Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/350

Rh 336 GEOLOGY [v1. STRATIGRAPIIICAL. qucnt. Among the more frequently recurring species of lamclli- [ st;-“ta 31,0;-9 the original 1,0ne_],(..13_t Ludlow, togethcx-with branchs the following may be named——t'urdioIu iutcrrupta, L’. s.'m'afa, Orthonofu. rigidu, 0. SL'I)(fS(lfL'(l[(l, and a munber of species of P(¢'I’i:‘le(l. The orthoceratitcs are nmnerous, as Urllwccras Ludensc, U. submululuhun, also species of 1']: ragmocnras and Liluitcs. The numbers of these straight. and curved eephalopods form one of the distinguishing features of the zone. At one locality, near I.eint- wardine in Shropshire, which has been proliﬁc in Lower Ludlow fossils, particularly in star-fishes and curypterid crustaceans, a fragment of the fish 1’teraspi.s was discovered in 1859. This is the earliest trace of vertebrate life yet detected. It is interesting to note that the 1’tcra.s;vis does not stand low in the scale of organiza- tion, but has allinities with our modern stm'gcon. (1).) .13/1m=.s-fry Limestone is a dark grey somewhat earthy concretionary limestone in beds from 1 to 5 feet thick. Where at its thickest it forms a conspicuous feature, rising above the soft and denuded Lower Ludlow shales and, owing to the easily removable nature of some fuller’s earth on which it lies, it has here and there been dislocated by large landslips. It is still more inconstant than the Wen lock limestone. Though well developed at Aymestry it soon dies away into bands of calcareous nodules, which ﬁnally disappear, and the lower and upper divisions of the Ludlow group then come together. The most characteristic fossil is the Penlamerus I{ni_r,Ihtiz'; other common forms are I?/L_z/nclwnella ll'2'lsom', Lin;/ula Lewisii, Stroplwmena euglg/p/ta, Belleroplwn tlilatatus, I ’terz'nea. Sozve7'b_z/3, with many of the same shells, corals, and trilobites found in the Venlock limestone. Indeed, as Murchison has pointed out, except in the less number of species and the occurrence of some of the shells more characteristic of the Upper Ludlow zone, there is not much palzeontological distinction between the two limestones.‘ (41) Upp-=r Ludlow I€ock.—In the original Silurian district described by Murchison, the Aymestry limestone is covered by a calcareous shelly band full of I-.’lL_;/nclzonella mu-icezlu, sometimes 30 or 40 feet thick. This layer is succeeded by grey sandy shale or mudstone, often weathering into con- cretions, as in the Lower Ludlow zone, and assuming ex- ternally the same rusty-brown or greyish olive-green hue. Its harder beds are quarried for building stone; but the general character of the deposit, like that of the argillaceous portions of the Upper Silurian formations as a whole in the typical district of Siluria, is soft, incoherent, and crumb- ling, easily decomposing once more into the original mud, and presenting in this respect a contrast to the hard ﬁssile and often slaty sl1ales of the Lower Silurian series. Many of the sandstone beds are crowded with ripplemarks, rill- marks, and annelicl—trails, indicative of the shallow littoral waters in which they were deposited. One of the upper- most sandstones is termed the “ F ucoid Bed,” from the nmnber of its cylindrical sea-weed—like stems. It like- wise contains numerous inverted pyramidal bodies, which are believed to be casts of the cavities made in the muddy sand by the rotatory movement of crinoids rooted and half- buried in the micaceous mud? At the top of the Upper Ludlow rock near the town of Ludlow, a brown layer occurs from a quarter of an inch to 3 or 4 inches in thick- ness, full of fragments of ﬁsh, Pterg/golus, and shells. This layer, termed the “ Ludlow Bone-bed,” is the oldest from which any considerable number of vertebrate remains has been obtained. In spite of its insigniﬁcant thickness it has been detected at numerous localities from Ludlow as far as Pyrton passage, at the mouth of the Severn—a distance of 45 miles from north to south, and from Kington to Ledbury and Malvern —-a distance of nearly 30 miles from west to east ; so that it probably covers an area (now largely buried under Old Red Sandstone) not less than 1000 square miles in extent, yet it appears never to exceed and usually to fall short of a thickness of 1 foot. Fish remains, however, are not conﬁned to this horizon. They have been detected in 1 Siluria, p. 130. '-’ Siluria, p. 133. some minute globular bodies believed to be the sporangia of a lycopod. These, with some other plant remains from the satire district, are the earliest traces of land vegetation yet found. The higher parts of the Ludlow rock consist of ﬁne yellow sandstone and harder grits known as the Downton sandstone. Originally the whole of these tlaggy upper parts of the Ludlow group were called “ Tilestones ” by .lurchison, and being often red in colour were included by him as the base of the Old lied Sandstone, into which they gradually and conformably ascend. bndoubtedly they show the gradual change of physical conditions which took place at the close of the Silurian period in the west of England, and brought in the deposits of the Old lied Sand- stone. But as their organic contents are still unequivocally those of the Ludlow group, they are now classed as the uppermost zone of the Silurian system. A considerable suite of organic remains has been obtained from the Upper Ludlow rock, which on the whole are the same as those in the zones underneath. Vegetable remains, some of which seem to be fucoids, but most of which are probably ter- restrial and lycopodiaccous, abound in the Donnton sand- stone and passage-beds into the Old Red Sandstone. Corals, as might be supposed from the muddy character of the de- posit, seldom occur, though Murchison mentions that thc en- crusting form _4lrcolilcs _/ibrosus may not inl'requently be found enveloping shells, Cyclcrncnza corullii and 1l[u.7'clr1'so:u'a corullii being, as their names ilnply, it.s favourite habitats. Some anne- lides (scrpztlitcs longz'spz'nus, Cor-nulites smj)-uI(17'z'us, Tcntaculilcs tennis, and Trachydcrma coriacclt) are not uncommon. The crus- tacea are represented chiefly by small ost.racods (1)'cy7'1Ichia Kltuleui, Lepe7'clit1'¢t onarginata, E-ntomis tubcrosw), and by species of (.'c-rul[c- cm'is, I)2'cl_y0ca-ris, Euryplcr-us, Ilcmiuspis, I’tc7'ygotus, and Mylo- mtrus; the trilobites having still further waned, though 1/: ma- lonotus I{mIght2'i, Encrirmrus pmzclatus, I’hacops .Do2mu'7r_I/1'u-, and a few others still occur, and even the persistent C'alg/mcnc 1>’lu-mcn- bachii may occasionally be found. Of the bmchiopods the most abundant forms are I-.’hyneho7zclla 71 ucu Ia, Ch0m'lc's slriatclla, _l)iscina 7-ugata, and Li-ngula cornea. The most characteristic la1nelli- branchs are Ortlzonota amygdalina, G'0'ni0];h07'(L cy7nbu_/'ormz's, Ptc'rz'7zca lincata, P. retroﬂcara; some of t.hc commonest gastcropods are lfurchisonia corallii, 1-’latysc}1is'n2a Irclicitcs, a11d Ilolupcllu. obsoleta. The orthoceratit.cs are specifzcally identical with those of the Lower Ludlow rock, and are sometimes of large size, 7'lltoecru.s- bullatmn being specially abundant. The ﬁsh remains consist (f bones, teeth, shagrcen-like scales, plates, and fin-spines. They include some plagiostomous (placoid) forms (flzcloclus, shagreen- scales, Sphagodus, skin, Unchus, spines) and some ostiacostcans (C'cphal(Lsp2's, Auchcnasjﬁs, and Ptcruspzis-). In the typical Silurian region of Shropshire and the adjacent counties, nothing can be more decided than the lithological evidence for the gradual disappearance of the Silurian sea, with its crowds of graptolites, trilobites, and brachiopods, and for the gradual introduction of those geographical conditions which brought about the deposit of the Old Red Sandstone. The ﬁne grey and olive—colourcd muds, with their occasional zones of limestone, are suc- ceeded by bright red clays, sandstones, cornstones, and con- glomerates. The evidence from fossils is equally explicit. Up to the top of the Ludlow rocks the abundant Silurian fauna continues in hardly diminished numbers. But as soon as the red strata begin the organic remains rapidly die out, until at last only the ﬁsh and the large eurypterid crus- taceans continue to occur. Turning now from the interesting and extremely import- ant though limited area in which the original type of the Upper Silurian rocks is developed, we observe that whether we pass northwards or south-westwards the soft mudstoncs and thick limestones give way to hard slates, grits, and ﬂagstones, among which it is scarcely possible sometimes even to discriminate what represents the Wenlock from what may be the equivalent of the Ludlow group. It is in Denbighshire and the adjacent counties that this change becomes most marked. The Tarannon shale above de- scribed passes into that region of North Wales, where it forms the base of the Upper Silurian formations. It is covered