Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/349

Rh sILURIAN.] the persistence of so thick a band of rock between what were supposed to be continuous and conformable formations should have been unrecognized until it was proved by the detailed mapping of the Geological Survey. 2. Wenloc/.' (r'roup.—(a.) Wool/Lope Limeslone.—In the original typical Upper Silurian tract of Shropshire and the adjacent counties, the Upper Llandovery rocks are overlaid by a local group of grey shales containing nodular lime- stone which here and there swells out into beds having an aggregate thickness of 30 or 40 feet. These strata are well displayed in the picturesque valley of Woolhope in Here- fordshire, which lies upon a worn quaquaversal dome of Upper Silurian strata rising in the midst of the surrounding Old Red Sandstone. They are seen likewise to the iiortli— west at Presteign, Nash Scar, and Old lladnor in lladnor— shire, and to the east and south in the Malvern Hills (where they include a great thickness of shale below the limestone), and .[-ay Hill in G loucestersliire. These strata have yielded many characteristically Upper Silurian fossils, among which may be mentioned Jiumustus ]J’cu'rz'ensz's, Ilomalonotus del- 1:/zinoccp/cctl us, 1’/mcops cauclatus, Atrypa 7'etZcularis, Ort/Lis «'allz'{/runznuz, ;S'trop/zomcna imbreac, It’/13/nc/Lonella borealis, 1.’. Wo'lsonL', E’ 160211.]?/t((fZtS sczclptus, Or!/toceras cumulatum. It is a characteristic of the older Pal-aeozoic liinestones to occur in a very lcnticular form, swelling in some places to a great thickness and rapidly dying out, to reappear again perhaps some miles away with increased proportions. 'l‘his local character is well exhibited by the Voolhope limestone. Where it dies out, the shales underneath and intercalzited with it join on continuously to the overlying Wenlock shale, and no line for the Woolhope group can tlieu be satisfactorily drawn. The same discontinuity is strikingly traceable in the Wenlock limestone to be iminedi- ately referred to. (/I.) ll'enlut-7; Slzale. This is a group of grey and black line shales, traceable from the banks of the Severn near Coal- brook Dale across Radnorshire to near Carinartlien—a dis- tance of about 90 miles. The same strata reappear in the protrusions of Upper Silurian rock which rise out of the Old lied Sandstone plains of Gloucestershire, Herefordshire, and Bloninouthshire. In the Malvern Hills they were estimated by Professor Phillips to reach a thickness of 640 feet, but towards the north they thicken out to 1000 or even H00 feet. On the whole the fossils are identical with those of the overlyinglimestone. The corals, however, so abundant in that rock are here comparatively rare. The brachiopods (of the genera Lc[»z‘a>na, 02'!/zis, b'trop/zomcmt, _ll)‘_i/1111, and If/z_2/nclconella) are generally of small size- Urt/zis bilubu, 0. /1 _I/britlu, and the large ﬁat 0. rustica, being characteristic. Of the higher mollusca thin-shelled forms of Or!/zoccras are specially abundant. Among the trilobites, /.'n«'rz'mIrus pmu'ta.f1(s, 'ca7'iolcu'z's, C’al_2/mane BlumenbacIu'1', C’. iubuwzlosa, I’/mcops ccmdatus, and P. lon_r/icaudat-us are common. The G'ru):tolz'z‘lzus prioclon, so frequent among the Bala beds of the Lower Silurian series, also occurs in the “'enlock shale. (.'rcI.plolil7¢us Flcm2'n_r/ii is here a. characteristic species. ll'cn.IucIc Limcslunc is a thick—bedded, sometimes ﬂaggy, usually more or less concretionary limestone, grey or pale pink, often highly crystalline, occurring in some places as a single massive bed, in others as two or more strata separated by grey shales, the whole forming a thick- ness of rock ranging from 100 to 300 feet. As its name denotes, this stratum is typically developed along Wenlock Edge in Shropshire_. where it runs as a prominent ridge for fully 20 miles, also between Aymestry and Ludlow. It like- wise appears at the detached areas of Upper Silurian strata above referred to, being specially well seen near Dudley (whence it is often spoken of as the Dudley limestone), Woolhope, Malvern, May Hill, and U51: in Monmouthshire. GEOLOGY 335 A distinguishing characteristic of the 'enlock limestone is the abundance and variety of its corals, of which 53 species have been described. The rock seems indeed to have been formed in part by massive sheets and bunches of coral. Ainono cli-ai'actei'istic species are 11al_:/sites catenularia, Ilcliolites ‘£72103 stinctus, II. tubulatus, Altcolites Labcchci, Iv'cwositcs aspcm, F. ﬁbrosa, F. Golltla7wli«:u, (,'a3m'tcs jzuu'p¢"rz'nus, Syringopom fascz'cu- laris, and Omphyma tm-bhmtum. The crinoids are also specially abundant, and are often beautifully preserved: I’c1'iecIzocrz'7ms, monil§fo7'mz's is one of the most frequent species; others are C7'otalo- crz'nu.s 7'u_r/osus, C'_:/allmcrinus go7Lz'odact_:/lus, and jllarsupzhcriuus ccvlutus ; with several cystideans, as I’.s‘cu(loc'rim'tcs quacl7'z]'asciatus. The crustaceans include numerous trilobites, among which we miss some of the persistent Lower Silurian genera, such as Asaphus, 0;/ygia, and T rizmclcus, none of which ascend into the Wen- lock group. The most abundant trilobitc is the loiig-lived Caly- mcnc Blmncnbrcc/n'L', which ranges from the Llandcilo ﬂags up to near the top of thc I_Jppcr Silurian formations. It occurs abundantly at Dudley, where it received the name of the “ Dudley Locust." Other common forms are 1i'ncrinzn'us punctutus, E. e:arz'ola7'z's, I’hacops caudatus, 1’. Dozcnz'7igz'w, 1’. Slolrcsii, Bumastus Barricnsis, IIomalo- notus dclphinoccphalus, and C-'kcirurus lu'mucronatus. The brachio- pods continue to be abundant; among typical species may be noted Atrg/pa 7'ctz'cularz's, Jllcristclla tzmzida, .S'pz'r2:/‘tr eleratus, S. plwatcllus, Iilzynclwnella boraalis (very common), 12. cwncata, R. II'ilsom', Orthis clcgantula, 0. rztstica, Strophonzcna 7-hoznboiclalis, and Pcnl(mm'us galcatzts. The laincllibraiichs are not well repre- seiitcd; but several species of Ptcrinca are abundant, with Gram- mysia cingulata, and some species of 1|[odz'olopsz's and Ctcnodonta. The gasteropods are most characteristically marked by 8 or 9 species of Euomplzalus, 3 or more of Jllm-chisom'a, with species of I’lcu7'oloma7'2'a, Acroculia, and Cyclozicma. The ceplialopods are confined to few genera, Lz'tuz'tcs, Actinoccras, Cyrtoccms, Ortlioccras, and P/zragmoccrzw ; of these the orthoccratites are by far the most abundant both in species and individuals. Urthoceras amiulalum is the most common form. The ptcropods appear in the beautiful and very abundant Conularia Sozccrbyi, and the hcteropods in the common and characteristic Ijcllcrophon IVc7zlockcnsz's. 3. Ludlow Gro2zp.—This series of strata consists essen- tially of shales, with occasionally a calcareous band in the middle. It graduates downward into the Wenlock group, so that when the Wenlock limestone disappears the Venlock and Ludlow shales form one continuous argillaceous formation. It is in this united form that the two groups stretch to the south—west through Brecon and Carniarthen. The Ludlow rocks are typically seen between Ludlow and Ayniestry. They appear likewise at the detached Silurian areas from Dudley to the mouth of the Severn. They were grouped by Murchison into three ZOIIBS. (a.) Lower Ludlow Reels. This is a group of soft dark- grey to pale greenish-brown or olive sandy shales, often with calcareous concretions. Much of the rock, however, presents so little ﬁssile structure as to get the name of mudstone, weathering out into concretions which fall to angular frag- ments as the rock crumbles down. It becomes more sandy and ﬂaggy towards the top. From the softness of the shales this zone of rock has been extensively denuded, and the Wenlock limestone rises up boldly from under it. An abundant suite of fossils has been yielded by these slialcs. No fewer than 18 species of star-ﬁshes, belonging to 6 genera, have been described (Protastc-r, like the brittle-stars of the British seas, Palwocoma, I’alastc7-2'-na). A few graptolites occur, particularly the persistent G7'(I11tol2'tIms priodon (common), 0'. colmms, and G. Flcmz'7zgz'z'. A few of the Wcnlock corals survive in the Lower Ludlow rock, but the ccnditioiis of deposit were evidently un- favourable for their growth. The trilobites are less numerous than in older beds; they include the venerable Cu.I_z/mcnc ]Ilu1nc7zL(zcIz2'i, Plzacops caudatus, and its still longer-tailed variety I’. longi- caudatus; also Acidaspis L’7'z'gILti17, Homalonotus clclphmoccplzalus, and CypIeaspz's 7ncgaIo;'-s. But other forms of crustacean life occur in some number. As the trilobites bccrin to wane numerous pliyllopods appear, the gcinis C'c7'atz'ocarz's being represented by 10 or mo1'e species. Large eurypterids now make their entrance upon geological history — Eur;/ptcrus, I’tcr_1/gotus, and Hc12zz'(zs])_is. Though bracliiopods are not scarce, hardly any seem to be peculiar to the Lower Ludlow rock, the Lingula lata-, which Murcliiso_ii suggested might be peculiar, having been obtained ‘from what is supposed to be representative of this group of strata in Westmorc- land. Rlzynclzonclla Wilsoni, Spirifcr c.rpor_7'c_ctus, S(roplzo_7ncna euglyplza, Atrypa 7-c(z'cular2's, and C/cauctcs mmuna are not nifre-