Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/363

CARBONII-‘EROUS.] The chert bands which form such marked horizons among these limestones are counterparts of others found abundantly in the Carboniferous Limestone of England and Scotland. They have been recently studied by Hull and Hardman, who have found them full of siliceous replace- ments of calcareous foraminifers, crinoids, &c., and who regard them as due to a chemical alteration on the floor of the Carboniferous sea. Portions of the limestone have a dolomitic character, and sometimes are oolitic. Great sheets of mclaphyre, felstone, and tutf, representing volcanic eruptions of contemporaneous date, are interpolated in the Carboniferous Limestone of Limerick and other parts of Ireland. As the limestone is traced northwards it shows a similar change to that which takes place in the north of England, becoming more and more split up with sandstone, shale, and coal—sea1ns, until, at Ballycastle, it presents exactly the characters of the coal—beariug part of the forma- tion in Scotland. .l[z'll.s-tone G’;-i(.— -This name is given to a group of sand- stones aml grits, with shales and clays, which runs persist- ently through the centre of the Carboniferous system from South Wales into the middle of Scotland. In South Wales it has a depth of 400 to 1000 feet; in the Bristol coal- field, of about 1200 feet. Traced northwards it is found to be intercalated with shales, fire—clays, and thin coals, and, like the lower members of the Carboniferous system, to swell out to enormous dimensions in the Pennine region. In North Staffordshirc, according to Mr Hull, it attains a thickness of 4000 feet, which in Lancashire increases to 5500 feet. These massive accumulations of sedinient were deposited on the north side of a barrier of Silurian and Cambrian rocks, which, during all the earlier p irt of the Carboniferous period, seems to have extended across central England, and which was 11ot submerged until pirt of the Coal-measures had been laid down. North of this gre.1t area of deposit the Millstone Grit thins away to mt more than 400 or 500 feet. It continues a compara- tively insignificant formation in Scotland, attaining its greatest thickness in Lanarkshire and Stirlingshire, where it is known as the Moor Rock. In Ayrshire it does not exist, unless its place be represented by a few beds of sandstone at the base of the Coal-measures. The Millstone Grit is generally barren of fossils. When they occur they are either plants like those in the coal- be:1ring strata above and below, or marine organisms of Carboniferous Limestone species. In Northumberland, indeed, it contains a band of limestone undistinguishable from some of those in the Yoredale group and Seaur lime- stone. C-'o((l—.l[eIlsu7'cs. This division of the Carboniferous sys- tem consists of numerous alternations of grey, white, yellow, sometimes reddish sandstone, dark-grey and black shales, clay-ironstones, fire-clays, and coal—scams. In South lVales it attains a n1a.'imun1 depth of about 12,000 feet; in the Bristol coal—field it is 5090 feet. But in these districts, as in the rest of the Carboniferous areas of Britain, we cannot be sure that all the Coal—1neasures originally deposited now remain, for they are always unconformably covered by later formations. In some localities, indeed, the denudation must have been very great, for the next overlying system of deposits (Permian) is found resting even on the Carboni- ferous Limestone. In North Staffordshire the depth of the Coal-measures is about 5000 feet, which in South Lancashire increases to 8000. These great masses of strata diminish as we trace them eastwards and north- wards. In Derbyshire they are about 2500 feet thick, in Northumberland and Durham about 2000 feet, and about the same thickness on the west side of the island in the 'hitehaven coal-field. In Scotland they attain a maximum of about 2000 feet. GEOLOGY The Coal-measures are susceptible of local subdivisions indicative of different and variable conditions of deposit. The following tables show the more important of these :——- GLA.I’.GA.'SllIP.E. Upper series, more than 5 Sandstoncs, shales, &c., with 26 coal- 3400 feet ........... .. ( seams. Pennant (irit, 3246 Hard thick-bedded sandstones &c., 15 feet .................. .. coal-scams. L ' . ' . 4’ 0 t . . 05;)-’%r l Shales, ironstones, and 34 coal-seams. Millstone Grit. SOUTH L.-T(‘-ASIIIRE. Upper, 1680 to 2000 3 Shales, Spirorbis limestone, ironstone, feet ................... .. sandstoile, and thin coal—seams. Middle 3000 to 4000 5A great series of strata, with numerous ’ " ( valuable coals. Lower or Gannistcr, Flags, shales, and three or four thin coals, 1400 to 2000 feet with floors of “ gam1istcr." Millstone Grit. CE.'TI:AL SCOTI...'D. _ ‘ Red sandstones and clays with occasional 150 feet and up“ ards" l thin coals and Spirorbiie limestone. Upwards of 2000 feet Sandstones, shales, and fire-clays, with in Lauarkshirc . ironstones and coals. lloor—rock or Millstone Grit. The numerous beds of compressed vegetation form the most remarkable feature of the Coal-measures. Each of them is usually underlaid by a seam of fire—clay, represent- ing the soil on which the plants grew. Hence they mark successive terrestrial surfaces, which, after enduring for a longer or shorter period, were carried down beneath the water and covered over with sand and silt. There is11o modern formation that affords a close parallel to that of the ancient coal—growtl1s. The nearest analogy is furnished by the mangrove swamps alluded to already on p. 290. These masses of arborescent vegetation grow out into the sea as a belt or fringe on low shores, and form a matted soil which adds to the breadth of the land. Their roots spread in the salt water where marine organisms are abundant. The coal-growths no doubt also flourished in salt or at least brackish water; forsuch shells as A viculopecten,Anthracosia, and Goiziatites are found lying on the coal or in the shales attached to it. The vegetation of the Carboniferous period must have been luxuriant and varied. Upwards of 500 species of plants have been obtained from the Carboniferous rocks of Britain, yet these must represent but asmall portion of the original flora. The most abundant forms were ferns and lycopods. Niiiiieroiis reed-like plants likewise played a conspicuous part in the general vegetation of the low grounds, while apparently on the drier and more elevated tracts (for their remains are less frequently met with) large coniferous trees flourished. The ferns strikingly resemble many modern forms. Among the more frequent genera were Sphmoptcris, Pccoptcr-is, Alrthoptcris, 1'cm'optcrz's, and Cycloptcris. The lycopodiaccous plants attained the size of forest trees. Among the more typical forms were many species of Lqddodnzclroaz, the stems and seed-cones of which are scattered through the Carboni- fcrous strata. Some of the coal-seams are largely composed of the spores of these plants. Szyillaria, rcgarded by some as allied to the cycads, by others as more akin to the lycopods, is represented by numerous species. The roots known as .S'tz'gmarz'a abound in most fire-clays, showing how largely the coal consists of trees having roots of this kind. The Eqztzlsctacca: show themselves in the form of numerous C'aIamitc._9 which, though resembling in general aspect our modern horse-tails, differed in many points of structure, and notably in their gigantic size. Coniferous trees occur chiefly in the form of stems and branches. They include the genera. Dado.r_1/Ion, ./Iraucarz'o.1'yIon, and Pinitcs. Small nut-like fruits ( Trigonocarjnmz) abound in some beds, and are now believed to be the fruits of ycw—like conifers. The presence of monocotyledonous plants is proved by the I’otlzon'tz's referred to above (p. 347). The animal remains of the Coal-measures are compara- tively few. In certain bands, particularly of ironstone, numerous mollusca occur,_and similar forms are to be occa-

I sionally met with iii the shales.