Geology of the Environs of Petersburg . 
397 
proportion to the distance from the original mouth of the river 
at which the depositions commenced, contributes to render the 
progressive increase of this, as of other deltas, apparently less rapid 
in these days, than it must have been formerly. 
It has been proved by actual examination, that of three successive 
depositions which form the present bar, and whose slow but 
constant increase is become already a material disadvantage to the 
port of Petersburg, the first consists of small pebbles and all the 
coarser particles, the greater specific gravity of which has caused 
them to fall on the slightest relaxation of the velocity of the current; 
the second of a finer sand, and the third of the finest silt or lighter 
particles of earth, which have not been deposited until the stream, 
meeting the waters of the gulf, has thrown them wherever the 
eddies or still waters have become incapable of carrying them any 
further. It is not improbable, that the materials which form the 
basis of the older islands may be pebbles of a still larger size ; even 
the isle of Cronstadt itself, which is also alluvial, may be accumulated 
over some great heap of boulders.* The surfaces of all the islands, 
however, are covered with a fine sand, which has floated the highest, 
and which prevents the ascertaining of this point. 
These alluvial depositions are certainly very considerable, for a 
river whose whole course amounts not to more than seventy-five 
versts ; especially if it is considered that its waters flow directly 
from a vast lake in which all their previous impurities must have 
been left. In consequence, its waters are perhaps purer than those 
of any river of the same size. Its supply of water from this 
source is, of course, very steady, and the streams it receives are too 
* When the waters of the gulf are low, numbers of vast boulders maybe seen stretching 
in reefs and banks to a great distance from the shore. The shape of the isle of Cronstadt 
agrees exactly with the idea of the alluvial tails described by Sir James Hall in the 
Trans. R. S. Edinb. 1813. 
