Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/345

Rh cAMBnI.A.N.] appear in the second fauna), has been found by M. Barrande higher than his primordial zone. Among other organisms in this primordial fauna, the brachiopods are represented by 2 species (Urt/z-is and Orbic-ula), the pteropods by 5 (T/Lesa), and the echinoderms by 5 cystideans. Scm2(li11(z.via.—II1 Norway the vast masses of Archaean gneiss (Tellemark) are overlaid by schists, red sandstones, and conglomerates. These are termed the Sparagmite formation, and have hitherto proved barren of fossils. They are covered, however, by beds containing Dirty/enema Norve_r/icmn, which may represent the primordial zone of Barrande. In Sweden the sparagmite formation has been more productive of organic remains. It is there represented by a sandy zone not more than 50 or 60 feet thick—a poor equivalent for the great mass of strata in the Cambrian system of Wales. It was originally termed the Ifqr/2'0 1v'7zco27«_la.ru-m by Angelin, from the fucoids alone found in it. In more recent years, however, its- list of organic re- mains has been considerably increased; 12 species of plants, chieﬂy fucoids, but including some (]]oplz_z/ton) of higher grade, 9 species of annelides, 4 brachiopods, a pteropod, a bryozoan, a coral, a crinoid, and a sponge have been obtained. Above the strata containing these organisms comes a zone which has yielded 77 species of primordial trilobites, including the genera A_r/nostus (19 species), Cano- /:or_z/)»/cc (13), Olerms (21), Parado.ricles Nonrn ..-lMERICA.——ROCl{S corresponding in position and in the general character of their organic contents with the Cambrian formations of Britain have been recognized in diﬁcrent parts of the United States and Canada. They appear in Newfoundland, whence, ranging by Nova Scotia and New Brunswick, they enter Canada, the northern parts of New York, Vermont, and eastern Massachusetts. They rise again along the Appalachian ridge, in Wisconsin, hlinnesota, Missouri, Arkansas, Texas, and Georgia. Vest- wards from the great valley of the Mississippi, where they have been found in many places, they reappear from u11der the Secondary a11d younger Palzeozoic rocks of the Rocky Mountains. They have been divided by American geo- logists into two formations-—(.1) Acadian, a mass (2000 feet) of grey and dark shales and some sandstones; and (2) Potsdam (or Georgian), which attains in Newfoundland a depth of 5600 feet, but thins away westward and southward till in the valley of the St Lawrence, where it was studied by Logan and his associates of the Geological Survey of Canada, it is only from 300 to 600 feet thick. Among the organic remains of the North American Cambrian rocks fucoid casts appear in many of the sand- stones, but no traces of higher vegetation. The Acadian formation has yielded primordial trilobites of the genera Pm-rziloacicles, Conocor_z/plze, A_r/nostus, and some others ; brachiopods of the genera Lingulel/u, 1)e'scz'na, Obolella, and Orz‘/zis ; and several kinds of annelide—tracks. The Potsdam rocks contain a few sponges, the earliest forms of graptolite, some brachiopods, including, besides the genera in the Acadian beds, Obolus, Camarella, and Ort/n'sz'na ; some pteropods (I[_z/elites or T/zeca) ; two species of Ortlzoceras ;. annelide tracks ; trilobites of the genera Conocoruz/plze, .-lgnostus, 1)iI.'e.’ocepImZus, Olcncllus, Pt‘:/clzaspis, C/z(m'oce- plmlus, .»1_r/laspis, and I /lcen 2/rus. M. B-arrande has called attention to the remarkable uni- formity of character in the organic remains of his primordial zone over the continents of Europe and America. He pub- lished in 1871 the subjoined table, to show how close is the parallelism between the proportions i11 which the different classes of the animal kingdom are represented.1 1 T-rt./ubitcs, Prague, 1871, p. 196. Since the publication of this table the progress of research has increased the number of species from some localities; but the general facies of the primordial fauna has not been materially affected thereby. GEOLOGY 331 Crus-. .. Inferior mcmlm Molluslm. mussel ' U! ., 2 Countries. 3| ,5 ea g _ E ' S tr: ' ‘ '6 c "3 * 0
 * 7_’i3I€§§§~§.§‘§§g;»

e 2 '5 -'.-'::«.;.s.~.<.::.:s:—-: o 5-’ §c~|o<::~..:c:.-Eo'z§}§ 1. Bohemia ................... .. 27. 5 2 1 " 40 2. Spain ...................... .. 9 1 2 6 . 1 19 - ,- Iicgioncs ,.,_, 3. Seandrnaua A and], E H _, 2 8 4 96 4. Entr- Menevian .... ..l .. laiid Harlech, part i 33 1 4 4 ’ 6 ' 1 2 58 5. Newfoundland ........... .. 2 . . . . . . . . . . 2 6. New Brunswick ......... .. 18 . . . . . . 6 . 1 . 25 7. New York ................ .. 5 , , _ , _ , , , _ , 5 8. Braintrce(Massachusetts)! 1 1 1172 110 4!14. 2 28 5 8 :2=246 SILURIAN. The important system of rocks next to be described was first investigated by the late Sir ll. Murchison in Wales and the bordering counties of England. He found it to be char- acteristically developed over the tract once inhabited by the Silures, an ancient British tribe, and he thence chose the name of Silurian as a convenient designation. It there passes down conformably into the Treinadoc slates at the top of the Cambrian series, and is covered conformably by the base of the Old Red Sandstone. GREAT BRITAIN. ~—In the typical area where Murchison's discoveries were ﬁrst made he found the Silurian rocks divisible into two great and well—1narked series, which he termed Lower and Upper. This classiﬁcation has been found to hold good over a large part of the world. The subjoined table shows the present arrangement a11d nomenclature of the various subdivisions of the Silurian system. Feet. 7. Ludlow group ........................... .. 1,950 B. Upper Silurian. 6. 'enloek group .......................... .. 1,600 5. Upper Llandovcry group ............. .. 1,500 4. Lower Llandovcry group ............. .. 1,000 A- Sllum  lfi‘§i‘..?éif.%ifit}S‘?.%‘.‘fTT¥’.:::::::::::::::: 3:288 1. Arenig or Stiper Stone group ....... .. 4,000 Approximate average thickness = 18,550 A. Lower ;S'ilm'1'an. 1. A7'e72ig or Stiper Stone G’-roup.—These rocks consist of dark slates, shales, ﬂags, and bands of sandstone, They are abundantly developed in the Arenig mountain, where, as originally described by Sedgwick, they contain masses of associated porphyry. Throughout that district they have been deposited at a time when streams of lava and showers of volcanic ashes were thrown out in great quantity from submarine vents. They contain an abund- ant suite of organic remains (184 species), of which only 13 species are common to the Tremadoc beds below. Trilobites occur of the genera 4E_r]Zina, Agnostus, A7)lpy.z', Bar-randea, Calg/mene, C/u3irm'us, Illce72ops2'.9, Illcen-us, 0_r/_2/_r/ia, P/zacops, and T2-iizuclcus. Three species of ptero- pods (Conularia, T /zeca), 18 species of brachiopods (Ling- ztla, Lingulella, Obolella, Discina, Siplzonotreta, Ortluis), 8 lamellibranchs, 3 gasteropods, and 5 cephalopods have been found; but the most abundant organisms are the graptolites, of which the Arenig rocks of St David’s, in Pembrokeshire, have yielded 48 species, which belong to 20 genera, including Dz'a7_2/22zo_r/raptzls, T etrngraptzzs, D1plo- graptus, Dendrof/r(Iptus, and Callog7'aptzzs.2 Altogether 3 Hicks, Quart. Journ. Gcol. Soc., xxxi. 167; H0Pki11SC'11 and L3-P‘ worth, '2‘b1'rl., p. 635.