LOWER SILURIAN SURMOUNTED BY DEVONIAN. 
32* 
clear and unambiguous sections of the Volkof and the Siass have already demon- 
strated, that Lower Silurian limestone, similar to that of the hills of Czarskoe-celo, 
is at once surmounted by a red formation with ichthyolites of the Devonian age. 
This occurs at a very few miles only to the south of the section on the Popofka, 
and only thirty-five versts to the south of St. Petersburgh. In his work on the en- 
virons of St. Petersburgh, Strangways, who must have closely surveyed this district, 
has indicated in his map the occurrence of a “ red earth of doubtful character” on 
the sides of the Ishora river, at the hamlet of Ontoleva. 
The spot thus indicated appears, however, to have escaped the notice of geolo- 
gists during many intervening years ; for since the notice of it by Strangways, no 
one seems to have explored this locality until M. Eicliwald recently visited and 
described its rocks. At that period (1843, subsequent to our two first visits to 
Russia), and after tbe existence of the true Old Red or Devonian along the southern 
frontier of the Silurian rocks had been established and even laid down by Colonel 
Helmersen in a small general map, there could be little difficulty in asserting, that 
this red earth of Strangways formed really part of the Devonian system. But 
M. Eichwald had then found no fossils in it. Such, however, were soon afterwards 
discovered by one of our own party (Count Keyserling), accompanied by M. Worth, 
the account of which result has already been published in the last volume of the 
Mineralogical Society of St. Petersburgh. This sketch further contains an account 
of the extension of these beds high up the Ishora, thus connecting them with the 
great mass of the Devonian system which we had formerly recognised and shall 
hereafter describe. As this discovery was made long after our chapter on the 
Devonian strata and a large portion of this volume were printed, and as another 
member of our party has since visited the principal localities (August 1844), we 
deem it essential to give a short account of the order of the strata with a brief 
allusion to their contents, particularly as the latter have thrown new light upon the 
fauna of the Devonian system. 
Inclining very slightly to the south or south-south-east, the pleta limestone of 
the plateau of Czarskoe-celo, here of greenish and reddish colours and loaded with 
Orthoceratites, and passing by Grafskaya Slavenka, is overlaid by other beds of 
somewhat similar structure, as exposed near the hamlet of Ontoleva. These upper 
strata are so perfectly conformable to the lower, that if unquestionable Devonian 
remains had not been found in them, they might have passed for some ambiguous 
and hitherto undescribed member of the Upper Silurian group. At Ontoleva, 
F 
