Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/346

Rh 332 78 species of hydrozoa have been obtained from the British Arenig rocks, but none from any older strata. This sudden and great development of these organisms gives a distinctive aspect to the Arenig rocks. It con- tinues in the overlying Llandeilo group, so that the graptolites form in Britain a convenient character by which to mark off the (,‘-anibrian from the Lower Silurian fauna. 2. Llandrilo Flag Groz(p.—Dark argillaceous ﬂags-tones, sandstones, and shales, some parts often calcareous. These beds were first described by Murchison as occurring at Llandeilo, in Carmarthenshire. They reappear on the coast of Pembrokeshire, and at Builth, in Radnorshire. Up to the present time they have yielded 227 species of fossils. Of these 13 are common to the Arenig below, 82 to the Caradoc or Bala above, while 145 are peculiar. The hydrozoa are still the most abundant forms, 94 species being here met with, no fewer than 81 of these being confined to Llandeilo rocks, and only 9 passing down into the Arenig group. Of crustacea 44 species have been obtained. These include the characteristic trilobites —Amp_yJc mulus, .ls.r([;Ims t_z/rcumus, Barrandea Cordai, C’al_z/mene rlztplicatcl, C. C’ambrensz's, Cheimrus SecI_«]wicIt'z'i, 0_r/_2/_r/ia Buclzii, Trimccleus concentricus, T. Lloydii. The brachiopods number 37 species, including the genera Orthis, Leptorna, Strophomena, Lingula, Siphonotreta. The lamellibranchs are represented by 6 species, the gasteropods by 10 (.l[:u-clzisozzia, 0'3/clonema, Lo.ronema.), the heteropods by 7 (lklleroplzon), the pteropods by 2 (('onularz'a, T/ceca), the cephalopods by S (0I't/l0C€7‘(1S, Cyrtoceras). A remarkable feature in the history of the Llandeilo rocks in Britain was the outbreak of volcanic action abundantly in North Wales and in Cumberland. Vast piles of lava and ashes were thrown out, which even to this day remain in mass suﬂicieut to form groups of important hills, as Cader Idris, Aran Mowddwy, the Arenigs, and the Moelwyns in Wales, and Helvellyn and Scaw Fell in Vestmoreland and Cumberland. 3. Cmwloc or Bala G'ro2zp.—Under this name are placed the thick yellowish and grey sandstones of Caer Caradoc in Shropshire, and the grey and dark slates, grits, and sandstones round Bala in lferionethshire. In the Shropshire area some of the rocks are so shelly as to become strongly calcareous. In the Bala district the strata contain two limestones separated by a sandy and slaty group of rocks 1400 feet thick. The lower or Bala limestone (25 feet thick) has been traced as a variable band over a large area in North Wales. It is usually identiﬁed with the Coniston limestone of the Vestmore- land region. The upper or Hirnant limestone (10 feet) is more local. Bands of volcanic tutf and large beds of various felsitic lavas occur among the Bala beds, and prove the contemporaneous ejection of volcanic products. These attain a thickness of several thousand feet in the Snowdon region. A large suite of fossils has been obtained from this formation :-—the sponges represented by Sp]z(E7'osp0n_gIia and other genera; the graptolites by Digzlograptzzs pristis, 0'raptolit/ms prioclon, and G’. Sedf/u'z'cl-z'2', &c.; the corals by species of Ileliolites, Favosites, Jfontzrulipora, 1l¢cl_2/- sites, Petraia; the echinoderms by encrinites of the genera C'_2/atlzocvinus and G'l_2/ptocrinus, by cystideans of the genera Ifclzinosphcerites and Sphceromles, and by star-ﬁshes of the genera Prllreaster and Stenaster ; the annelides by Serpulitrs, Tentaculites, and numerous burrows and tracks; the trilo- bites by many species of the genera I’/mcops, C’/teirzu-us, C'_I/bele, Lie/ms, .lrz'(Ia.s-pis, Cal_1/7m=na=, I.’emopleur2'r.les, Asa- phus, Illrenus, .imp_z/.r, and Trinucleus; the polyzoa by Fenestella, Glcmrozzrmze, and 1’lz'lodz'ct_uu.-. the brachiopods GEOLOGY [v1. STRATIGRAPIIICAL. by .~ftr_z/pa, Ii’/z._z/m-/tomllrl, Imptwrza, Ortlzis (many species), Strophomena, I)z'scz'na, and Lz'n_r/ulu ; the lamellibranchs by .l[odz'olops[s, JI_z/tilus, 1’«(14t’(Irr'((, Plerinea, Orllzonota, and Ctenodonta; the gasteropods by .l[urrIzison2'a, Pleura- tomaria, It’a1:histo7n((, C_:/clonema, 11'Il07lI})]l.((fI(8, .ll:u-Iurm, Ilolupea ; the pteropods by Cmmluria, Tlzecu, and ]:'c«-u/z'- omplzalus; the heteropods by various species of ];’([ft’)'n]r](0)( and the cephalopods by many species of 0)‘!/mc¢'ras, with forms of C_7/rtoceras and 1.z'tm'tes. 4. Lower Llamlover_z/ Group.——ln North Wales the Bala beds about 5 miles SE. of l-J‘-ala Lake begin to be covered with grey grits, which gradually expand southwards until they attain a thickness of 1000 feet in South Wales. These overlying rocks are well displayed near the town of Llan- dovery, where they contain some conglomerate b-.1nds_. and where M r Aveline detected an unconformability between them and the Bala group below them, so that the subter- ranean movements had already begun, which in Wales marked the close of the Lower Silurian period. Else- where they seem to graduate downwards conformably into that group. They cover a considerable breadth of country in Cardigan and Carmarthenshire, owing to the numerous undulations into which they have been thrown. Their chief interest lies in the transition which they present between the fauna of the Lower and Upper Silurian for- mations. They have yielded in all about 128 species of fossils, whereof 11 are peculiar, 93 are common to the Bala group below, and 83 pass up into Upper Llandovery rocks above. Some of the peculiar fossils are -'z'«Iu/ilcs fcwus, Jleristella crassa, Jf. °m_r/ust1_'fro7zs, and .l[urr/'u'.sum'r.t an_r/ulatu. Among the forms which come up from the Bala group and disappear here are the corals 1l«.=l[olitcs interstinrtus, Petmia s-ubr_l2(plz'cata, and Faro.»-itcs uspera; the trilobites Lichas laaratus and 1”(t’7l?(S Bozvmamzi ; the brachiopods Orthis Acfonice and 0. insularis ; the gastero- pods Jfurclzisonia _r/3/rogonia and Cyclone7mt c2'ebn'slrz'«'1; and the cephalopod Ortlzoceras temzz'cz°nctu-m. But many of the Lower Silurian forms continue on into the l'pper Llandovery beds. From the abundance of the peculiar brachiopods termed Pe7lt((7ne2"us in the Lower, but still more in the Upper Llandovery rocks, these strata were formerly grouped together under the name of “I’entamerus beds.” Though the same species are found in both divi- sions, Pentamerus oblongus is chiefly characteristic of the upper group and comparatively infrequent in the lower, while ;S'tricIclandim'a (Pentamerus) lens abounds in the lower but appears more sparingly in the upper. The Lower Silurian rocks, typically developed in Val<-s, extend over nearly the whole of Britain, though largely buried 1llltl('1' more recent formations. They rise into the hilly tracts of 'cstmoreland aml Cumberland, where they consist of the following subdivisions in descending order :- (Lower Llandovery not represented.) Coniston Limestone and Shale  = Volcanic series: tuffs and lavas without any intermixture of ordinary sedimentary strata ex- cept at the base, 12,000 ft ..... .. _ _ Skiddaw Slates, 10,000 or 12,000 ft. 2 3 “‘.f.‘I§:_§mJ 1...,.1»;p3 base not seen .................... .. Lingula 1,1 ags_ Apart from the massive intercalation of volcanic rocks these strata present considerable lithological and palzeontological ditl«-r- ences from the typical subdivisions in 'ales. The Skiddaw slates are black or dark-grey argillaceous, and in some beds san-ly rocks, often much cleaved though seldom yielding workable slates, some- times soft and black like Carboniferous shale. As a. rule they are singularly unfossiliferous, but in some of their less cleaved and altered portions they have yielded about 40 species of graptohtcs (chiefly of the genera D2'(lyn1o_qr(Iptus, l)i'pIo_(/raplus, IJ2'rImgr(r}vI2ts, T(‘l7'(1_f])'(1])ll(S, I’/z_1/llogmptus, and CIz'mnco_I/'r(q:tus) Lz'1:_r]uIa Inrvis, traces of annelides, a few trilobites (.-EgIz'na, /1_r]nostus, 4-f.lI]I]lItS, &c.), some phyllopods (C'aryocm'is), and remains of plants (1)’u1I1o- trcphis, &c.). In many places the slates have been m<-tumorphosed, passing into chiastolitc-slate, mica-schist, amlalusite-schist, &c., Bala beds. Part of llala, whole of Llandeilo, and perhaps part of Arenig formation.