432 
SILURIAN ROCKS UPON THE AI. 
limestone with its usual fossils, that rising out from beneath them is itself under- 
laid by calcareous and quartzose masses which, as developed at Alina, we believe 
to be of Devonian age. The next group, extending from thence to Pristan, is 
unquestionably Upper Silurian, for we found in it Pentamerus Bashkiricus, Favosites 
Gothlandica, and Stromatopora concentrica. To the east of Pristan, a great convo- 
lution is followed by a break, and schistose and quartzose rocks with some green- 
stone are thrown over with a reversed dip. We consider these rocks, extending 
from Silkia to Satkinsk, to be Lower Silurian, because although we did not detect 
in them any characteristic organic remains, they possess the original elements of 
rocks of that age and contain at intervals courses of limestone, as already explained 
in the traverse to Yuryusensk. Knowing, as we do, from the sections of the Arctic 
Ural, that unquestionable Lower Silurian rocks exist in this chain, it is a fair 
inference that these quartzose and schistose grauwackes, which here succeed to 
true Upper Silurian, are also of that older age. In continuing the section, to 
Satkinsk and to the east of it, we find that the rocks in question are there over- 
laid by copious calcareous masses, often in the form of black dolomite, which lie 
in a trough, associated with much greenstone and trap. Near the eastern limit of 
this trough is the Zavod of Kussinsk, where crystals of talc abound in the schist, 
and thus indicate an approach to the zone of high metamorphism. 
The banks of the Ai, however, near this place expose fine cliffs of limestone, 
which, though we could discern nothing but Encrinites in them, we suspect to be 
of Upper Silurian age. The lower beds are thin flagstones, the middle beds are 
thick, exhibiting a singular concretionary structure and remarkable undulations in 
the joints ; whilst rounded elliptical forms, as if derived from large obliterated 
fossils, Pentameri'?, protrude at intervals from the surface. The uppermost beds are 
thin-bedded like the lower, of red, green and yellow colours, and have a hard schis- 
tose aspect. The associated shales are changed into finely laminated, hard argillo- 
calcareous schists, which are occasionally welded together into compact rocks, with 
gaping breaks transverse to the lamination, the whole formation having a strongly 
altered aspect. 
These rocks constitute the external fringe of the still more highly metamor- 
phosed masses, which occupying all the region around Zlataust, have before been 
alluded to. Between the Zavods of Kussinsk and Zlataust, other bands of altered 
limestone with quartzose grits occur at Kuvashi, associated with greenstone and 
micaceous schist, which has been converted into Lydian stone. As in many other 
