Geology of the Environs of Petersburg. 427 
The clay district is usually covered by a yellowish sand, which I 
conceive to be principally formed in the same manner as that above 
mentioned. It is sometimes ferruginous, as on the first hill on the 
Wyborg road ; sometimes rather argillaceous, as on the banks of the 
stream that flows through Goupchina, and on the hill of Penty. Its 
most argillaceous form resembles the blue clay as nearly as the 
sandy form does the sand in situ ; and is equally difficult to distin- 
guish from the original formation. On the Tzarcoe Gelo es- 
carpment it is used for bricks, and these, as well as on the cliffs 
opposite Cordelova, covers the upper beds of the Pleta limestone. 
This occasioned its due share of difficulty in making out the 
structure of the country ; till, happily, in the last mentioned 
situation, I found rolled pieces of granite and other rocks buried in 
this clay, which, it was then clear, could only be a superficial or 
diluvian deposit. The decayed felspar of the primitive boulders may 
perhaps enter into the composition of this argillaceous variety, as 
the sand formed by the disintegration of their quartzose and mica- 
ceous particles does into that of the sandy one. 
That which lies above the limestone is a light brown earth, 
containing fragments of the subjacent rock, though in general in a 
very small proportion to the primitive pebbles. Its thickness is 
sometimes very considerable, particularly on the flat tops of the 
hills ; at Grasnoe Celo, &c. 
In the country immediately south of Petersburg, behind the bank 
of gardens on the left of the road to Strelna, the diluvium consists of 
a whitish dry clay, sometimes slightly sandy, lying in beds or layers 
of half an inch or an inch in thickness, to the depth of twelve or 
thirteen feet. They are nearly horizontal, except that the upper 
layers seem to follow the outline of the present surface of the earth, 
and where they join the vegetable mould are rather disturbed ia 
