RANGE TO THE NORTH-EAST — SECTIONS OF THE MGRA AND YITEGRA. 47 
and Moscow, we may now describe it in its extension, first to the east-north-east, 
and afterwards to the west-south-west, concluding our account by a sketch of its 
range to the south-east, or into the central governments of Orel and Voroneje 
(see Map). 
From what has been said of the transformation of the Silurian rocks in their 
course to the north-east (p. 19), it will not be expected that much instructive 
evidence is to be obtained, from that region, of the lowest Devonian beds. 
In our journey to Archangel, however, we traced the middle and upper mem- 
bers of the system in many places 1 , the latter being invariably capped, as in 
the Valdai Hills, by the carboniferous limestone 3 . At the river Mgra, about four 
versts south of a post-house on the high road to Vitegra, are light-coloured and 
reddish mottled, micaceous, incoherent grits and siliceous flagstones, in which 
we discovered scales of Diplopterus ? and Glyptosteus, associated with portions of 
the jaw of a new genus to which Professor Agassiz lias given the name of Cteno- 
ptychius 2 . To the south and west of the Lake Onega the strata of this age are 
more or less incoherent sandstones, which in parts have the aspect of the New 
Red Sandstone of Western Europe, though they are in general more flaglike. 
These are the beds which we conceive to have been altered by the intrusion 
and eruption of the trappean rocks of Petrozavodsk and the northern regions ; 
for at the south-western end of the Lake Onega, the sandstone is soft and inco- 
herent, and when followed on the same level is found suddenly to become a hard 
siliceous rock, wherever it is in the proximity of the greenstone, which abounds to 
the north of the river Svir (pp. 18 and 19). 
The eastern banks of Lake Onega a little to the south of the embouchure of the 
river Andoma present cliffs, about 150 feet high, composed of red and green 
marls, which pass into incoherent, variegated sandstone, resembling on the whole 
both the New and Old Red Sandstones of England 8 . Owing to their fragile nature, 
' We are indebted to a collection made by Mr. Strangways, and now in tbe museum of Dr. Buckland 
at Oxford, for a tine specimen of a very peculiar ichtbyolite found in the red sandstone between N. La- 
doga and Tichvin, and to which Professor Agassiz has given the name of Placosteus meandrims. 
3 This locality is on the left bank of the stream, about one mile above the saw-mills, where (in the heart 
of Northern Russia) we met with a most intelligent director of the works, who possessed a well-assorted 
small library and philosophical instruments, in a neatly arranged establishment, and who welcomed us 
with the hospitality so characteristic of all Russians. 
3 See ‘ Silurian System,’ in which the lithological identity of the Old and New Red Sandstones in 
certain parts of England is much insisted on, pp. 27 and 55. 
H 2 
