408 
The Hon. Mr. Strangways on the 
often under water. In such situations, the blue clay is uniformly 
washed out to the depth of about five or six inches ; the large veins 
preserving that height round the holes thus formed. As the holes 
are gradually excavated, the upper part of the walls which surround 
them, not being strong enough to resist the stream, are carried 
away. The whole presents a sort of honey-combed appearance on 
a large scale, which I have endeavoured to express in the sketch. 
This may also- be very well seen in the bed of the Ivanofka, and 
of the Crasninka above Grasnoe Cabac, the whole bed of which 
stream appears as if hollowed out of one vast septarium. 
The three rivers abovementioncd are the only ones in which 
I have found this singularity of the blue clay ; it is not observable 
in the banks of the Ligovca, nor in those of the Chorny Rechka, 
either above or below Coupchina. In the former, we find only 
blue clay without veins, and in the latter, no blue clay whatever : 
the whole depth of its banks being composed of yellowish diluvian 
gravel. 
The only good exposure of the blue clay, on the right bank of 
the Neva, is in a pit, whence the clay is taken for brick-making, by 
the side of the small stream called Chernavca in the great map of the 
environs of Petersburg, just where it quits the picturesque church- 
yard of Okhta. It is somewhat singular, that though visible here at 
so high a level, it should not be laid open in the lower grounds ; the 
sand which forms the bed of the Okhta river is rather argillaceous, 
especially just below Mourina ; as is that found on piercing the 
bogs east of Riabova, and that which forms the bottom of the 
larger lake of that name: but these being members of the same 
formation, it must happen, as in analogous cases, that they pass 
into and blend with one another. 
South of the Neva, it skirts the shore of Peterhof and Oranien- 
