Geology of the Environs of Petersburg . 
441 
* Within each bog, usually near the centre, is almost always 
found a lake or small pool, gradually filling up. In many cases, 
as at the Finnish lake Lindo Jervy, in the woods of Riabova, the 
spot where land ends and water begins is not to be determined, 
except the gradual dwindling of the trees is taken as a criterion. 
Stunted birch and alder bushes grow into the water, and encroach 
every year on the retreating waters of the lake. Some such cause 
may be connected with the quantities of old wood found in many 
places beneath the superficial soil, as on the lower Coirovca and 
Crasninca, in a soft and moist state, dark brown, and easily cut in 
all directions with the spade, like cheese, f 
Blocks of a hardish, white, rugged stone, are found on the banks 
of the Tosna: they resemble tufa, but I cannot discover it there 
in situ. 
It remains for me to describe more minutely a few of the more 
remarkable spots situated within this district, and to which I should 
wish to draw the attention of any future geologist who may happen 
to visit the environs of Petersburg. 
* Phosphate of iron is not the only alluvial metallic substance found in this district. 
The waters of this neighbourhood, (especially of the sandy part of it) are very commonly 
impregnated with oxyd of iron, a character by no means unusual throughout all the north 
of Russia. This iron is deposited in great quantities at the bottom of almost all the lakes, 
bogs and rivers, on the Carelian side of the Neva, more sparingly on the other. Large 
masses have been raised even out of the bed of the Neva itself. It is in the state of bog 
or marsh iron ore, and supplies some iron works. Where the waters are very strongly 
charged with iron, they communicate a bright red or orange colour to pieces of wood 
which they meet with in their course: the iron itself being often deposited in the interior 
of the branch or between the bark and the wood, by very slow' infiltration. This is a 
beautiful instance of what is vulgarly called petrifaction. Branches of the birch afford 
better examples than those of any other tree. 
+ A layer of old wood is also frequently found under the soil throughout the flat 
country cast of Riabova. 
3 k 2 
