Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/366

Rh 3.’ 2 . GEOLOGY outpouring of great sheets of quartz-porph_vr_v, granite-porphyry, porphyrite, aml mclaphyre, with abundant interstratiﬁcations of various tulfs, not infrequently enclosing organie remains. From the very nature of its component materials, the ltothliegcndc is comparatively barren of fossils, a few ferns, ealamites, and remains of coniferous trees are found in it, particularly towards the base, where indeed they form, in the Mansfcld district, aeoal-seam about 5 feet thick. The plants, all of terrestrial growth, on the whole rescmblc gencrically the Carboniferous flora, but seem to be nearly all speci- fically distinct. They include forms of Calanu'tcs (0. gigas), Astero- plcyllitcs, and ferns of the genera. Splwnopteris, A Ictlaoptcrfs, .'curoptcrz's, Odontoptcris, with wcll—prese1'vcd siliciﬁed stems of tree-ferns (Psaro.u'us, Tubicaulis). The conifer Il’ulcIu'a. (IV. pimformis) is specially characteristic. Fish remains oceur sparingly (Ambl_z/ptcrus, Palazoniscus, Jcanthodcs), and traces of lab_vrintho- donts (Archegosaurus Dcclzcni) have been met with. The Zechstein group is characterized by a suite of fossils like those of the Middle Permian of England. The Kupfer-schiefcr contains numerous ﬁsh (PaZa:om'scus Frcislcbcn-1', Platysomus gib- I/osus, &c.) and remains of plants (coniferous leaves and fruits and sca—wecds). This deposit is believed to have been laid down in some enclosed sea-basin, the waters of which, probably from the rise of mineral springs connected with some of the volcanic foci of the time, were so charged with metallic salts in solution as to become unﬁt for the continued existence of animal life. The dead ﬁsh, plants, &c., by their deeay, gave rise to reduction and pre- cipitation of these salts as sulphides, which thereupon enclosed and replaced the organic forms, and permeated the m11d at the bottom. This old sea-floor is now the widely extended band of copper-slate which has so long and so extensively been worked along the flanks of the llarz. After the formation of the Kupfer-schiefer the area must have been once more covered by clearer water, for the Zech- stein contains a number of organisms of which Prod uctus Izorrizlus, .S'p1':'zfcr mzdulrtlus, Sh'o1)l2alosia G'0I(lfussi, ScIu'::0(Ius obscurus, and I<'cnrsl4'lla rct{fornu's are eommon. Renewed unfavourable condi- tions are indicated by the dolomite, gypsum, and rock-salt which succeed. Professor Ramsay, reasoning upon such phenomena as developed in England, has connected them with the abundant labyrinthodont footprints and other evidences of shores and land, as well as the small number and dwarfed forms of the shells in the Magnesian Limestone, and has speculated on the occurrence of a long continental )(’l‘lO(l. in Europe, during one epoch of which a number of salt inlanil seas existed wherein the Permian rocks were accumu- latcd. He compares these deposits to what may be supposed to be forming now in parts of the Caspian Sea. NORTH AMERICA.—Tl1e Permian system is hardly repre- sented at all in this part of the globe. In Kansas certain red and green clays, sandstones, limcstones, conglomerates, and beds of gypsum lie conformably on the Carboniferous system, and contain a few genera and species of molluscs (Bal~evellz'a, Jfyalina, d:c,), which occur in the European Permian rocks. III. SECONDARY OR MESOZOIC. TRIASSIC. It has been already mentioned that the great mass of red rocks, which in England overlie the Carboniferous system, were formerly classed together as N cw Red Sandstone, but are now ranged in two systems. We have considered the lower of these under the name of Permian. The general facies of organic remains in that division is still decidedly Palaeozoic. Its brachiopods and its plants connect it with the Carboniferous rocks below. Hence it is placed at the close of the long series of Palaeozoic formations. When, however, we enter the upper division of the red rocks, though the general lithological characters remain very m11cl1 as in the lower group, the fossils bring before us the advent of the great Mesozoic ﬂora and fauna. This group there- fore is put at the base of the Mesozoic or Secondary series. It is called Trias, because in Germany it consists of three well-marked subdivisions. But the old name, New lied Sandstone, is familiarly retained for it by many geologists in England. The term Trias, like Dyas, is unfortunately chosen, for it elevates a mere local character into an import- ance which it does not deserve. The threefold subdivision, though so distinct in Germany, disappears elsewhere. GREAT Br.1'r.I.'.——T:'iassic rocks occupy a large area of [v1. s'rR.Tn:n.rIIIc.u.. the low plains in the centre of Fhigland, ranging thence northwards along the flanks of the Carboniferous tracts to Lancaster Day, and southwards by the head of the Bristol Channel to the south-east coast of Dcvonshire. They have been arranged in the following subdivisions :- Penarth lu-ds.——llcd, green, and grey marl.-, and " White Lias." (New lied ll:11'l.—licd and grey shales and marls, with beds of rock-salt and gypsum (Es!/cc/'[(t and Forum in (fora). Lower K0111»-1‘ Sandstone.—Thinly laminated llllt a- ceous sandstones and marls (watcistones) passing downwards into white, brown, or reddish sand- stones, with a base of calcareous conglomerate or L breccia. ..'anting in England (Muschelkalk of Ge-rman_v). Upper Mottled Sandstone.—.,'oft bright-rcd variegated sandstones, without pebbles. Pebble beds.—llarder reddish-brown sandstones with + quartzosc pebbles, passing into conglomerate; with a base of calcareous breccia. Lower Mottled Sandstonc.—Soft bright-red and variegated sandstone, without pebbles. Like the Permian red rocks below, the sandstones and marls of the Triassic series are almost barren of organic re- mains. Hence the subdivisions in the foregoing table are based on mineral characters, and could not therefore be relied on as a guide in districts outside of the English area. Indeed, extraordinary differences in the development of the different members of the series occur, even within that area, as may be seen from the subjoined table, which shows the variations in thickness from north-west to south-east :— Rha-tic .. UpperTrias 4 or Keuper. Middle ..... .. and Lower Trias or Buntcr. and W. ‘. _ - _ Cheshire. shin. k. I-‘ect. Feet. Fcct. KC“ or Red marl .................. .. 3000 S00 71'” P ' Lower Keupcr sandstone 450 200 150 Upper mottled sandstone 500 50-21 '0 absent Buntcr. Pebble beds .............. .. bw—750 100-300, 0-100. Lower mottled sandstone 200-500 0—1f|0| absent ' Hence we observe that, while towards the north—west the Triassic rocks attain a maximum depth of 5200 feet, they rapidly come down to a ﬁfth or a sixth of that thickness as they pass towards the south-east. Recent borings in the south-eastern counties show that the Triassic rocks are there absent altogether. It is evident that the source of supply of the sediment lay towards the north. This is further borne out by the character of the pebble-beds. These are coarsest towards the north, and, besides local materials, con- tain abundant rolled pebbles of quartz which have evidently been derived from some previous conglomerate, probably from some of the Old Red Sandstone masses now removed or concealed. The Trias everywhere rests uneonformabl y on the rocks underneath it, so that, although the general physical conditions as regards climate, geography, and sedi- mcntation, which prevailed in the Permian period still continued, great terrestrial movements had, in the incan- while, taken place, whereby the Permian sediments were upraised and exposed to denudation. Hence the Trias rests now on Permian, now on Carboniferous, and sometimes even on Cambrian rocks. Moreover, the upper parts of the Triassic series overlap the lower, so that the Keuper groups come to rest directly on Permian or Carboniferous rocks. One of the most interesting features in the English Trias is the occurrence of beds of rock-salt which have long been proﬁtably worked. T he uppermost subdivision of the Keuper, consisting of red marls, has a wide distribution, for it can be traced from the coast of Lancashire to the Bristol Channel, and covers a larger area of surface in the central counties than the rest of the Trias and the whole of the Permian sandstones combined. Even as far south as the