SECTIONS NEAR VERKNI AND NIJNI TROITSK. 
153 
Troitsk, or from east to west, the beds with Producti are seen to be overlaid by a 
very considerable thickness (400 to 500 feet) of white marlstone, flaggy limestone, 
marl and sandstone ; the white marlstone occasionally predominating in the upper 
parts of the hills. We might offer many detailed sections of the structure of the 
hills around these Zavods. We could not, however, propose any one as a type of 
general succession ; for, as before said, no two sections, made even in the same 
parallel and at very short distances from each other, will be found to agree. As 
the beds throughout the whole of this tract are very slightly inclined, and ap- 
proach as near to horizontality as may be, with very few signs of dislocation, 
and as the numerous watercourses which fall into the Kidash expose fine natural 
sections, we convinced ourselves, that, for the most part, the variety of structure 
was due to expansions and contractions of mineral matter upon the same horizon ; 
in short, to the system of inosculation represented in the coloured section, PL Ilf 
fig. 1, and in the tabular view which accompanies the Map. 
The Productus limestone, in its lowest and thickest beds, is sometimes a hard, 
thick, flaglike magnesian limestone, of conchoidal fracture, splitting into very 
large flags : in some places, strata containing the same fossils are calcareous grits, 
in others they are white limestones. Occasionally they are tufaceous, though 
usually the uppermost beds only assume the latter character, and they are then 
associated with a great thickness of whitish and greenish beds of marl and marl- 
stone, with few fossils, except Modiolae. Sometimes indeed (as at Metaftamak, 
see opposite section), the Producti are found in calcareous grits or sandstones ; at 
other times they are united together in matted masses, and form shelly, tertiary- 
like beds, six to eight inches thick, harder than the strata with which they are 
intercalated, and with which they alternate in escarpments from 150 to 200 feet 
high. Again, at Nijni Troitsk, the same shells occur in a brown shale surmounted 
by white limestone, but in no instance have they as yet been found in the over- 
lying beds of cupriferous sandstone and conglomerate associated with the bones of 
Saurians. This group is also diversified by the presence of flint and chert, which 
occur both in the form of thin continuous layers and small concretions, like those 
of the chalk of Western Europe. The siliceous bands exhibit, indeed, all the 
shades of variety, from a coarse chert to the finest resinous silex, and as in the 
carboniferous limestone of Russia, they sometimes contain fossils. 
The red argillaceous shale styled “ leber thon” by Major Von Qualen, contains 
no organic remains, nor have the white marls and tufaceous limestones afforded 
