Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/361

Rh l——»_ ['.'LRBO.IIFEI‘.OUS.] singularly barre.i of org-aiiic rein=_iins. They seem to have been laid down with great slowness, and without di:’sturbance, in enclosed basins, which were not well ﬁtted for the support of animal life, though fragmentary plants serve to show that the adjoining slopes were covered with vegetation. In the basin of the l"ii-th of Forth, however, the group presents - dilfcreiit litliologieal aspect and is abundantly fossilifcrous. lt .'h«-re usually coiisists of yellow, grey, and white sandstones, with blue and hla--k sliiles, clay-iroiistoiies, limestoncs, “ cciiiciit-stoiies," and oewasional seams of coal. The sandstones forin excellent building stone.-:, the city of lﬂdiiiburgli having been luiilt of them. .‘.»nie of the shalesare so hitiiiiiiiioiis as to yield, on distillation, from Iii) to 40 gallons of crude pe,ti-olcuni to the ton of shale; they are I'UllS(‘tlllt-llll_Y largely worked for the inanufaeture ot' mineral oils. 'l'lie liin--stones are usually dull, yellow, and close grained, in seams s--luloin more than a few inches thick, and graduate by addition of carbonate of iron into ccniciit—stone; but 0H‘11SiOllalll_V they swell out into thick lenticular niasscs like the well—kiiowii limestone of liur-lie. llouse, .- 3 long noted for its reinark:iblc fossil fishes. This liinestoiie appe irs to he inainly made up of the crowded cases of a small ostraeol Cl'll~'.t:ll'C:lll (L--prrr.Ii'liw Ulce/ii, var. S«_'oto-L’m'di'ga.l- uni’. . The coil-se.nns are few and commonly too thin to be woikable, though one of them, known as the Houston coal, has heeii mined to some extent in Liiilitligowsliire. The fossils of the cciiieiit-stoiic. group indicate an alternation of fresh or hrackisli- w-;iter an-l inarine conditions. They include numerous plants, of which the most abiiii laiit are .‘pIzeiinptci'i's (Iﬂiuis, Lcpi'Ilo(lcndron i’ two or three species), Lt’/u'zlusti-ubils 'r¢u'z'(rl:1'l1's, .1rrmc(Irz'0.i:yI0:z. Seine. of the Sll:llI'.~‘- near l'I«liiihurgh have atforded a few specimens of a true inonoeotylclon allied to the modern Pothos (1’otIioci'z‘r;s h'ruiiln21i'). s'tl'JL Oll crustaceans, chiefly the I.rpr;)'(Zz'£z'a above men- ti-iin-d, crowd inzniy of the slialcs. 'ith these are usually as- s.) -late-l abuiialznit tiaecs of the presence of fish, either in the form --feoprolites or of sc.ile;:, bones, plates, and teeth. The following are ('li:11‘:11 teristie species: I3’/un2'eIzth3/S slriolutus, E. 11012130721", 1:]: 'cZ[:i[cl(’li_i/s‘ orn //T.s‘.s'i'/iziis, .'uizatopl_i/cIu'us (lrcciiockii, Ifuryiiolus er- iirtlus, I.’/u':'nrlus 1[!7h"")‘fI', .l[r_r/«I13.-Iitlzys sp., 03/I'(IC(l)lf]ll(.S‘ tuber- eulrz/rI~', ('l:.z~,’_:/-Iu'r19 gm-tinrllus. At intervals throughout the group ni.iriiie lioi'i7,oii-r. on -ur, usually as shale bands marked by the presence. of such distinctively Carboniferous Limestone species as .',i'rorlu's cm-Iioiir./7'i'i(.¢, Iﬁsciiia oziticla, Lingula sr1i1(u/uform is, 1'.’ llcrop/10.1,tlcrirsswtics, and Orllioccras cyﬁiulracczuiz-. One of the most interc’-‘ting features in the eemeiit-stoiie group of the lJ:1.<lIl of the Firth of Forth is the prodigious nuni'ocr and variety of the associated volcanic masses, and the proofs which they exhibit that, at the time when that group of strata was accumulating, the region of shallow lagoons, islets, and coal-growths was dotted over with iii- iiunierablc active volcanic vents. The eruptions continued into the time of the C‘-.irboniferoiis Limestone, but ceased before the deposition of the Millstone Grit. The lavas are clii-cﬂy varieties of basalt-rocks, sometimes coarsely crystal- line and even gr-anitoid in texture, and graduating through niterinediate stages t.) true close-grained compact basalts, which neither externally nor in microscopic structure differ from basalt of Tertiary date The basement group of the Carboniferous system in Ireland is evidently a prolongation of "the Scottish cement- stone group. In the south of this island, however, a very distinct and peculiar development of the Lower Carboni- ferous rocks is to be l‘Cl11'.1l“l{Ctl. Between the top of the Old ltcd Sandstone and the base of the Carboniferous Lime- stone there occurs in the county of Cork an enormous mass (fully 5000 feet) of black and dark—grey shales, impure limestones, and grey and green grits and true cleaved slates. 7 - o . . [0 these rocks the name of Carboniferous Slate was given by Griffith. They contain iiuinerous Carboniferous Linic- stone species of bracliiopods, ecliinoderms, &c., as well as traces of laiid-plants in the grit bands. Great though their tlnckness is in Cork, they rapidly change their litliological character, and diminish in mass as they are traced away from that district. In the almost incredibly short space of lo miles, the whole of the 5000 feet of Carboniferous Slate of Baiitiy Bay have disappeared, and at Kcninare the Old Red Sandstone is followed immediately and con- formably by the Limestone with its underlying shale. Mr GEOLOGY 347 J ukes held that the Carboniferous Slate is the equivalent of part of the Devonian rocks of Devon and Cornwall. (,'m'boizz:/'erous Linzes-tone.—'1'lie Lower Limestone shale is overlaid coiiforinably by a thick mass of limestone, one of the most distinctive members of the British Carboniferous system. On referring to a geological map of England it will be seen that from N ortliumberland southwards to the low plains in the centre of England there runs a ridge of high ground, foi'iiied by a great aiiticliiie, along which the Cd.l'lZ)UIllfUl‘l1S Limestone appears at intervals from under- iieatli higher members of the system. In this northern Carboniferous area, of which the axis is known as the Pcnnine Chain, the limestone attains its maximum develop- ment. Iii one portion of the district it reaches a depth of 4000 feet, and yet its actual base is nowhere seen. This Pcnnine region appears to have been the area of maximum depression during the early part of the Carboniferous period in Britain. Traced towards the south west, the limestone diminishes to sometimes not more than 500 feet in South Wales. 1'orthwards, losing its character as a massive cal- careous formation, it is split up by iiitercalations of sand- stone, shale, coal, &c., until actual limestone becomes a very subordinate member of the series in central Scotland. In the Carboniferous areas of the south-west of England and South ‘Vales, the limits of the Carboniferous Limestone are well deﬂiied by the Limestone Shale below, and by the Farewell Rock or Millstone Grit above. In the Penninc area, however, the massive limestone is succeeded by a series of shales, liniestones, and sandstones, known as the Yoredale group. These cover a large area and attain a great tliick- ness. Iii North Staﬁ'ordshirc they are 2300 feet, which, added to the 4000 feet of limestone below, gives a depth of 6300 feet for the whole Carboniferous Limestone series of that region. In Laiicashire the Yoredale rocks attain still more stupendous dimensions, Mr Hull having found them to be no less than 4500 feet thick. Both the lower or main (Scaur) limestone and the Yoredale group pass northwards into sandstones and shales, with coal-seams, and diminish in thickness. It is not impossible, as already suggested, that the cement-stone group of the Calciferous Sandstones of Scotland may represent, not only the thin Lower Limestone shale, but also some of the older parts of the English Carboniferous Limestone. Where typically developed, the Carboniferous Limestone is a massive well—bedded limestone, chiefly light l)lul.‘-‘ll: grey in colour, varying from a compact honiogeneous to a distinctly crystalline texture, and rising into ranges of hills, whence its original name “ Mountain Limestone.” It con- tains occasional scattered irregular nodules aiid nodular beds of dark cliert. It is abundantly fossiliferoiis. The fossils commonly stand out on weathered surfaces of the rock, but microscopic investigation shows that even those portions of the mass which appear most structureless consist of the crowded remains of marine organisms. The limestone may be regarded as derived entirely from the organic debris of a sea-ﬂoor. Diversities of colour and lithological character occur, whereby the bedding of the thick calcareous mass can be distinctly seen. Here and there a more marked crystal- line structure has been superindueed ,' but the most distinct examples of met-ainorphic change are those where the rock has been converted into what is termed “ diiii-stone. ’ This alteration is frequent in the mining districts of Yorkshire and the neighbouring counties. It consists in thc doloinitiz- ation of the rock along either the lines of bedding or the joints. Thus changed, it becomes a yellowish or brown crystalline dolomite, which runs vertically through the mass of the limestone along some definite joint, in courses of 20 or 30 fatlionis in width. Such a inetamorphisri iiinst have been effected by water percolating along the line of the joint, and affecting the rock for some distance on either side. In