44 
LAKE ILMEN — KOROSTINO — VALDAI HILLS. 
under the superficial detritus which obscures the edges of the Volkof and of its 
great feeder the Tigoda 1 . 
It may here be observed, that the ichthyolites of the lower beds of the system, 
whether from these localities or those of Tchudova and Kalipi-polist before men- 
tioned, differ specifically from the forms which we shall afterwards point out in the 
uppermost strata in the Valdai Hills (on the Priutchka, a tributary of the Msta). 
The Glyptosteus and Diplopterus are genera common to the north of Scotland and 
Russia, but the species in these lower beds, G. reticulatus and 1). rugulosus, are, ac- 
cording to the recent determinations of Prolessor Agassiz, unknown in the British 
Isles. We shall presently, however, indicate the existence of several species of 
ichthyolites in the middle and upper beds of the system which are identical with 
Scottish types. 
The central members of the Devonian system consist of red and green argilla- 
ceous marls or clays, limestones, both flag-like and concretionary, and courses of 
flag-like sandstone and grit, and for the most part little coherent. Gypsum is 
disseminated at intervals, and salt-springs issue from the deposit. The lower 
parts of the Valdai Hills are composed ol these central members of the system : 
they consist ol red and green marls, in which concretions of impure limestone and 
calcareous flags are only to be detected at intervals, as on the banks of the Msta, 
in certain cuttings of the Moscow chaussee, and occasionally in ravines at short 
distances liom it. At and near the Imperial summer-palace ol Rorostino the 
banks of the southern end of the Lake Ilmen, and the edges of a ravine at 
Porogi, present cliffs from forty to fifty feet high. The upper beds are greyish and 
purplish, compact, calcareous flags, which split into numberless small cubes, and 
weather externally to a yellowish colour. They are often spotted with blood-red 
circles, and their surfaces are much diversified by long, tubular, incurvated bodies. 
These calcareous strata having a maximum thickness of about thirty feet, rest on a 
deep red, rough limestone, charged with many prevalent Devonian shells. Among 
these are several of the forms noticed on the banks of the Volkof near Prussino, 
with another species, which, though not detected in that locality, is associated 
with the same group in the Devonian limestones of the Boulonnais in France and 
elsewhere ; viz. the Spirifer Verneuillii (Murch.). Besides the common shells, 
1 The beds here described are those which lie in the trough before mentioned, and from beneath which 
the Silurian rocks of the Vloia rise to the surface (see woodcut, p. 30). 
