460 
SECTION FROM VERCH-URALSK TO STERLITAMAK. 
and purple colours with schists and conglomerate, which latter rise up with an 
easterly dip and form the Ala-tau. From this ridge we descended into a broad 
depression on the west, occupied by shale, and covered with dense foliage, on the 
other side of which rises boldly the Akri-tau. Knowing that this was the last 
western ridge of the mountains, our anxiety increased as we approached it, to 
discover some organic remains, by which a key might be had to explain the age of 
the numerous undulations over which we had passed, from the Silurian axis of the 
chain. The shale ol the valley to the west of the Ala-tau, forming a broad anti- 
clinal, bends over to the west and passes beneath the Akri-tau, thus exhibiting a 
valley of elevation. Near the little hamlet of DerikK, towards the eastern end of 
this depression, we found the Favosites polymorpha, thus indicating that these cal- 
careous shales were either of the Upper Silurian or Devonian age. The lowest beds 
of Akri-tau on the west side of the valley, are grey psammites, inclined westwards 
at 32°, and the overlying mass of the mountain is made up of very striking ledges 
of dingy green and yellowish-brown, slightly micaceous sandstone, much resembling 
the Ludlow rocks of England, where they contain little calcareous matter, as on 
the banks of the Wye. This external resemblance is carried so far, that the sur- 
faces of the rocks are often covered with the same purple film of iron ore so 
common near Ludlow. Towards the western slope of the Akri-tau these beds are 
surmounted by red, gritty, fine conglomerate, reddish hard quartzose, green spotted 
sandstone, with hard purple and grey rocks, the whole of which are really un- 
distinguishable from many examples of the Scottish Old Red Sandstone. Having 
before us such analogies in mineral succession, from Upper Silurian through Old 
Red Sandstone, little more was now wanting to complete the belief, that the rocks 
were really such, than to find them surmounted by the carboniferous limestone, 
and in this we were not disappointed. 
The strata which succeeded, and in perfect conformity, to the red sandstones, 
were dark-coloured limestones, with Productus striatus (Fisch.), P. Valdaicus, 
P. antiquatus (Sow.), Lithostrotion fioriforme, &c. These are followed by carbo- 
naceous grits, which are partially dislocated, and after a fault, the section terminates 
towards the plains of the Permian system, exposing a great thickness of white- 
veined limestones, of light and dark-grey colours, with courses of black flint, the 
whole charged with numerous true carboniferous fossils, including the well-known 
Spirifer Mosquensis (Fisch.), Terebratula lamellosa ( Spir . id. Lev.), and corals, &c. 
The fortunate discovery of these fossils, in the uppermost band of these mountains, 
