411 
Geology of the Environs of Petersburg . 
of the Ishora, a little above the village of Samsonovsky, or Oloseeo 
Mekky, from which it is divided by a narrow ravine without water. 
At the river’s edge appears the common blue clay before described, 
which forms the base of a wide meadow occasionally overflowed 
by the river. On reaching the foot of the cliff, it passes into a 
sandy mixture, gradually acquiring a greater degree of purity as it 
approaches the upper beds. The most remarkable character is that 
of a brilliant white quartzose sand, of the finest quality, in which 
are excavated small caves, supported by pillars left in the rock, for 
the purpose of procuring sand for writing, or for hour-glasses. 
It has just sufficient coherence not to fall in. Next above this 
white sand is a bed of the same substance, differing only in its 
colour, which is a bright and beautiful yellow. These two beds, 
which are sufficiently obvious, may also be seen on the banks of 
the Tosna, Slavenca, and Popovca ; also at Mishkina, and in the 
neighbourhood of Pavloosk. 
It is in this part of the series that there occurs a bed which 
I must mention here, though I have only found it in situ at the 
rocks above Nicolskoe on the Tosna. It is a yellowish white 
sand, differing but little from those sorts just described, excepting 
that it contains organic remains. These are only one large species 
of chamite, in very good preservation, usually of a brownish colour, 
and retaining the original polish of the shell. Where found in situ,, 
it is of very loose texture, readily crumbles between the fingers, so 
that it is impossible to bring away a characteristic specimen ; and on 
weathering, offers some indications of a mammillated or botryoidal 
structure, which, it will be seen hereafter, is a remarkable feature in 
another member of this formation. It was first found in rolled 
pieces, and in small quantity, near Tzarscoe Celo ; I have since 
found it in large rounded blocks and pebbles of great hardness, and 
