Geology of the Environs of Petersburg. 4 07 
clay, which merits particular attention. It is best seen in the sides 
and bottoms of the ravines which the streams Ivanovka, Coirovka, 
and Crasninka, have cut through that bank of diluvian gravel which 
skirts the Peterhof road from Petersburg to Strelna. The natural 
channel of the Ligovca has been so obscured in this part by artificial 
works, that the clay is not seen to advantage in it. 
There is a low cliff on the right bank of the Coirovca, between 
the village of New Ligova and the Peterhof road:* Immediately 
below the soil are some thin layers of a whitish clay, which cover 
a layer of pebbles slightly cemented together, and just hard enough 
to project a little beyond the beds above and below it. Thus far 
all seems to be diluvian, and agrees in character with that which 
is found immediately beneath the soil in all the neighbourhood. 
But under this we find the clay of the country, of its usual blue 
colour, with the exception of the upper part in contact with the 
bed of pebbles, which is yellowish ; as are also certain veins by 
which the blue clay is traversed in various directions, though but 
seldom horizontally. These veins are from three to six inches, or 
even more in width ; their substance is a yellowish clay somewhat 
like that which intervenes between the blue clay and the dilu- 
vian. Although these veins are not hard, they are of a firmer 
consistency than the blue clay which they intersect; they are 
usually very easily divisible in the direction of a ferruginous line 
which runs down the middle of each, and which separates, or is 
cut through, in exact conformity with the yellow clay veins 
themselves. The relative hardness, or rather solidity, of the blue 
clay and its accompanying veins, is beautifully shewn in every 
instance where they occur in the bed of the stream, or in places 
Vol. V. 
* See Sketch of the clay veins on the Coirovca. 
3 F 
