462 OBSERVATIONS UPON THE ORIGINAL STRUCTURE OF THE URAL. 
an intense movement having been communicated to the central ridge, certain wave- 
like undulations to which the whole has been subjected, were necessarily most rapid 
towards that disturbing centre, and gradually died away as they receded from it. 
In short, the views which American geologists have so admirably worked out in 
the Apalachian chain 1 may, we think, be considered equally striking in the South- 
western Ural. Thus, in the tract between Verch-Uralsk and Bieloretzk, highly 
metamorphosed strata, with many intrusive rocks, are thrown about in rapid 
undulations, forming high mountains, steep slopes and deep valleys (see coloured 
section, PI. IV.). Next we meet with Silurian strata rolling over in numerous 
folds, which expand into the wider troughs of Uziansk and Petrofsk. These un- 
dulations becoming wider, and the ridges decreasing in altitude as they recede from 
the axis, the whole series terminates on the west, in the broad trough between the 
Akri-tau and Tcheke-tau. Again, with this expansion and retrocession from the 
chief axis, and with the diminution in altitude, the crystalline characters of the 
chain gradually disappear. The limestones part with their saccharoid and slaty 
aspect, the mica schists pass into micaceous flagstones, the quartz rocks into con- 
glomerates and psammites, and thus the observer is regularly conducted, with few if 
any discordant or unconformable junctions, from a crystalline nucleus into ordinary 
sedimentary masses. Though partaking more of a general description than we 
could wish, such we feel confident is the true picture of this portion of the Ural. 
Interstratified as these palseozoic sediments have been, in other parallels, with 
large bands of igneous matter, which we believe to have been coeval with their 
accumulation, and afterwards cut through in many places by intrusive rocks which 
have altered their original character, and often highly mineralized them, it is im- 
practicable to draw a well-defined base-line for this greatly contorted, broken and 
often inverted series. In one spot only, have we detected true Lower Silurian 
shells : even the upper members of that system are, for the most part, mere detached 
masses of limestone, which, along the eastern flank of the chain, being pierced on 
all sides, and surrounded by eruptive rocks with much serpentine, constitute one 
of the chief auriferous zones, to which we shall presently advert. In several sec- 
tions on both flanks of the chain, we have, indeed, shown a succession through a 
large Devonian series to carboniferous limestones ; whilst in other places, as in the 
eastern steppes, the lower of these two formations is either covered up by eruptive 
1 “ On the Physical Structure of the Apalachian Chain,” by Professors W. B. and H. D. Rogers. 
(Trans. Assoc. Amer. Geol., 1840-42, p. 474.) 
