SECTION OF THE OKA — OREL TO OTTRADA. 
57 
the little stream the Optika, at Ivanofsk, nineteen versts from Orel, where the 
yellow, sandy beds, with sub-concretionary courses of a sort of lumachelli, contain 
bones of fishes. At the village of Ottrada, the cliffs on the right bank of the 
Oka expose excellent sections, particularly in a lateral ravine north of the village. 
Ascending from the river edge the strata are seen to consist of 
10 . 
1. Thin-bedded shelly limestones of greyish colours, with surfaces marked by fucoid-like bodies, as at Tchudova and neighbourhood ; like which 
also, these beds contain Spirifer Archiaci, S. Anmofli ( nob.), Leplatna m&nbrnrtacea (PhiU.), Trrcbratula ventilabrum (PhillO, Orthis etriatulus 
CSchloth), Area Orellana (nob.), Natwa like the species at Orel, with internal casts of Cirrm, Serpula omphatoies, Encrinites, and many small 
fragments of ichthyolites. Among the overlying beds (+) in the adjoining ravine we met with two or three alternations of the same thick-heddcd, 
impure, small concretionary limestone, to which we have partially alluded at Orel, and which we shall again describe at Mtzensk. 
Towards the summit of the section, where the yellow strata (for the whole have 
a prevalent yellow tinge) are lost under drifted sands, clay and detritus, one 
of the subordinate limestone beds ( p ) excited our attention, being made up of 
myriads of a minute shell, which seems to bear an analogy to the Venus gemma 
of the shores of North America. This fossil is associated with fine, pisolitic, 
ferruginous concretions, like those in the upper beds near Orel, and also with grains 
of siliceous sand, both white and black. Another course is composed of small 
serpuline bodies. 
Among the calcareous and flag-like beds of this system, some weather w ute , 
and others, not exceeding a quarter of an inch thick, are chocolate- coloured, com- 
pact, siliceous limestones. These strata range to near Mtzensk, on the Zuclia, a 
tributary of the Oka. 
J li. 
The left bank of the river, on which 
that town is built, offers a most striking 
section of the concretionary, dolomitic, 
grotto-like limestone, which we noticed at 
Orel and Ottrada, and which we here saw 
expanded into three distinct bands (+ + ), 
as represented in this woodcut. 
