31 
DIP, UNDULATIONS, AND DISLOCATIONS. 
exposed for several miles on the banks of this straight-flowing stream, can recog- 
nise, without hesitation, their order and succession, as indicated in the little pre- 
ceding woodcut. Looking southwards from the abrupt cliff - at Petropavlosk, he 
sees the upper beds of the grey Silurian rocks declining gradually in the distance 
till they are overlapped by the red Devonian beds. 
Whilst we speak of a very slight, general inclination to the south-south-east, we 
must explain, that we believe it to be often accompanied by broad undulations, 
sometimes leaving the strata in almost horizontal positions, and at other times 
producing domes and troughs like those on the Volkof and Vloia, to which we have 
just adverted. It will hereafter appear, that such undulations occur also in the 
Devonian rocks of Courland, and in other districts ; and we would now show, that 
in certain tracts, the movement of the strata has proceeded to such an amount 
as to produce considerable dislocations. 
Although, therefore, we have spoken of the horizontality of the Silurian rocks, we 
must, therefore, except from this remark some of the beds which are exposed in 
the ravines of the hilly grounds watered by the Pulkovka brook to the south of 
St. Petersburgh, i.e. between the Duderhof Hills and Czarskoe-celo. The sections 
of Mr. Strangways established by clear evidences, that the strata on the sides of 
this rivulet are fractured and thrown into rapid undulations 1 , which M. Pander has 
since sought to prove might be explained by subsidence and derangement of the 
inferior clay. To us, however, it is manifest, that the dislocations on the Pulkovka 
cannot be referred to such a cause. The breaks and repetitions seen along the 
sides of the little valley, from above the village to a verst or two below it (as ex- 
pressed in the following woodcut), can, we contend, be alone explained by elevatory 
movements 2 . 
SECTION ALONG THE PULKOVKA BROOK. 5*. 
E.N.E. W.S.W. 
s. Drift, and northern erratic blocks. b. Ungulite sandstone. 
d. Limestone. b*. Ungulite sandstone in hard concretions. 
c. Bituminous schist. a. Blue clay. 
1 Geol. Trans., old series, vol. v. pi. 25 and 26. 
* Colonel Helmersen has well remarked, that the blue clay rises to a much higher level to the south of 
St. Petersburgh than in Esthonia, and he accounts for this fact as well as for these dislocations by eleva- 
tion. — Ann. des Mines de Russie, 1838, p. 102. M. Pander was formerly disposed to attribute the con- 
tortions and dislocations on the Pulkovka to subsidences caused by the conflagration of the bituminous 
and pyritous schists beneath the limestone ; hut we apprehend that with the present amount of evidence 
he would no longer adhere to that idea. 
