244 
GREAT JURASSIC BASIN OF THE LOWER VOLGA. 
becomes of course problematical. Iu tracing their western limit, we discover it in 
the bed of the river Alatyr and its affluents, and we have already shown that these 
strata reappear on the Oka near Jelatma. In that parallel the Jurassic strata have 
certainly not spread out so far to the south as the meridian of Saratof, for they are 
entirely absent in the government of Voroneje, where we saw cretaceous strata 
reposing at once upon Devonian rocks. Lastly, the northern limit of the great 
Jurassic band we are now considering, is defined by a line which, passing from 
about twenty versts below the source of the Piana, terminates upon the Volga 
about fifty to sixty versts north of Simbirsk, and a little below Tetiuslii. We infer 
that Jurassic beds are to be found on the road from Arzamas to Simbirsk, because 
we observed Belemnites in the bed of the Piana. Near Barnukova, indeed, these 
beds are seen in situ in the banks of the rivulets around the village of Kamarinof, 
about twenty-five versts east of Ardatof, and they continue to range without 
interruption to the river Sura, where they disappear beneath the chalk, and only 
emerge from beneath that formation in the deep valley of the Volga at Simbirsk. 
If, as we presume, there is no objection to the idea, that the argillaceous beds with 
Ammonites, mentioned by Pallas in so many places on the Alatyr, near Vassili- 
Maidan, &c., and which we refer to the Jurassic age, are prolonged beneath a 
vast detrital cover, and are united with the strata described by us upon the Oka, 
it will readily be admitted, that this basin having the form of a triangle, the 
acute point of which extends to near Jelatma, and the two sides of which range 
to the Volga, is the largest continuous mass of this age in the centre of the Rus- 
sian empire. (See Map.) 
After thus seeking to trace the limits of this great Jurassic mass, we may now 
speak of its composition and fossil contents. It is a fact, to which we must again 
advert, that the Jurassic formation of the north and centre of Russia is every- 
where composed of dark-coloured pyritous shale or clay, and of sands, sandstone 
and marlstone, and very seldom contains solid and calcareous beds. Thus, for 
example, in all the basin of which we are treating, we know of no calcareous 
matter except in the concretions or discontinuous courses of impure argillaceous 
limestone, of greyish and blue colour, which we can best compare with the cement 
stones of the Lias. In his journey from Arzamas to Penza, Pallas pointed out 
these concretions near Vassili-Maidan, in an affluent of the Alatyr, and at other 
places. They are, in fact, well known to modern geologists as occurring in many 
argillaceous deposits of very different age. The country between the Piana and 
