Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/344

Rh ‘ I .3 30 suite (73 species) of organic remains. The trilobites include the genera .-fgzzostm, A napolcnus. Conocoryplic, Dz'lrcIocr_phal us, 1'.'r-inn_2/s, Ulcnus, and 1’urmIo.rz'dcs. 'l‘he earliest phyllopods (II_1/nmzocuris) and hetcropods (Bcllcrophonl occur in these beds. The brachiopods include species of Lingulcllo (L. Dru-isii), Discina, Obolclla, and Ortlzis. The pteropods are represented by three species of Them. Several annelides ((,'ru::iana) and polyzoa (Fcncstclln) likewise occur. According to a careful census by iIr Etheridge, the Lingula ﬂags may be grouped into three zones, each characterized by a peculiar assemblage of organic remains. The lower division contains 37 species, of which 9 are peculiar to it. The middle zone has yielded 5 species, 2 of which (Conocoryplw buccphala and Lingulclla Diwisii) pass down into the lower division, 1 (Kutorgia. cingulata) into the upper, and 2 (Lingula squamosa and Bellam- phon. C'ambrcnsi.s) are peculiar. The upper zone has yielded 40 species. Of these 9 pass up into the Trcmadoc beds, while 2 (Lingulclla lcpis and L. I)ai~z'sii) continue on into the Arenig group. 4. Trcmadoc .S'Iatcs.—-This name was given by Scdgwick to a group of dark grey slates, about 1000 feet thick, found near Tremadoc in Carnarvonshire, and traceable thence to Dolgelly. Tlu-ir importance as a geological formation was not recognized until the discovery of a remarkably abundant and varied fauna in them. They contain the earliest erinoids, star-ﬁshes, lainelli- branehs, and cephalopods yet found. The trilobites embrace 14 genera, among which, besides some, as Agnost-us, Conocoryplze, and Olcnils, found in the Lingula ﬂags, we meet for the first time with A2i_(/clina, 1fS'l])]Hl.S‘, C'licz'rzn'-us, Ncscm'ctus, Niobc, Ogygia, I’sil0— cqzltalus, &c. The same genera, and in some cases species, of brachiopods appear which occur in the Lingula ﬂags, 07-this Carausii and Lingulclla Drwisii being common forms. Mr Hicks has dcscribel 1'3 species of lamellibranclis from the Tremadoc beds of Rainsay Island and St Davids, belonging to the genera C'lcn0- donta, Pttlccarca, G13/ptarca, Daciclia, and Jlodiolopsis. The cepha- lopods a1'e represented by 01'thoccras scriccmn and C_i/rtoccras prwcox; the pteropods by Thom Dcwidii, T. opcrculata, and C'0-nu- Iaria Ifomfrayi; the echinoderms by a beautiful star—tish (I’aIas- tcrina 7'amsr'gcnsz's) and by a criiioid (Dendrocrinus C'ambrc1zsz's).‘ Careful analysis of the fossils yielded by the Tremadoc beds suggests a division of this formation into two zones. According to a census by _Ir Etheridge, the Lower Trcmadoc rocks have yielded in all 56 species, of which 9 pass down into the Lingula ﬂags and 10 ascend into the Upper Tremadoc zone, 31 being peculiar. The Upper Tremadoc beds contain, as at present ascertained, 33 species, of which 9 are peculiar, and 13 or possibly 15 pass up into the Arenig group. It is at the top of the Upper Treinadoc strata that the line between the Cambrian and Silurian systems is here drawn. According to Professor Ramsay, there is evidence of a physical break at the top of the Treinadoc beds of Wales, so that on a large scale the next succeeding or Arenig strata repose uiiconformably upon e.verything older than themselves. Mr Etlieridge also shows that the palzeontological break is nearly complete, only about 7 per cent. of the fossils of the one series passing over into the other. Out of 184 known Arenig species, not more than 13 are common to the Tremadoc beds underneath. Besides these important facts the character of the Arenig fauna strongly distinguishes it from that of the formations below, and further supports the line of division here adopted between the Cambrian and Silurian systems. In the north-West of Scotland a mass of reddish-brown and cliocolate-coloured sandstone and conglomerate (at least 8000 feet thick in the Loch Torridou district) lies uiicon- formably upon the fundamental gneiss in nearly horizontal or gently inclined beds. It rises into picturesque groups of mountains which stand out as striking inonuinents of denudation, seeing that the truncated ends of their com- ponent ﬂat strata can be traced even from a distance forming parallel bars along the slopes and precipices. The denuda-- tion must have been considerable even in early Silurian times, for the sandstones are unconformably overlaid by quartz-rocks and limestones containing Lower Silurian fossils. No trace of organic remains of any kind has been found in the red sandstones themselves. They were at one time regarded as Old Red Sandstone, though Macculloch, 1 Hicks, Quart. Juurn. h'»;'nz'. Sm‘ x:.-ix, 39, 4., GEOLOGY [V1, STRATIGRAPIIICAL. and afterwards Hay Ckniningham, pointed out that they underlie parts of the schistose rocks of the northern lligli- lands. The discovery by Mr C. W. Peach of Lower Silurian shells in the overlying liniestones showed that the 1113.5:-l'8 red sandstones of western Ross aml Sutherlaml could not be paralleled with those of the eastern tracts of those counties, but must be of older date than part of the Ll:n1- deila rocks of the Lower Silurian period. Sir R. Murchison classed them as Cambi-ian—an identification which has much support in the lithological reseiublancc between these rocks of the iiorth-west Ilighlands and much of the Lower Cambrian system of Wales. In the soutli—east of Ireland masses of purplish, red, and green sliales, slates, grits, quartz—rocks, and si-hi.~ts occupy a considerable area and attain a depth of 14,000 feet with- out revealing their base, while their top is covered by ini- coiiforinable formations (Lower Silurian and Lower Carboni- fcrous). They have yielded Old/zamia, described originally as a sertularian zoopliyte, but now regarded by many palzeontologists as an alga; also numerous burrows and trails of ainielides (llistioclerma Ilibcrizzcmn, .lrenir-olilrs di¢I_I/mus, A. sparsus, Ilcmf//itozzirz poecilu). N0 Upper Cainbrian forms have been met with in these "Irish rocks, which are therefore placed with the Lower C'ainbrian, the unconforniability at their top being regarded as equivalent to the interval required for the deposition of the intervening formations up to the time of the Llandcilo rocks, as in the nortl1—wcst of Scotland. Some portions of the Irish Cam- brian series have been intensely metamorphosed. Thus on the Howth coast they appear as scliists and quartz-rocks', in Wexford they pass into giiciss and granite. In West Galway Mr Kinahan has described a vast mass of schists, quartz-rocks, and liinestones (8000 feet and upwards) pass- ing up into schistose, hornblendic, and unaltered rocks con- taining Llandcilo fossils, and he agrees with (lritlith and King in regarding these as probably Cambrian. Ile suggests that they are Upper Cambrian, which would imply that Upper Cambrian rocks pass conforinably into the Llandeilo formation without the occurrence of the thick Arenig i'oc-ks of Vales. In a difficult country, however, broken by faults and greatly inetainorphosed, an unconforinability might easily escape detection. CONTINENTAL EUi:orE.——-According to the classification adopted by M. Barrande, the older Palzeozoic rocks of Europe suggest an early division of the area of this con- tinent into two regions or provii1ces,——a northern province, embracing the British Islands, and extending through North Germany into Scandinavia, on the one hand, and into Russia on the other, and a central-European province, including Bohemia, I"rance, Spain, I’ortugal, and Sardinia. Bo/z.emia.——The classic researches of M. Barrande have given to the oldest fossiliferous rocks of Bohemia an extra ordinary interest. He has made known the existence there of a remarkable suite of organic remains representative of those which characterize the Cambrian rocks of Britain. At the base of the geological formations of that region lie the Arcliaean gneisses already described. These are over- laid by vast masses of schists, conglomerates, quartzites, slates, and igneous rocks, which have been more or less inetainorphosed, and are singularly barren of organic remains, though some of them have yielded traces of anne- lides. They pass up into certain grey and green ﬁssile shales, in which the earliest well-marked fossils occur. The organic contents of this zone (litagc (‘) form what ll. Barrande terms his primordial fauna, which contains 40 or inore species, of which 27 are trilobites, belonging to the characteristic Cambrian genei'a—I’cLrado.ridcs ([2), A_(/)zo.~'- tus C’0noc0r_z/1;/Le (4), 1:'llz'p.s0cq;/talus (2), 1/]/ll?'(.C('1)/((1, us (2), Arioncllus (1), Sue  Not a single species of any one of these genera, save A_r/nosius (of which 4 species