Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/370

Rh 3.36 In the .lrom.E or Oxronn OoLI'rI-:s, the Oxford Clay, so called from the name of the county through which it passes in its course from the coast of Dorsetshire to that of York- shire, consists mainly of layers of stiff blue and brown clay. In its lower portion lies a marked zone of calcareous abund- antly fossiliferous sandstone, known, from a place in Wilt- shire, as the Kelaways lock, which, after dying out in the midland counties, reappears on the Yorkshire coast. This zone contains about 150 species of fossils, of which nearly a half are found in lower parts of the Jurassic series, and about the same number pass upward into higher zones. Among its characteristic forms is Ammouitcs Callocicusis. The Oxford clay, from the nature of its material and the conditions of its deposit, is deﬁcient in some forms of life which were no doubt abundant in neighbouring areas of clearer water. Thus there are hardly any corals, few echinoderms, polyzoa, or brachiopods. Some lamellibranchs are abundant, particularly (lrypluea and 0.,~trca (both forming sometimes wide oyster-beds), Lima, -1:-icula, Pcctcn, Aslurlc, Trigonz'¢l.—tl1c whole having a great similarity to the assemblages in the Lower Oolite formations. The ga.~.teropods are not so numerous as in the calcareous beds below, but belong mostly to the same genera. The ammomtes are numerons,—A. 1)unca.ni, A. J (sun, .1. Lrun.bcrt2', and .1. oculutzas being characteristic. Of th-- belemnites, which also are fretpient, B. Imstalus is found all the way from Dorsetslnre to Yorkshire. Spines and teeth of placoid ﬁshes and entire specimens of Lcpidotus are occasionally to be met with. The reptiles, besides Iclztlq/osa.1u'us, Jfcyalosazwus, ]’lcs2'o- sa1u'u>‘ (4 species), Stciwosamws, and Jtlm.nzphorhynchus, comprise also P.'cr'v)sauru.s—a marine sanrian with large head, short neck, pa«l"lles similar to those of Plesiosanrns, approaching the type of the ichthyosanri-ans, but even surpassing them in size. The Coralline Oolite can likewise be traced, with local modiﬁcations and partial interruptions, across England fro111 Yorkshire to 1)orsetshire. It is 11an1ed from its beds full of masses of coral. It consists of three zones,——a lower calcare- ous grit, a central rubbly limestone with corals (the true "coral rag” of William Smith), and an upper calcareous grit, which, though fcebly represented further south, attains importance in Yorkshire. It is frequently entirely made -up of comminuted shells, urchins, corals, and other marine organisms. The corals include the genera Isastrwa, Timm- nustrceu, and T leecos2m'lz'a. The urchins belong to Uiclcms, IIcnu'cizl«u'is, Pz'yurus, Pigaster, and other genera. There are likewise A mmonites, Belemnites, and J'azm'li. The UPPER or PoaTL_xND OOLITES bring before us the closing epochs of the long Jurassic period in England, with the records of some of the physical revolutions which led to this change. At their base lies the Kimeridge Clay, so named from the locality on the coast of Dorsetshire where it is so well exhibited, and whence it is traceable continuously, save where covered by the Chalk, into Yorkshire. Like the Oxford ‘Clay below, it is distinguished by its thickness, persistence, and peculiar organic remains. Illollusca appear in greatly diminished variety ; 07-3/plztrrz. 'rz'rgu.la, Us-trrca dcltoitlca, Astartc Ilartwcllcnsis, and C'(Lr(lizun striatul um are characteristic species. The reptiles are the most important of the palaeontologieal contents of this zone. They include remains of turtles, 5 species of II:hllz._1/osrmrus, 5 of Pleiosaurus, 8 of Plcsz'o- saurus, C'r:!c0.3azI.rus, ]lI'ega.losam‘us, and the croeodilians Simeo- sazI.ru.s, Tclcosaurus, and Goniopltolis. The Portland beds are so named from the isle of Portland, where they directly succeed the Kimeridge Clay. A feeble representative of them is believed to overlie that clay on the Yorkshire coast, but it is in the southern counties that they attain their chief development. They consist, at Port- land, of a lower sandy set of beds about 150 feet thick, and of an upper calcareous zone (containing the well-known lime- stone so largely used for building purposes under the name of Portland stone) about 70 feet thick. The fossils, which very commonly occur as mere empty casts, in- clude as characteristic species Isastrcca oblonga, Cm-di-um cliss2'mz'lc, Trz'_qom'a gibbosa, aml T crcbra Portlandica. There oeeur also remains of some of the great Oolitie sanrians. The Purbeck beds, so named from the Isle of Purbeck, where they are best developed, are usually connected with G 1'] O L O G Y [V1, S'l'P..TIG I1.- PI! ICA L. the foregoing formations as the highest zone of the J urassic series of England. But they are certainly separated from the rest of that series by many peculiarities, which show that they were accnlnulatcd at a time when the physical geography and the animal and vegetable life of the region were undergoing a re1n-ark-.1ble change. They have been arranged in three groups. The lowest consists-of fresh-water limcstones and clu_'.~:, with layers of ancient soil contain- ing stumps of the trees which grew in them. The middle group comprises about 130 feet of strata with marine fossils, while the highest division shows a return of fresh-water eomlitions. Among the indications of the presence of the sea is an o_-.~:ter bed (0s[rca (1 is- torla) 12 feet thick, with 1’;-ctrn, Jlurliolu, Jricu/rt, T/u-m*1'rI, .'c. The fresh-water bands contain still living genera of la:-ustrine an-l tlnriatile sl1ells—I’a.hIrl[)m, .L1'mna'u, 1’lunorlz2'.-, I'll;/mt, l'u/ruin. Unit), and C-yclas. I'umerons fishes, both plaeoid and ganoid. haunted these Pnrbeck waters. Many insects, blown off fr-an th- adjacent land, sank and were cmtombed and preserved in the till- careons mud. These include c )lL‘01)tl‘I'Oll.‘, orthoptcrous. lnmipler- ons, ucnropterous, and dipterous forms. Remains of several reptile-. chiefly ehelonian, b11t including the Jurassic crocodile (.'om'u1a/to/is, have also been discovered. But the most remarkable organic re- mains of the l‘urba-ck beds are those of 10 g"m-ra and :25 slim-in-s of marsnpial mammals, from the size of a mole to that of a pole- eat. The_v are believed to have been mostly iuse«-ti'orous. One of them (Triconotluit mrrjor) is regarded by Owen as carnivorous, and probably about the size of the existing Ila.»-5/zu-us nu/zq/ri of Australia. These mammalian ru-mains occur, almost wholly as lower jaws, in a stratum about 5 inches thick lying near the base of the Middle Purbeck group. CO.'TI.'EST.L lZL'1:or1:.—Jurassic rocks cover a vast area in central Europe. They rise from under the Cretaceous formations in the north-east of F rancc, whence they range southwards down the valleys of the S-aone and llhone to the Mediterranean. They appear as a broken border round the old crystalline nucleus of Auvergne. Eastwards they range through the Jura Mountains up to the high grounds of Bohemia. They appear in the outer chains of the .lp:5 on both sides, and on the south they rise along the centre of the Apennines, and here and there over the Spanish peninsula. Covered by more recent formations they under. lie the great plain of northern Germany, whence they range eastwards and occupy large tracts in central and eastern Russia. According to Nenmayr, three distinct geographical regions of deposit can be made out among the Jurassic rocks of Europe. The hlediterranean province, embracing the Pyrenees, Alps, and Carpathians, with all the tracts lying to the south. One of the biological characters of this area was the great abundance of ammonites belonging to the groups of 11¢-tn-o1;l¢_z/lli (I’/z_z/llocerus) and 1"1'mbrz'uti (L_2/tocerus). The central European province, comprising the tracts lying to the north of the Alpine ridge, and nnirked by the comparative rarity of the ammonites just mentioned, which are replaced by others of the groups I n_/Iuli (.1 .-);izIo- ceras) and Oppellia, and by alnmdant reefs and masses of coral.  The boreal o1'Tlns.sl.'n1 province, comprising the middle and north of linssia, Spitzbergeu, and Greenland. The life in this area was much less varied than in the others, showing that in Jurassic times there was a perceptible dim- inution of temperature towards the north. The ammonites of the more southern tracts here disappear, together with the corals.- In France the following arrangement has been made of the J urassie rocks, the subdivisions nearly corre-‘~ponding to those first proposed in England :——- (Purbeek beds not recognized.) Terrain Portlandien (Calcaire tachcte de lloulogne). ,, Kimeridgien (Argile de llonllenr). ,, Corallieu (Calcaire C-orallien). ,, Oxfordien (Oolithe dc Tronville). ,, Callovien (Argiles de Dive-s= Kelaways Rock). ,, Bathonien (Oolithe do-(‘£1611 = Cornbrash, Terre it fonlon). ,, Bajocicn on Oolithe I nferienrc (Oolithe de llayenx). ,, 'l‘oareicn (Marnes Liasiques snpérienre= Upper Lias). ,, Liasien(.Marnes et calcaires a Belemnites= Middle Lias). ,, Sine’-murien (Calcaire in Gryphées= Lower Lias).