of the harder sandstone 
beds, those rapids commonly called the falls of Pella. The 
encreased rapidity of the stream is occasioned also by its being 
suddenly raised, as it were, on a step of this sand, independently of 
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