Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/354

Rh 340 A. Lower Silurian. F(3) Cincinnati (lludson River) group (Syrhzgopora, I{aI_i/sites, Diplograptus prz's(1's, Ptcri-nca demissa Le )f ena scricea). (2) Utiea grdup—1—lJtica shale.‘ G’raptoli'(Ims a.mpIc.rz'- I. Trenton Trenton . . I . _ . . mulzs, Trmuclcus I oriuation. limestone. c0m_mh_l.cm_ 0r”“.s (1) Trenton Black River u,smd1.nm_l.‘a’ ,,[m__ group Bl.lf"eSt_°"c' chisonia, Conularia, ii dsey e _ _ limcsbne 02 thoceras, Cy? to- L ' ccras, &e. f (3) (‘hazy group—Chazy liiiiestone(.l[aclurca nmgna, J1. Logan 1', Orthoceras, Illa’-nus, Asagih-us). ('2) Quebec group (upwards of 100 species of trilo- bites of genera Agnoslus, AI)1p_1/.1‘, Amphion, C'onoco7‘_1/phe, Dﬂccloccplmlits, I Ilcenus, Asa- phus, &e., more than 50 species of graptolites). (1) Calciferous group (graptolites, Lingulella acu- minata, Leptccna, C’o)wca.rd-iznn, Opltilcta com- pacta, Orlhoccras prz'mz'gcnizmz, 14 species of trilobites of the genera Amphion, Batlzyirrus, L Asaphus, Coiiocorg/phe). Potsdam formation, representing Cambrian (see ante, p. 331). The most recent researches of Mr Selwyn and his colleagues of the Geological Survey of Canada have shown that in the so-called Quebec group have been included a number of formations of very dilferent stratigraphical positions. lle recognizes in it three distinct groups :—(1) at the base a group of crystalline seliists ; (2) a group of sandstones and slates with abundant volcanic rocks, probably Cambrian ; and (3) Lower Silurian slates, sliales, liinestoiies, quartz- ites, sandstones, and conglomerates. He objects to the introduction of new names to denote systems of rocks, and thinks that Laurentian, Hui'oiiiaii, Cambrian, and Silurian meet all present requirements. 1. Canadian 4 Formation. DEVOSIAN AND OLD RED SANDsTo.'i5. In Wales and the adjoining counties of England, where so full a development of the Silurian system was originally discovered and worked out by Murchison, the abinidaiit marine fauna comes to an abrupt close when the red rocks set in at the top of the Ludlow group. From that horizon upwards in the geological series we have to pass through some 10,000 feet or more of barren red sandstones and niarls, until we again encounter a copious marine fauna in the Carboniferous Limestone. It is evident that between the disappearance of the Silurian and the arrival of the Carboniferous fauna very great geographical changes must have occurred over the area of Wales and the west of England. The sea must have been excluded from the area, or at least must have been rendered unﬁt for the existence and development of marine life over the area in question. From the striking contrast between the general facies of life in the Silurian and that in the Carboniferous system it is manifest that the interval between them must have been of long duration. The geological records of this vast interval are still only very partially unravelled and interpreted. At present the general belief among geologists is that, while in the west and ii0rth—west of Europe the Silurian sea-bed was upraised into land in such a way as to enclose large inland basins, in the. centre and soutli-west the geographical changes did not suffice to exclude the sea, which continued to cover more or less completely that region. In the isolated basins of the nortli—west a peculiar type of deposits termed the Old Red Sandstone is believed to have accumulated, while in the shallow seas to the south and cast a series of marine sediments and limestones was formed to which the name of Devonian has been given. It is thus supposed that the Old Red Sandstone and the Devonian represent different geographical areas, with different phases of sedimentation and of life, during the long lapse of time between the Silurian and Carboniferous periods. That the Old I-‘ted Sandstone does really represent this prolonged interval can be demonstrated by innumerable sections in Britain, where its lowest strata are found gradu- GEOLOGY [vI. STRATIGRAPIIICAL. ating downward into the top of the Ludlow group and its highest beds are seen to pass up into the base of the Carboniferous system. But the evidence is not everywhere so clear in regard to the true position of the l)evoiiian rocks. That these rocks lie between Silurian and Carboni- ferous formations is indeed sufliciently clear. lut it is a curious fact that where the Lower Devonian beds are best developed the Upper Silurian forni-.itions are scarcely to be recognized, or if they occur, they can hardly be separated from the so—ealled 1)evoniaii rocks. It is therefore quite possible that the lower portions of what has been tcrnied the Devonian series may in certain regions to some extent represent what are elsewhere recognized as uiidoulited Lud- low or even perhaps 'enlocl{ rocks. We cannot suppose that the rich Silurian fauna died out abruptly at the close of the Ludlow epoch. “'0 should be prepared for the dis- covery of younger Silurian rocks than the latest of those in Dritain, such as M. Barrande has shown to exist in his lit-age H. The rocks termed Lower Devonian may partly represent some of these later phases of Silurian life, it" they do not also mark peculiar geographical conditions of a still older period in Upper Silurian time. On the other hand, the upper parts of the Devonian system might in several respects be claimed as fairly belonging to the Carboniferous system above. The late Mr Jukes proposed a solution of the Devonian problem, the effect of which would be to turn the wliolo of the Devonian rocks into Lower Carboniferous, and to place them above the Old lied Sandstone, which would thus become the sole representative in Europe of the interval between Silurian and Carboniferous time.‘ In the following descriptions an aeeoimt will first be given of the Devonian type and then of the Old Red Sandstone. .1. Devonz'an. This name was first applied by Sedgwiek and llurchison to the rocks of North and South Devon and Cornwall, whence a suite of fossils was obtained which Lonsdale pro- nonneed to be intermediate in character between Silurian and Carboniferous. The relations of these strata to Silurian rocks cannot be determined from any section. They pass upward into Carboniferous strata. They have been arranged into three divisions, as in the subjoined table. fPilton and Piekwell-Down Group.—(li'ey slate with courses of impure limestone (l’ilton) passing down, into 'ellow brown, and TctlSall(lStO1lCS(B:H""V I’oint, Lpper 3 2 _ cc- Marwood), and a series of hard grey and red sand- L Down, Dulverton, Morte liay). Ilfracomhe Group. ——(ire_v iiiifossiliti-i'oiis slates (.lorte i lloe, Woolacombc, and Lee Bay) passing down into calcareous fossiliferous slates and limestoncs (Ilfra- comhe, Conibe lilartin, Toriluay, l’lymouth) resting on hard green, grey, and red grits, sandstones, and L conglomerates (llangiuan llill). {Lynton (’lronp.—Sol't slates with thin limestone and L qr ' sandstone bands (Lynton) resting on lowest S(‘lll>'l.5' mu ' 1 and red and grey iiiieaceous sandstones (Lynton, Lyn- moutli, Foreland, &e.). The total fauna of the British Devonian rocks numbers about 400 species. The middle group is the chief reposi- tory of fossils. Lowe;-.——Among the lower gritty slates and limestone bands of North Devon there are found, according to the detailed census compiled by _[r litlieridge, 18 species of fossils, comprising Favosiles cerricorm'.¢, Fenestella cmtzgurr, I);-[leis armam, S'p2'r7_'fer cmzalzferus, S. lcwicostus, dzc. Of these organisms 7 species are also found among the Devonian rocks of the Rhine, Belgium, or France. Mr Etheridge re- Middle .. i l 1 See his papers in Journ. Roy. (teal. Soc. Ireland (1865), i. pt. 1, new ser., and Quart. Journ. Geol. Soc., xxii. (1866), and his pamphlet on .l¢l¢lz'ti'o7ml Nutcs on liuclcs ~_f North Devon, &e., 1867.
 * stones aml iiiicaeeous llagstones at the base (l’iel<well-