413 
Geology of the Environs of Petersburg . 
to the Tosna. It forms large beds of various degrees of hardness, 
usually of a yellow colour, and more or less charged with iron. 
It may be found at Nicolskoe on the Tosna in every state 
intermediate between a simple botryoidal sandstone and a heavy 
mammillated iron stone. It is in situ on the Popovca a little above 
the village, and near the white sand. 
The succeeding beds become more argillaceous, and are of a 
dark brown colour inclining to green. They contain as much iron 
as those immediately below them, usually in the form of irregular 
masses of pyrites radiated in the interior ; sometimes in bunches of 
crystals, exhibiting both the cube and octohedron with their 
modifications, as well as the bronze red and brass yellow colours. 
The superior beds are thinner and more slaty ; their colour ash 
grey or purple passing into green and black. They are bituminous, 
and contain layers of fetid calcareous spar, disposed in nodules of a 
dirty brownish white colour, and radiated from the centre. They 
sometimes appear like rugged balls, the apices of the crystals 
projecting at the circumference. 
These pyritical and bituminous beds are always very thin and 
Sometimes much contorted. Their upper portion contains a 
vast quantity of green earth of very fine quality, which might 
be used for painting. These beds of green earth differ in character, 
the lower being argillaceous, passing into green clays immediately 
below them ; the upper, on the contrary, although in appearance 
the same, are hard, contain lime, and pass gradually into the lower 
and argillaceous beds of the Pleta limestone, through which green 
earth is also disseminated. 
The topography of this intermediate bed must be described under 
two different heads : one of which will comprehend its outcroppings 
from under the limestone, which it skirts throughout ; the other 
