Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/362

Rh 348 Derbyshire sheets of contemporaneous lava, locally termed “to.idstone,” are interpolated in the Carboniferous Lime- stone. The fauna of the Carboniferous Limestone of England is abund- ant and chai'acteri.stie. .'nnierons foriiiiiiiiifei's occur, Saeeamina ("artcri_being often very characteristic. The corals are_iiiiiiiero1is, embracing upwards of 30 genera and about 100 species. 'lliese include both simple cup corals, as Zaplirenti'.s‘, (L'lisi'opliyllum, and compound forms, like Jlrcolitcs and I'lu‘l1i'psastra;a. Crinoids are iiidividually in enormous numbers, many beds of limestone appearing to consist of little else than their fragiiientar_v stems and cups; Aetinocrinus, Platyerinus, Ibtcriocri-nus, CyatIwcr1'nu.s, are common genera. Three species of sea-urchiiis are known. Of the trilobites, so characteristic of the _older l‘al-aeozoic rocks, the last liiigernig forms are here found in three small genera ——Brachyn_2etopus, Plzillipsia, and (_:'rzjitlu'dcs. l’o_lyzoa abound, some portions of the limestone being almost entirely composed of them, the genera Frnrstclla, Sulcorctq)ora., Vincularia, Poly- pora, Diastopora, and Glauconomc being frequent. The braeliio- pods comprise 18 genera and 160 species, seine of the most coni- iiioii forms being Prorluctus, Spz'.1'1_'fcr, .’h_i/nehoncIIa., Atliyris, C’/wnctcs, Orlhis, Lingula, and Jjisciiia. But the higher iiiol- liisks now begin to preponderate over the braehiopods. The laiiiellibraiiehs number 49 geiiera and 334_ species, including forms of Avzculopcetcn, Leda, Aucula, iSanguznolztcs, Leptodomus, Sc.-liizozlus, Eclmondia, J[odi'0la., and Conocardimn. The gasteropods amount to 206 species belonging to 29 genera, among w iicli I:'m)m- phalus, Nalica, I’lcurotomar1'a, Jllaerochcilus, and Lc.z'07wma. are frequent. _ The genus Bellerophon. is represented by 23 species, tiinong which B. Lrcz. andB. dccussatusarefrequent. The eeplialopods number 148 species, belonging among other genera to Orthoccras, GEO L 0 ti Y Nautilu,.s', Dzlscitcs, and (}'on.iatL'tcs. About 100 genera of fishes, - chieﬂy from teeth and spines, have been described, as Psammorl-us, -Cue-hlioclu,s, Cladodus, Pctalod us, Rhiwdus, Ctcnoptyclz.ius, &c. Some of these were no doubt plaeoids which lived solely in the sea, but many, if not all of the ganoids, probably migrated between salt and fresh water; at least their remains are found in Scotland in [vi. STRATIURAPHICAL. of country. Hence it is evident that during certain epochs of the Cai'boniferous period a singular uiiiforinity of coii- ditions prevailed over a large region of deposit in the centre of Scotland. The difference between the lithological cliaracters of the Carboniferous Limestone series, in its typical development, as a great marine formation, and in its arenaceous and argillaceous prolongation into the north of England and Scotland, has long been a familiar example of the nature and application of the evidence fnriiislied by strata as to former geographical conditions. It shows that the deeper and clearer water of the Carboniferous sea spread over the site of Yorkshire, l)erbysliire, and Laneashire ; that the land lay to the north; and that, while the whole area was under- going subsidence, the maxininin iiiovemeiit took place over tlie area of deeper water. It was from the northern land that the sand and mud were derived, but the sediment during the time of the Carboniferous Limestone seeins in have sunk to the bottom before it could reach the great basin in which foraminifers, corals, criiioids, and mollusks were building up the great calcareous deposit. Yet the thin limestone bands, which run so persistently among the Lower Carboniferous rocks in Scotland, prove that there were occasional episodes during which the sediment ceased to arrive, and when the same species of shells, corals, and crinoids spread northwards towards the land, forming for a time over the sea—bottoiii a continuous sheet of calcareous ooze like that of the deeper water further south. These intervals of limestone growth no doubt point to times of - more rapid subiiiergeiice, perhaps also to other geographical strata full of land-plants, cyprids, and other indications of estuarine ' or tluviatile conditions. The Carboniferous Limestone series of Scotland presents a striking contrast to that of the typical formation in England. It consists mainly of sandstones, shales, ﬁre-clays, and coal-seanis, with a few comparatively thin seams of ' encrinal limestone. Its lower portions include the chief limestone bands. The thickest of these scams, known as the Hurlet or main limestone, is usually about 6 feet in thickness, but in the north of Ayrshire swells out to 100 feet, which is the most massive bed of limestone in any part of the Scottish Carboniferous system. It lies upon a seam of coal, and is in some places associated with pyritous shales, which have been largely worked as a source of alum. This superposition of a bed of marine limestone on a seam of coal is of frequent occurrence in the Carboniferous Lime- stone series of Scotland. Above the group of Lower Lime- stones comes a thick mass of strata containing many valu- able coal-seaiiis and iroiistoiies. Some of these strata are full of terrestrial plants (Lepicloclemlron, Sz'_r/illcu'z'.a, Stz'_«/maria, S'p/ie;2opleris, Ale!/aopteris); others, particularly the iron- stones, contain marine shells, such as Lz'u_r/ulu, I)z'srina,
 * Ledr_t, J1 _g/alina, ];'uomp/mlus. Numerous remains of fishes

have been obtained, more especially from some of the iron- stones and coals ((z'_yrrLc«uztIuz.~.-jbrmosus and other placoid ﬁn-spines, ll[e_a/alir:/it/z_I/s Ilibberli, It’/iizodus Ilibberti, with species of 1;'luniclith_g/s, .lcantIi0<Ies, C'tenopI_1/r/u'u.s. "Item-ains of labyrintliodoiits have also been found in this -group of strata, and have been detected even down in the Burdie House limestone. The highest division of the "C.irboniferous Limestone series consists of a group of sandstones and shales, with a few coal—seams, anrl three, -sometimes more, bands of marine limestone. =these liniestones are each seldom more than 3 or 4 feet 'thick, they have a wonderful persistence throughout the -coal-ﬁelds of central Scotland. As already mentioned, they can be traced over an area of at least 1000 square rniiles, and they probably extended originally over 9. con- siderably greater region. The Hnrlet limestone with its underlying coal can also be followed across a similar extent changes whereby the sediment was for a time prevented from spreading so far. Viewed as a whole, therefore, the Carboniferous Lime- stone series of N ortliuniberlaiid and Scotland contains the records of a loiig-coiitiiiiied but intermittent process of subsidence. The numerous coal-seanis with their under- clays were undoubtedly surfaces of vegetation that grew in rank luxuriance on the wide marine inud-ﬂats, and mark pauses in the subsidence. Perhaps we may infer the relative length of these pauses by the comparative thicknesses of the coal-seaiiis. The overlying and intervening sandstones and shales indicate a renewal of the downward inovenicnt. and the gradual inﬁlling of the depressed area with sedi- ment, uiitil the water once more shoaled, and the vegetation from adjacent swamps spread over the muddy ﬂats as before. The occasional liniestones serve to indicate the epochs of more prolonged or more rapid subsidence, when marine life was enabled to ﬂourish over the site of the submerged forests. lut that the sea, even though teiiaiited in these northern parts by a liniestone-making faniia, was not so clear and well suited for the development of animal life during some of these subiiiergeiices as it was further south, 1 seems to be proved by the paucity and dwarfed forms of Although ' the fossils in the thin liiiiestones, as well as by the admix- ture of clay in the stone. In Ireland the Carboniferous Limestone swells out to a vast thickness, and covers a large part of the island. It attains a maximum in the west mid soutli—wcst, where, according to Kinahan,1 it consists in Limerick of the follow- ing subdivisions :— Feet. Upper (Burren) Bedded limestone ......................... .. ‘.240 Limestone. (‘herty zone ................................ .. 20 Upper (Calp) Limestones and sliales ................. .. 1000 Limestone. Cheity zone .............................. .. 40 lower I ime_ Fencstcllu limestone ..................... .. 1900 ‘ t ‘ Lower chei-ty zone . ..................... .. 20 5 one """" " Lower slialy liiiicstoiies ................. .. 280 Lower Limestone Shale.   100 3600 1 Geology of Ireland, p. 72.