30 
SECTIONS ON THE YOLKOF AND SIASS RIVERS. 
4 *. 
g. Thin beds of impure limestone, alternating with dull red and green, concretionary marlstone, the whole covered 
by drift (h), containing granitic and other blocks.—/. Upper and thin-bedded limestone, without chlorite, 
and containing Orthaeeraiitex vaginafois, O. duple,*', Illanus crammudo, and Asnphus expunsus—e. Chlo- 
ritic limestone, in beds from six inches to one foot thick, of light grey colour, in part earthy, in part compact, 
and speckled throughout with numerous grains of dark green chlorite, some of which arc crystalline. Among 
the fossils are Orthi* calligrumma , 0. ohtusn , 0, purm (Pand.), and abundance of Trilobites, Amphvx expansus 
and Jllcenus crassicuudn. A few Sphaaronites are also present, and on the surface of the beds are some 
markings of fucoids ? — d. Bituminous schist, with concretions of calcareous spar. — c. Ferruginous grit with Un- 
gulites. — a and b. Yellow sands and sandstone, and whitish incoherent sandstone. The cliff is about sixty feet 
high. 
Other beds of highly argillaceous limestone, superior to those represented in the 
above woodcut, and forming the central mass of the deposit, occupy the cliffs 
between Isvosk and Petropavlosk, and are largely excavated for the manufacture 
of hydraulic cement. In these quarries we found several fossils similar to those 
of the lower limestone at Isvosk, viz. Asaphus expansus, Illcenus crassicauda, Ortho- 
ceratites vaginatus and 0. duplex ; and in addition to them, Orth, bacillus , Euom- 
phalus qualleriatus (Schloth.), Siphonotreta verrucosa (Lingula, Pand.), Lepteena im- 
brex (Pand.), &c. Fucoid-like bodies also occur upon the surfaces of the strata 1 . 
At Petropavlosk, where the cliff rises to a greater altitude than in any other 
part of the river-banks, still higher strata appear, and these, sloping away at an 
almost imperceptible angle to the north of the village of Bor, pass under certain 
red and green strata which are the lowest beds of the Devonian system. Like the 
highest strata, which are exposed in the hills of Duderhof near Petersburgh, these 
upper limestone beds on the Volkof contain Spheeronites in vast abundance, 
particularly the £>. aurantium, together with the Cheetetes Petropolitanus, and a few 
small Orthidee. 
On the Siass we observed a succession of the lower formations precisely similar 
to that which is exposed on the Volkof and at Rebrova, where quarries of the 
chloritic limestone afforded, among other characteristic fossils, a gigantic specimen 
of the Asaphus expansus. In recently ascending the banks of this river above the 
point of our united explorations in 1840, we also obtained unequivocal proofs of 
the Lower Silurian strata being at once overlaid by true Devonian beds between 
the villages of Siaski, Rodok and Buyanetz 2 . The lower strata there consist of 
calcareous flags alternating with red and green marl, overlaid by a limestone with 
small concretions, and loaded with Orthis calligramma (Dalm.), O. plana, O.inflexa, 
0. extensa (Pand.), Asaphus expansus, Orthoceratites vaginatus, Cheetetes Petropoli- 
1 Colonel Helmersen discovered traces of copper ore in these limestones of the Volkof. 
5 Observation of Count Keyserling. 
