405 
Geology of the Environs of Petersburg . 
tains. This tract is therefore at the northern extremity of what has 
been termed the Great Basin of the North of Europe. I shall begin 
the geological detail with the lowest stratum, and proceed with the 
rest in regular succession from below upwards, according to their 
order of superposition. 
* The basis or substratum of all this country is a blue clay, be- 
neath which no other bed has yet been exposed. The highest 
stratum, both with regard to geological and topographical position, 
is a limestone, which presents no appearance of having been covered 
by any other orginal formation. Between these two, occurs an 
intermediate bed, whose presence is constant, but whose characters 
are by no means equally so. It is, in fact, a transition bed partaking 
of the characters of that above it as well as of that below it ; to 
which are added some peculiarities of its own, which sufficiently 
distinguish it from the other two. In the blue clay there is no ap- 
parent sign of stratification, nor of organic remains ; in the lime- 
stone are abundant marks of both ; the intermediate bed in some of 
its forms is evidently stratified, in others that appearance is totally 
wanting ; its organic remains are scanty and rare. These three beds 
are strictly conformable, as may be seen by their stratification wher- 
ever that is visible. Although perfectly distinct in general character, 
they pass into each other at their points of contact. I consider them 
therefore as members of the same formation. 
*• There is some reason for apprehending that the blue clay, which I mention first as 
lowest in situ, does actually rest upon the primitive rock without the interposition of any 
other stratum. I shall have occasion to speak again upon this subject in the comparison 
of the secondary beds of this district with those of Sweden. 
