JURASSIC BASIN OF THE OKA. 
233 
Belemnite shale of the Volga having no stratigraphical connection with the inferior 
strata, it follows necessarily, that their age can be determined by their fossil contents 
only. Between Pies and Kineshma, the black shale is surmounted by thin couises 
of a ferruginous, concretionary marlstone, in the form of a rude oolite, passing here 
and there into a compact rock, and exposing on the whole a thickness of about 
fifty feet. At Kineshma, these marlstone beds disappearing, the black pyritous 
shale alone occupies the clift’ in a thickness of about thirty or forty feet. Wherever 
we detected these beds of shale, whether upon the Volga or upon its tributary the 
Unja, the banks of w'hich we examined in a journey from Ustiug-Veliki by Nikolsk 
to Kostroma, we found that they were characterized by the same organic remains. 
Near Makarief, on the Unja, the shales, more bituminous than upon the Volga, 
and equally reposing upon red marl and sand, are loaded with a vast profusion of 
flattened and iridescent Ammonites, chiefly the A. cordatus and A. virgatus, asso- 
ciated with Belemnites, all of them characteristic fossils of the Oxford oolite and 
clay. The same forms of Ammonites and the same Belemnites in abundance were 
observed at various spots along the Volga, and with them we also found the Gryphaa 
dilatata. 
Jurassic Basin of the Oka. — On the banks of the river Oka, at Oksevo, the first 
post station north of Jelatma, in the government of Vladimir, we met with a strong 
ledge offossiliferous Jura rocks. The lowest beds visible consist of black pyritous 
shale containing the Grypheea dilatata, with Belemnites. Intercalated with these 
strata are spheroidal concretions of calcareous sandstone, occasionally two to three 
feet long, which, when split up by the peasants as flagstones for their doorways, 
expose micaceous surfaces. The overlying masses consist of numerous concretions 
of yellowish, marly sandstone, and the whole are conformably surmounted by fer- 
ruginous sands with concretions of ironstone. In these upper strata we did not 
observe organic remains ; but by the analogy of similar beds near Jelatma and 
Moscow, of which we are about to speak, we believe they belong to the group 
in question. . 
The best natural sections of these strata are laid hare in several deep ravines on 
the left bank of the Oka, four versts above Jelatma and near the village of Inkino. 
Here as in the adjacent tracts, the superficial cover of detritus is very thick, but 
wherever this matter has been removed by falls of the cliff, as represented in the 
annexed woodcut, we met with the following beds in ascending order, and occupying 
a thickness of about 120 feet. 
