PECULIAR LITHOLOGICAL STRUCTURE OF THIS REGION. 
55 
contains very little sand, and is nowhere of a red colour. It is, on the contrary, 
made up of numerous alternations of flat-bedded, light yellowish limestone, some- 
times pure, sometimes earthy, and often so impregnated with magnesia (being oc- 
casionally even dolomitic), that it is frequently undistinguishable from the mag- 
nesian limestone of England, or the zechstein of Thuringia. When, travelling 
northwards from the plateau of Kursk, composed of cretaceous and tertiary rocks, 
we suddenly came upon these yellow calcareous cliffs on the river Oka, we were 
naturally disposed to believe that they were of the age of the magnesian limestone 
near Sunderland in England, or the zechstein of Germany, so completely did they 
resemble those rocks ; and with infinite surprise our first impression was cor- 
rected, by discovering that the fishes and shells which they contained were true 
Devonian types ! 
Sections along the banks of the Oka show the succession from the lower to the 
highest members, and the same light yellow colour still prevails. Some of the 
strata have a breccia-like aspect, caused by a rude concretionary action, which 
has formed hard, irregular nodules of impure limestone, the intervals being occu- 
pied by sandy or calcareous marl ; and where the latter disintegrates, the face of 
the cliffs presents a rough, sinuous and grotesque aspect, resembling the rus- 
tic-work in the basement story cf a Florentine palace. Courses of sandstone 
are rare, but they are seen near Orel, generally incoherent, and occasionally of 
greenish colour, but more frequently yellow or ferruginous. Sandy, yellowish 
limestones and fawn-coloured, sandy marls, form the chief dividing masses of 
the strong calcareous bands, which vary in thickness from mere tilestones to beds 
of two and three feet thick ; whilst hard, thick paving flags, of mottled light 
indigo and yellow colours, with way-boards of black and white mottled mails in 
the lower division (Orel), constitute the chief, if not the only material difference 
of colour, in these buff-coloured cliffs. The magnesian limestones and their asso- 
ciated marls (very rarely green and blue) are exposed at intervals all down the 
river Oka, occupying cliffs from sixty to eighty feet in height, from which the strata 
crop out or are laid bare in adjacent ravines. Shelly calcareous flagstones are 
prevalent in different stages, and some beds assume a chocolate hue, but red rocks 
are nowhere visible. 
Section of the Oka from Orel to Lichvin and Peremishl. Altei this general survey, 
we shall best convey to our readers an adequate knowledge of the structure of 
these rocks by describing the transverse sections of the Oka and the Don. And, 
i 2 
