Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/360

Rh 346 fragment of an angiospernioiis exogeii yet discovered. So abundant are these vegetable remains that in some layers they actually forin thin seams of coal. 'l‘lic interest of these remains of the most venerable .iiierieaii forests is heightened by the discovery of the fact that they were not without the hum of insect life.d The 1]]}0St ancient known relics of insect forms have been reeovere from t ie Devonian strata of few lhaiiiswiek. They ai'e. all iieuroptcrous wings, and have been referred by Mr St-udder of Boston to four species combining a remarkable union of characters now found in distinct orders of insects. In one fragment he observed a structure which he could only compare to the stridulating organ of some male Urtlioptrra. Auother wing indicates the existence of a gigantic Ephemera, with a spread of wing extending to 5 inches. In the shallow pools of the period some small crustaceans lived, the remains of which have been partially preserved. Among these is a minute, sliriiiip-like Euryptcrus, and a more highly organized forni named Amplu'pcltz's. That the sea had at least occasional access to the inland basins into which this abundant terrestrial vegetatioii was washed is proved by the occurrence of marine orgaiiisins, such as a small annelid (Spirorbis) adhering to the leaves of the plants, and (in Gaspé and Nova Scotia) by the occasional appearance of brachiopods, especially Lingula, .S‘pz'r{fcr, and ('/condos.‘ C-u-:i3o.'irEi>.oUs. This great system of rocks has received its name from the seams of coal which form one of its distinguishing features both in Europe and in North America. In Europe it is most completely developed in the British Islands. Elsewhere on that continent it occurs in patches, either lying in hollows of older rocks, or exposed by the removal of overlying formations. GREAT BRIT.-I‘_I.—-Tlle area within which the Carboni- ferous rocks of Britain occur is sufficiently extensive to contain more than one type of the system, and thus to cast much light on the varied geographical conditions under which these rocks were accumulated. In prosecuting the study of this part of Britisli geology, it is soon discovered, and it is essential to bear in mind, that, during the Carboiii- ferous period, the land whence the chief supplies of sedi- ment were derived rose mainly to the north and nortli-west, as it seems to have done from very early geological time. While therefore the centre and south of England lay under clear water of moderate depth, the north of the country and the south of Scotland were covered by shallow water, which was continually receiving sand and mud from the adjacent northern land. Hence vertical sections of the Carboniferous formations of Britain differ greatly according to the districts in which they are taken. The subjoined table may be regarded as expressing the typical subdivisions which can be recognized, with modiﬁcations, in all parts of the country. (Red and grey sandstones, clays, and sometimes breccias, with occasional seams and streaks of coal and spiroi-bis limestone (Cytlwrc inﬂata, Spirorbis carlxoiuuius). Middle or chief coal-bearing series of yellow sand- stones, clays, and shales, with numerous work- able coals (Anllwaco.-.-1'((, .111!/mtcomya, L'c'_I/ric/1 fa, Esthcria, .S'pL'/'orbis, &c.). Gaiinister beds, flagstones, scales, and thin coals, with hard siliceous (gaiiiiister) pavements (0r!Iz.0- ccms, Goniatitcs, Posidonia, Ariculopcctcn, Lin- L _r_/ula, &e.). I _ Millstone Grit % Grits, flagstones, and shales, with thin seams of Coal-mea- sures . .... .. coal. V Yoredalc group of shales and grits passing down into dark shales and liiiiestoiies (G'om'utz'tcs, Ari- culopccten, Posidoiwmya, Lingula, Dz'sc['/La, &c.). Thick limestone in south and centre of England Carboniferous and Ireland, passing northwards into sandstones, shales, and eoals_(abundant corals, polyzoans Liinestonc 4. . I “Cries brachiopods, lamelhbranclis, &e.). Lower Limestone Shale of south and centre of Eng- land (marine fossils like those of overlying lime- stone), passing northward into the C3l('lfCl'OllS Sandstone group of Scotland (marine, estuarine, L and terrestrial organisms). 1 Dawson's Actzclil/.'/L f-'«ol(»_r/3/, chaps. xxi. and xxii. GEOLOGY [VL STR.-'l‘IGRAI‘IIICAL. In the article COAL (vol. vi. p. 49) an account has been given of the principal coal—ﬁelds of the world; likewise a diagram (p. 48) representing the chief subdivisions of the Carboniferous system in Britain, as the rocks are traced from north to south. Base of the ;.'_z/stein.-—Iii the south-west of England, and in South Wales, the Cai-boniferoiis system passes down coii- forniably into the Old llezl Sandstone. The passage beds consist of yellow, green, and i'eddisli sandstones, of green, grey, red, blue, and variegated niarls and sli-ales, sonietinies full of terrestri-.il plants. They are well exposed on the Peni- brokesliire coasts, marine fossils being there found even among the argillaceous beds at the top of the lied Sandstone series. They occur with a thickness of about .300 feet in the gorge of the Avon near Bristol, but show l.ss than half that depth about the Forest of Dean. At their base there lies a bone-bed containing abundant palatal teeth. Xot far above this horizon plaiit-beariiig strata are found. Ileiice these rocks bring before us a mingling of terrestrial and iiiarinecoii- ditions. In Yorkshire, near Lowther Castle, Brougli, and in llavenstonedale, alternations of red sandstones, .-liales, and elays_. containing .S'ti',r/niart'a and other plants, occur in the lower part of the Carboniferous Limestone. Along the eastern edge of the Silurian hills of the Lake district the Old lied Sandstone appears here and there, and passes up through a succession of red and grey sandstones, and green and red shales and niarls, with plants, into the base of the Carboniferous Limestone. It is in Scotland, however, that this peculiar type of the basement Carboniferous rocks is best seen. In that country the lowest subdivision of the Carboniferous systein, known as the Calciferoiis Sandstones, consists of red, white, and yellow sandstones, blue, grey, green, and red niarls or clays, blue and black shales, thin coals, seams of liinestonc and ceiiieiit—stone, and abundant volcanic rocks. The red sandstones lie at the base, and pass down into the Upper Old Red Sandstone, in which, as has been already pointed out (ante, p. 344), true Upper Old Red Sandstone ﬁshes are found, while there occur also bands of liinestone full of true Carboniferous Limestone corals and biacliiopods. Hence it is evident that the Carboniferous Limestone fauna had already appeared outside the British area before the close of the Old Red Sandstone period. It was when the peculiar geographical conditions which prevailed during that period tin-ally ceased, and the sea began to spread over the ancient lakes and land of Britain, that the abundant Carboniferous fauna invaded the area. The Ctllclftriilts Sandstones of Scotland may therefore represent a portion of the Carboniferous Limestone of lingland. Over the greater part of the south and centre of Scotlaiid the lower red sandstones are surmounted by a series of con- teniporaneoiis volcanic rocks. Successive sheets of poi‘- phyrites and tuffs forin long ranges of hills from Arr-an and Bute on the west to the mouth of the cstiiary of the Forth on the east, and from the C‘-aiiipsie Fells on the north to the heights of Liddesdale and the English border. These volcanic sheets sometimes reach a thickness of 1500 feet. That they belong to the Carboniferous system is shown by the occurrence of shales and sandstones (with Carbonifei'oiis plants) at their base. They show that the early part of the Carboniferous period in Scotland was inai-_ked by a prodi- gious volcanic activity, which, on its cessation, was followed by the prolonged subsidence required for the accumulation of the Carboniferous system. The rocks succeeding the volcanic zone are termed the cement-stone group. In Berwicksliire and the west of Scotland they consist of thin bedded white, yellow, and green sandstones, grey, green. blue, and red clays and shales, with thin bands of a pale argillaceous liniestone or ceineiit-stone. Seams of gypsuni occasionally appear. These strati are. on the whole,