421 
Geology of the Environs of Petersburg . 
although they separate without much difficulty : this circumstance 
renders this stone very unfit for paving, the use to which it is 
commonly applied in Petersburg. This form does not appear to 
have been favorable to the preservation of organic remains, as only 
small fragments of an encrinus, the vertebras of which have a 
cinquefoil perforation, and terebratulites, and some indistinct traces 
of other bodies, are found in it. 
In the quarry at Alexandrovsky, close to Tzarscoe Celo, thin 
seams of brown clay alternate with the beds of limestone. In this 
quarry has been found, though but rarely, a body supposed to be 
the head of that encrinus whose remains are found sparingly in the 
neighbourhood. It is a sort of irregular polyhedron, the angles of 
which are marked with treble ribs, and the faces slightly fringed 
with lines issuing from those ribs.* It has been called E. Paradoxus. 
Sometimes the limestone of the upper beds is of a dark red 
colour, in which case the orthoceratites and other animal remains 
are often white. I have never seen them so in fresh specimens, 
which are as red as the rock they come out of, but only in places 
which had been many years cut and used as marble, which its 
superior hardness and compactness qualifies it for. I cannot 
therefore answer for their colour on being taken from the quarry, 
but it is not impossible that the organic remains lose their colour 
on exposure to the air, as is the case with those of the black 
marbles of Kilkenny in Ireland, and Caermarthen in South 
Wales. These red limestone strata may be examined with great 
ease on the summit of the cliffs along the left bank of the Ishora, 
from the quarries opposite Cordelova to Podolova. These, as well 
* Bodies much resembling those here described, are figured in the plates which 
accompany Mr. Cumberland’s account of the fossils and strata near Bristol. Geol. Trans. 
to!. 5. pi. 2. f. 8. &c. 
