UNDULATIONS OF LOWER SILURIAN AND DEVONIAN STRATA. 30* 
tanus, &c. This perfectly characterized Lower Silurian band is at once conform- 
ably surmounted by other red and green marls with calcareous flags, followed by 
micaceous brownish-red sandstone and other marls and flags l , and in this group 
are the following true Devonian types, viz. Orthis striatula (Schloth.), Terebratula 
Livonica (V. Buch.), Spirtfer muralis (nob.), Ortho ceratites cochleatum and Serpula 
omphalotes ; with which species are mingled fragments of ichtbyolites, such as Den- 
drodus and a remarkable species, of which we shall speak in the next chapter as 
having been been first found on the river Siass by Mr. Strangways, Placosteus 
meandrina (Ag). (See note, p. 47.) 
The same flaglike structure of the upper calcareous beds, which is apparent on 
the banks of the Volkof and the Siass, is also seen on the Vloia, a small tributary 
of that river, and at about twenty-five versts north of the junction before alluded 
to, where these Silurian flags rise out by an undulation, from beneath a trough of 
Devonian strata, and occupy a low dome near the village of Possobea, as repre- 
sented in this figure. 
5 . 
N. S. 
Possobea. 
Lower Silurian. Devonian. Dome of Lower Silurian flags. Devonian. 
These uppermost Silurian beds are yellowish and white, sandy, calcareous flag- 
stones, in parts having a delicate green tint. They contain several fossils which 
clearly refer them to the Lower Silurian group ; such as Orthis adscendens, and 
two other species of this genus ; together with a Terebratula and a Trilobite 
closely approaching to the Asaphus expansus. 
Dip, Undulations, and Dislocations of the Strata. — It was in the quarries of the 
hydraulic limestone, north of Petropavlosk, that we first ascertained the very slight 
inclination of the strata to be towards the south-south-east. Large surfaces of the 
rock being laid bare, we perceived that the rain-water, which had recently fallen, 
flowed slowly to the south-south-east, and lodged against the edges of the unquar- 
ried rock in that direction 2 . The clear continuous section of the Volkof obviates, 
however, the necessity of such a test, even where the clinometer may fail to 
register the inclination ; for though some difficulty may exist, when one vertical 
face of rock only is examined, the geologist who views at a coup -d’ ceil the strata 
1 M. Pander, we may further observe, has recently observed a similar junction of Lower Silurian 
with Devonian on the banks of the river Loya, a feeder of the Lake Ladoga, thirty-six versts above 
Schlusselburg. 
- A similar experiment gave a like result in the limestone quarries on the river Siass. 
