iforme_. In the higher beds _Holoptychius_ and other
well-known fishes of the Old Red Sandstone occur. Followed still
farther to the south, as far as the watershed between Orel and
Voronezh, the Devonian rocks lose their red colour and sandy
character, and become thin-bedded yellow limestones, and dolomites
with soft green and blue marls. Traces of salt deposits are indicated
by occasional saline springs. It is evident that the geographical
conditions of the Russian area during the Devonian period must have
closely resembled those of the Rhine basin and central England during
the Triassic period. The Russian Devonian rocks have been classified
in Table II. There is an unquestionable passage of the uppermost
Devonian rocks of Russia into the base of the Carboniferous system.

TABLE II.

+---------------------------------+-----------------+------------------+-----------------------------+
| North-West Russia. | Central Russia. | Petchoraland. | Ural Region. |
/ +---------------+-----------------+-----------------+------------------+-----------------------------+
U | | Red sandstone | Limestones with | Limestones with | Domanik slates | Cypridina slates, Clymenia |
P | | (Old Red). | Spirifer | Arca oreliana. | and limestones | limestones (Famennien). |
P < | | Verneuili and | Limestones with | with Sp. | Limestones with Gephyoceras |
E | | | Sp. Archiaci. | Sp. Verneuili | Verneuili. | intumescens and |
R | | | | and Sp. | | Rhynchonella cuboides |
| | | | Archiaci. | | (Frasnien). |
\ +---------------+-----------------+-----------------+------------------+-----------------------------+
M / | Dolomites and limestones | Marl with | Limestones and slates with |
I | | with | Spirifer Anossofi | Sp. Anossofi (Givetien). |
D < | Spirifer Anossofi. | and corals. | Limestones and slates with |
D | | | Pentamerus baschkiricus |
L | | Lower sandstone (Old Red). |