Geology of the Environs of Petersburg. 489 
in the garden, opposite the north front of the palace at Peterhof, 
and many smaller ornamental buildings which decorate the environs 
of Petersburg, are constructed of this stone. Though coarse and 
porous, it is the best building stone in this part of Russia : its 
colour being a fine yellowish white, and its substance light and 
easily worked. Unfortunately it is hardly durable enough to 
withstand the vicissitudes of spring or the severity of winter in a 
climate where the thermometer often ranges 24° of Reaumur, up and 
down, partly above the freezing point, partly below, in as many 
hours. It may be added, that stone building is as yet so very rare 
in Russia, that this material is neglected in many other situations 
besides this. 
I have only been able to find two species of shells, sometimes 
preserved in this stone, never in great quantities. They are a 
species of the helix and a buccinum, both fresh-water varieties, and 
yet to be found in a recent state on the banks of the river. There 
is a ford which crosses the river nearly in the middle of the valley, 
formed by two ledges of this rock over which the water falls as 
clear as crystal. In this situation they seem at first to be rocks of 
the soil, and any thing but recent depositions. The quarries and 
the flat land in which they lie are now never covered but by high 
floods. Trees and thickets are scattered over the surface of the tufa, 
remarkable for some handsome species of Campanulas and some 
dwarf willows uncommon in other parts of the country. 
The greatest depth to which this bed has been quarried is only 
six feet ; it probably does not reach much lower, although the 
substratum is not laid bare. 
It is remarkable that, of all the waters of this country, those 
which deposit tufa, are apparently the purest. Perhaps the lime is 
held in perfect solution* and the sediment is to be considered as 
Vol. V. 3 K 
