Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/348

Rh 334 belt of Lower Silurian rocks runs from the coast of Down into the heart of Roscommon and Longford. This belt is evidently a. pro- longation of that in the southern uplands of Scotland. It is marked by the occurrence of similar dark anthraeitic shales crowded with graptolitcs. The richest fossilifcrous localities among the Irish Lower Silurian 1'0.-ks are found at the Chair of Kildare, 1’ortranc near Dublin, l‘omeroy in Tyrone, and Lisbcllan in F(1'- managh, where small protrusions of the older rocks rise as oases among the surrounding later formations. Portloek brought the northern and western localities to light, and lIu|‘chison pointed out that, while a number of the trilobites (Trimtclcus, Pluzcops, Caly- mcne, and Illtrnus), as well as the simple plaitcd Orthidcr, .L(‘1l(ﬂ'7lCU, and Slrophomeme, some spiral shells, and many Ortlzoccrata, are specifically identical with those from the typical Caradoc and Bala. beds of Shropshire and Wales, yet they are associated with peculiar forms, first discovered in Ireland, and very rare elsewhere in the British Islands. Among these distinctive fossils he cites the trilobites, Rcmoplcuridcs, Ifarpcs, A mphion, and Bronlcus, with the smooth forms of Asaphus (Isotelus), which, though abundant in Ireland and America, seldom occur in 'alcs or England, and never on the Continent.‘ In the north and west of Ireland a large area of surface is occu- pied by crystalline rocks—gneiss, sehists, quartz--rocks, limestone, granite, 3:e.—which are manifestly a continuation of those of the Highlands of Scotland. Thcy rnn south—wcstward parallel with the belt of unaltered Lower Silurian rocks from which, in some places, as in county Tyrone, they are only a few miles distant. The district of Pomeroy, so rich in Silurian fossils, promises to afford the greatest light on the interesting but ditlicult problem of the metamorphism of the Lower Silurian rocks of the Scottish Highlands and the north-west of Ireland. It will be seen from the evidence furnished by the sections in Vest Mayo (p. 337) that the metamorphism must have taken place prior to the deposition of the Upper Silurian formations of the west of Ireland. B. Upper Siluﬁctn. The formations which in the British Islands are classed as Upper Silurian occur in two very distinct types. So great indeed is the contrast between these types that it is only by a comparison of organic remains that the whole can be grouped together as the deposits of one great geological period. In the original region described by Murchison, and from which his type of the system was taken, the strata are comparatively ﬂat, soft, unaltered, consisting mainly of somewhat incoherent sandy mud with occasional bands of limestone. But as these rocks are followed into North Wales, they are found to swell out into a vast series of grits and shales so like portions of the hard altered Lower Silurian rocks that, save for the evidence of fossils, they would naturally be grouped as part of that more ancient series. In Westmoreland and Cumberland, and still further north in the border counties of Scotland, also in the south—west of Ireland, it is the North Welsh type which prevails, so that in Britain the general lithological charac- ters and minute palaeontological subdivisions ascertained in the typical Silurian district are almost conﬁned to that limited region, while over the rest of the British area for thousands of square miles the hard sandy and shaly type of North Vales is prevalent. Taking ﬁrstthe Silurian tract of thesouth-west of England, and the east and south of Wales, we ﬁnd a decided uncon- formability separating the Lower from the Upper Silurian formations. In some places the latter are found passing across the edges of the former, group after group, till they come to lie directly upon the Cambrian rocks. Indeed, in one district between the Longmynd and Wenlock edge, the base of the Upper Silurian rocks is found within a few miles to pass from the Caradoc group across to the Lower Cambrian rocks. It is evident, therefore, that in the Welsh region very great disturbance and extensive denudation preceded the commencement of the deposition of the Upper Silurian rocks. As Professor Ramsay has pointed out, the area of Wales, previously covered by a wide though shallow sea, was ridged up into a series of islands, round the margin of which the conglomerates at the base of the Upper GEOLOGY ['I. STRATIG 1t.I'1IICAL. Silurian series began to be laid down. This took place during a time of submcrgence, for these conglomeratic and sandy strata are found creeping up the slopes and even capping some of the heights, as at Bogmine, where they reach a height of 1150 feet above the sea.‘-’ The subsidence probably continued during the whole of the interval occu- pied by the deposition of the Fpper Silurian strata, which thus were piled to a depth of from 3000 to 5000 feet over the disturbed and denuded platform of Lower Silurian rocks. Arranged in tabular form, the subdivisions of the Upper Silurian rocks of 'ales and the a_l_joining counties of England are in descending order as follows-:- Base of Old Red Sandstone. '1‘ilcstones. Upper Ludlow Rock. Aymcstry Limestone. Lowcr Ludlow Rock. I Wenlock or Dudley Lime- 3 I O0 . Ludlow group stone ..................... .. Dcnbighshire ‘.3. Vcnlock group Vcnlock Shale ........... .. — Grits of Voolhope or Barr Limc- stone and Shale ....... .. Tarannon Shale. May llill Sandstones. Lower Llandovcry lloeks. North Wales. 1. Upper Lland- ovcry group... 1. Upper LImz(l0ver_2/ G'roup.—(a.) .l[u_y Ilill Smu/stones. —The position of these rocks as the true base of the Upper Silurian formations was ﬁrst shown in 1853 by Scdgwiek, who named them the May Hill Sandstones from the locality in Gloucestershire where they are so well displayed. Appearing on the coast of Pembrokeshire at M-arloes Bay, they range across South 'ales until they are overlapped by the Old Red Sandstone. They emerge again in C‘armar- thenshire, and trend north—eastward as a narrow strip at the base of the Upper Silurian series, from a few feet to 1000 feet or more in thickness, as far as the Longmynd, where as a marked conglomerate wrapping round that ancient Cambrian ridge they disappear. In the course of this long tract they pass successively and unconformably over Lower Llandovcry, Caradoc, Llandeilo, and Cambrian rocks. They consist of yellow and brown ferruginous sand- stones, often full of shells, which are apt to weather o11t and leave casts. Their lower parts are commonly con- glomeratic, the pebbles being largely derived from older parts of the Silurian formations. Here and there, where the organic remains become extraordinarily abundant, the strata pass into a kind of sandy limestone, known as the “ Pentamerus limestone,” from the numbers of this braehiopod contained in it. The species of fossils found in the May Hill Sandstones number about 230. Among these are some traces of fucoids: sponges ((.'h't.nu, Ischadilcs); the widely diffused (.'7'c1plul1'th1zs prioclon; a number of corals (Patrizia, IIcli0lz'trs, Fa'cosz'lz's, IIaI_:/s1'tcs, S'_:/n'11_q(»p0i'(t, &c.); a few erinoids; some annelides, particularly the T (7ztaculz'Ir.s- (I.n_r/I1'cus', which is abundant; a number of trilobites, of which I’lzrzc0ps Stnkcsii, P. IVcarcri, Encrinums pzuzctatus, and Ca.lymcm'1>’ltmzc71- bacltii are common ; numerous braehiopods, as A try]/"a hc172z's]rIz.crz'm, A. rcticularis, Pc7z(a.mr'r-us oblongus, .5'tricIclamlim'a lirata (S. ltus also occurs), Lcptama Iransrcrsalis, Orlhis callz'gra7mna, 0. elegan- tula, 0. rcvcrsa, Stropho77zc7za comprcssa, S. pcctrn, and Ijugzrluv parallcla; lamellibranchs ofthc mytiloid genera Orlhonola, .111/tz'Iu.s', and ]l[otlz'olopsz's, with forms of 1’tcrinca, (llcnoclowzln, and Lyra- dcsma; gasteropods, particularly the genera 1|[urclu'.<'onz'n, l’Iruru- lomaria, C3/cloncma, Ilolopclla; and cephalopods, chiefly (Irma- ccmta, with some forms of At-tivzoccrus and ]'hmgmom'as, and the old species Lituitcs cornu.-arictis. ((2.) Turannon S/zalc.—Above the Upper Llandovcry beds comes a very persistent zone of ﬁne, smooth, light grey or blue slates, which has been traced down the whole length of Wales from the mouth of the Conway into Carmarthcn- shire. These rocks, termed the “ paste-rock” by Sedgwick, have an extreme thickness of 1000 to 1500 feet. Barren in organic remains, their chief interest lies in the fact that 1 Siluria, p. 174. 9 Physical Geology qf Britain, p. 91.