Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/356

Rh 342 'l‘huringia.3 The characteristic IInIo1»(_ueIu'us nel-iIz'ssz‘n1m‘ has re- t-.-ntl_v been detected in the l’sammite dc ('ondroz. which in lielgium f-_n'ms a characteristic sandy portion of the Upper Devonian rocks. These are interesting facts, as helping to link the Devonian and Old lied Sandstone types together. But they are as yet too fev and unsupported to warrant any large deduction as to the corula— tions between these types. It is in the north-cast of Europe that the Dc'onian and Old Iied Sandstone appear to bc united into one system, where the lime- stones and marine organisms of the one are interstratilied with the fish-bearing sandstones and sltales of the other. In liussia, as was shown in the great work 1iussi'.t curl llzc Ural _l[o1Lntains by Mar- chison. De Vcrneuil, an.l Keyserling, rocks intermediate between the Upper Silurian and Carboniferous Limestone formations cover an extent of su1'face larger than the British Islands. This wide development arises not from tl1c thickness but from the undisturbed horizontal character of the strata. Like the Silurian formations above described, they remain to this day nearly as flat and un- altered as they were originally laid down. Judged by mere vertical depth. they present but a meagre 1'eprescntativc of the massive Devonian greywacke and limestone of Germany, or of the Old lied Samlstonc of liritain. Yet vast though the area is over which they form the surface rock, it is probably only a small portion of their total extent; for they are found turned up from under the newer fornutions along the flank of the Ural ehain. It would thus seem that they spread continuously ac1'oss the whole breadth of liussia in Europe. Though almost everywhere umlisturbed, they afford evidence of some terrestrial oscillation between the timeoftheir formation aml that of the Silurian rocks on which they rest, for they are found gradually to overlap Upper and Lower Silurian formations. The chief interest of the liussian rocks of this age lies in the. fact, first signalizel by Murchison and his associates, that they unite within themselves the characters of the Devonian and the Old Red Sandstone types. In some districts they consist largely of limestones, in oth -rs of red sandstones and marls. In the f)rmcr they present mollusks and other marine organisms of known Devonian species; in the latter they afford remains of fishes, some of which are specifically identical with those of the Old lied Sand- stone of Scotlan-l. The distribution of these two palzeontological types in liussia is traced by llurehison to the lithologieal characters of the rocks, and consequent original diversities of physical condi- tions, rather than to differences of age. Indeed cases occur where in the same haul of rock Devonian shells and Old lied Sandstone fishes lie commingled. In the belt of the formation which extends southward-s from Archangel and the Vhite Sea, the strata consist of sands and Inztrls, and cmtain only ﬁsh remains. Traced through the Baltic provinces, they are found to pass into red and green marls, clays, thin limestones, and sandstones, with beds of gypsum. In some of the calcareous bands such fossils occur as Orthis .5-triatula, .S',w'r[_/'c1'£Im. prism, Lcptteml, protluctoitlcs, S'1n'7'2_'f«'r cah'aratus, Spir- orbis omphrzloz'r_lcs, and 0I'UlI)C6]'(LS‘ sulgfuszformc. In the higher beds Iloloplyclzius an-l other well-known fishes of the Old lied Sandstone oeeur. Followed still further to the south, as far as thc watershed between Orel an-1 Woronesch, the Devonian rocks lose their red colour and sandy character, and become thin—beddcd yellow lime- stones, and dolomitcs with soft green and blue marls. Traces of salt deposits are indicated by occasional saline springs. It is evident that the geographical conditions of the Russian area during the Devonian period must have closely resembled those of the Rhine basin and central England during the Triassic period. The liussian Devonian 1'ocks have. been classified as follows :— lied and white sandstone and green marls,——n1nnerous ﬁsh remains, particularly Ilolnplyclzius nobilﬂs-sirlzzts, Glg/ptoslczts ﬂwosus, liiploptcrus macroccplmlus. S Limestones, clays, marls, dolomite, and gypsnm,—— numerous characteristic Devonian shells and crinoids, f_ also Iloloptychius 7mbil1'ssz'mus. f Upper. Middle .. In some districts red and green limcstones with red marls and Middle Devonian fossils; in others (North Livonia) sandstones and clays, with numerous fish remains of the genera Ostcolcpis, I)z'ptcrus, Iiiploptrrzts, Astcrolrpis, and others found also in the Caithncss l flags of Scotland. ‘There is an unquestionable passage of the uppermost Devonian rocks of Russia into the base of the Carboniferous system. The Deyonian rocks of North America are noticed at the end of the next sec Ion. Lower. ... B. Old Red Sandstone. Under this name is comprised a vast and still imperfectly described series of red sandstones, shales, and conglomerates, intermediate in age between the Ludlow rocks of the Upper Silurian formations and the base of the Carboniferous system ‘ Op. cit, 4'23. GEOLOGY [vI. s1‘1:-'1(:n..uu11c.I.. in Iiritain. T hcsc rocks were termed “ Old ” to distingui--h them from a somewhat similar series overlying the Coal- mcasures, to which the name New lied Sandstone was applied. 'hen the term Devonian was adopted, it speedily supplanted that of Old lied Sandstone, inasmuch as it was founded on a type of marine strata of wide geographical extent, whereas the latter term described what appeared to be merely a British and local development. For the reasons already given, however, it is desirable to retain the title O1 .1 lied Sandstone as descriptive of a remarkable suite of forma- tions to which there is nothing analogous in typical ])e-onian rocks. It is in Great llritain that the Old lied Sandstone of-Europe is almost entirely developed. This interesting series of deposits must from the first have been depositel in separate areas or basins, the sites of some of which can still be traced. Their diversities of sediment anl discrcpance of organic contents point to the want of any direct comnumication between them. It was maintained many years ago by Mr Godwin Austen, and has been more recently enforced by Professor liamsay, that tliC>'0 basllls were lakes or inland seas. The character of the strata, the absence of unequivocally marine fossils, the )l'CSCllf.'C of land- pl:u1ts and of numerous ganoid ﬁshes 'lll('ll have their modern representatives in rivers and lakes, s'igg«_-st and support this opinion, which has been generally adopted by geologists. The red arcnaccous and marly Luis which, with their fish remains and lan_l—plants, occupy a depth of many thousand feet between the top of the Upper Silurian and the base of the Lower Carboniferous formations, are re- garded as the deposits of a series of lakes or inland stas formed by the uprise of portions of the Silurian sea-floor. The length of time during which these lacustrinc basins must have existed is shown, not only by the thickntss of the deposits formed in them, but by the complete change which took place in the marine fauna between the close of the Silurian and the commencement of the C:11‘l‘oniferot1~‘pcriutl. The prolific fauna of the Vcnlock and Ludlow rocks was extirpated over the British area by the physical changes which produced the lake—basins of the Old lied Sandstone. 'hcn a marine popul-ation—crinoids, corals, and shclls—— once more oversprcads the area it is found to be completely different. So thorough a change must have dcmandegl a long interval of time. Murchison, who strongly advocated the opinion that the Old lied Sandstone and Devonian rocks represented differ- ent gcographical conditions of the same period, and who had with satisfaction seen the adoption of the Devonian cla.<~i- ﬁcation by Continental geologists, endeavoured to trace in the Old lied Sandstone of Britain a threefold tllvlsintl like that which had been accepted for the Devonian systcin. lle accordingly arranged the formations as in the sulajoined table. Yellow and red sandstones and conglomerates (I’l«r- ichtlq/s anajor, 1IoInpl_2/cllius 'noIu'lz'ssi2m(s, &e.)=llura Den beds. Grey and blue calcareous and bitumenous flagston--s, limestoncs, and rcl sandstones, and conglomeratts (Dfptcrus, Oslcolrpzs, ll.~tcrolepz's, .[(.'(lllf]lUi'fl3, J‘(t'r- _ 2'ch!h_r/s, &'.e.)-—-—-Caitlmess flags. ied and purple sandstones, grey flagstones, and coarse conglo1n1er:lttes (C'epIzalus~p1.‘s. 1’tcra.sy:i.s-. 1’(rI'g/_v/Uta.»-,. Arbroata f ags. Old Red Sandstone. as classificd bylllurchison. r- -4‘-*——-K Lower. Middle. Upper. It is important to observe that in no (llst1'i('t can these three subdivisions be found together, and that the so-called “middle” formation occurs only in one rcgio11—thc north of Scotland. The classification, therefore, does not rest upon any actually ascertained stratigraphieal sequence, but on an inference from the organic remains. The value of this inference will be estimated a little further on. All that can be affirmed from stratigraphical evidence in any Old lied Sandstone district in Britain is that a great physical and