128 Mr. Murchison on the Geological Structure 



5' 



land. Von L. Von Buch. 1840). Although then the clear strati- 

 graphical sequence is interrupted, there is no doubt that the equi- 

 valents, at least of the upper members of the Silurian rocks, exist 

 in these mountains ; and in tracing such into the South Ural, par- 

 ticularly by a transverse section from Verch-Uralsk to Sterlitamak, 

 the authors convinced themselves, from the presence of Orthidse, 

 Pentameri, &c, .that where not much interfered with by intrusive 

 rocks, the central deposits of the chain (usually however in the 

 state of slate and quartz rock) belong to the Silurian system, and 

 probably to its lowest divisions. 



The symmetry which is developed on the western side of the 

 water-shed is almost obliterated to the east by the greater frequency 

 of eruptive matter and the abundance of metamorphic and metalli- 

 ferous rocks. Thus in passing eastwards into Siberia on any paral- 

 lel, from Bogoslofsk, Nijny Tagilsk, Ekaterinburg, Miask or Verch- 

 Uralsk, no regular succession can be traced ; as large zones of 

 igneous and crystalline rocks intervene, and thus different members 

 of the palaeozoic series are met with upon the same strike. In some 

 spots however, notwithstanding all this confusion, transitions can 

 be traced from lower to higher formations. At Bogoslofsk, for ex- 

 ample, a passage may be observed from Silurian to Devonian strata j 

 and though all the formations are not in apposition to the east of 

 Ekaterinburg, the section of the river Isset clearly shows, that, after 

 various undulations, the Devonian limestones and schists on the 

 west are succeeded on the east by true carboniferous limestone 

 with large Producti, this latter deposit being in some instances 

 based upon conglomerates and grit. Whilst this succession is ex- 

 posed in a region penetrated by many points of eruptive trap and 

 porphyry, the whole of the less altered group reposing on mica- 

 ceous schists and other granitic rocks, a mass of Pentamerus lime- 

 stone is thrown up in an insulated tract at a small distance ; and 

 as this limestone is quite dissimilar from any visible in the* adjacent 

 gorges of the Isset, where the Devonian and carboniferous lime- 

 stones are fully developed, the authors conclude that it belongs to 

 the Silurian epoch. 



On the eastern flank of the southern Ural the ancient sedimen- 

 tary rocks occur in great undulations. At Troitsk in the steppes 

 of the Kirghis, or beyond a chain of granite separated from and 

 parallel to the Ural (see map), Silurian and Devonian limestones 

 occur, whilst at Cossatchi Datchi, close to the eastern flank of the 

 Ural, there is a small basin of palaeozoic rocks, the limestone of 

 which is proved to be true carboniferous, by containing a vast pro- 

 fusion of fossils, many of which are common to the VValda'i lime- 

 stones of Russia, and the mountain limestone of the British Isles and 

 Belgium. In following southward the eastern slopes of the chain 

 where they border on the river Ural, promontories of carboniferous 

 limestone rise up in undulations, supporting troughs of the coarse 

 carboniferous grits and conglomerates before alluded to; and on 

 passing the axis of the chain between Orsk and Orenburg, where it 

 dwindles to a small height, the same carboniferous group of lime- 



