Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/371

Rh cRETA.cEoUs.] In iiorth-western Germany the siibjoined classiﬁcation has been adopted. ,3 Purbeck group (Serpulit, Miiiidcr Merge], and Einibeck- ‘5 S ,; hiiuscr Platteiikalk). =5 _§ Kiineridge group (Upper, with Ammom'tcs gigas and Exo- £__-_.°.§ :5. gym z-irgula; Lower or Nerineen—Schiclitcn). ’ L with (r')-yp/Lcca dilatata-). { (‘lays with A-Imnonilcs ornalus. § '5‘: Shales with Ostrca K/Lorri, Amm. fcrrugiucus. 5;‘ E3 l Zone of Amm. Parkinsoni. -:_ 53 4 Coronaten-Scliicliten, clays with Bclcmnitrs g2'_qmi- 53 I Miilillc (nus, .-lmm. Ilumplzrcsimzus, Amm. Bra[kcn- -7‘: E l rizlgi.
 * _ :5 I Oxford group (Upper, with Cidaris ﬂorigcmmu: Lower,
 * Shalcs with Amm. macroccphal us.
 * _ { Upper i Cornbrash with AL-icula cclzinala, Amm. postcrus.
 * 5 g S Shales with I/zoccramus 12013/plocus, Amm. .1! ur-

vﬁ L Lower ch isoncc. I Clays and limcstones with Amm. opalinus. {Grey iiiarls with Amman itcs j urcnsis. Upper liltllmlllolls shalcs (I’osi-loiiieii-schiefer)_ with .-1mm. I 3/llzcnszs, .-I. comm mus, A. bzfrons, Position fa Bronni. 4 ( Clays with Amm. auzallhea. I II 2 Middle, Maris and liinestones with Amm. capr_z'cornus. ‘ Dark clays and feri'uginous iiiarls with Amm. brcvispina. { Clays with Amm. planicosta. Blue grey clays with A mm. Bucklanrli (Arieceii- Lower y seliichten)._ Dark clays with Amm. (mgulatus. Dark clays and sandy layers with A mm. plmwrbis l (]2sz'lo:zoti(s). liowcr or I Jnra (J ' NORTH AMEI‘:ICA.—S0 far as yet known rocks of Jurassic age play but a very subordinate part in North Anierciaii geology. Perhaps some of the red strata of the Trias belong to this division, for it is diﬂicult, owing to paucity of fossil evidence, to draw a satisfactory line between the two systems. Strata containing fossils believed to represent those of the European Jurassic series have been met with in recent years during the explorations in the western domains of the United States. They occur among some of the eastern ranges of the Rocky Mountains, as well as on the western side of the watershed. They have been recog- nized also far to the north beyond the great region of Azoic and Palzeozoic rocks in the arctic portion of the continent. They consist of liiuestones and iiiarls, which appear seldom to exceed a few hundred feet in thickness. The fossils include species of Pentacrinus, Jlonolis, Tr-igom'ct., Lima, Ammonites, and Belemnites. CRETACEOUS. The next great series of geological formations is termed the Cretaceous system, from the fact that in England and western Europe one of its most important members is a thick band of white chalk (crela). BRITAIN.—-—Tl1e Purbeck beds bring before us evidence of a great change in the geography of England towards the close of the Jurassic period. They show how the floor of the sea in which the thick and varied formations of that period were deposited came to be gradually elevated, and how into pools of fresh and brackish water the leaves, insects, and small niarsupials of the adjacent land were washed down. These evidences of terrestrial conditions are followed in the same region by a vast delta-formation, that of the Wcald, which accumulated over the south of England, while the ol-.ler parts of the Cretaceous system were being deposited in the north. Hence there are two types of that system, one where the strata are ﬂuviatile or estuarine, termed the Wealden type, the other where they are marine, known as the Neocomian type. Arranged in descending order the follfwiiig are the subdivisions of the English Cretaceous roc 's :— GEOLOGY 357 ( { Upper Chalk with ﬂints ,2 Chalk 4_ 533,3 §,g=;{1<(g;g1°g,gf,1,gts ........ ..6oo to 1200 ft. 3 8 LChloritic Marl ............. .. 2: § 4 Upper D *3 Green- Greenisli-grey sandstones and sands 40 ,, 150 ,, 5 sand Stiff blue clay with calcareous and pyritous nodules ..................... .. 100 ,, 150 ,, Flu'vz'alz'le Type. Jlarinc Type. Folkstone beds. 1 LGault. F 70to]0Oft. - {'75 ,,1OO ,, 80 ,, 300 ,, lSaudgate beds. . Hythe beds .... .. A C Atherfield clay. J '20 ,, 60 ,, on er reen- sand. Upper part of Speeton Clay .... .. 150 ,, Puiiﬁcld beds, Tealby beds, and middle part of Speeton Clay ............................. .. 150 ,, Veald Clay,1 000 ft Ilastiiigs beds 4 coiisistingof— Tuiibridge Wells Sand(140—38O Lower part of Speeton Clay .... .. 200 ,, Lower. Middle. Upper rft) , adhurst Clay (120—180 ft); Ashdown Sand L (400 or 500 ft). Lower Cretaceous or Neocomian.——The ﬂuviatile develop- ment of this series in the south of England consists of a great depth of sands and clays known generally as the Wealden series, from the Weald of Sussex and Kent, where they are best displayed. They precisely resemble the deposits of a delta, and this is borne out by their organic remains, which consist partly of terrestrial plants (Equisctum, Sphcnoptcris, Alclhqplcris, Th-uytcs, cyeads, and conifers), and fresli-water shells ( Unio, 10 species; C’y1'c7za, 5 species; with Cyclas, Palmlina, Mclania, &c.), with a few estuarine or inarine forms as 0.»-trca and ]|[_i/tilus, and ganoid ﬁshes (Lcpidotus) like the gar of American rivers. Among the spoils of the land ﬂoated down by this river were the careases of huge deinosaurian reptiles (Iguanodon, H ylazosaurzls, jllcgalosaurus), of the long necked plesiosaiirs, and of winged pterodaetyles. The deltoid formation in which these remains occur extends in an east and west direction for at least 200 miles, and from north to south for at least 100. Hence the delta must have been not less than 20,000 square miles in area. It has been compared with that of the Quorra ; in reality, however, its extent must have been greater than its present visible area, for it has suffered from denudation, and is to a large extent concealed under more recent formations. The river probably descended from the north-west, draining a vast area, of which the existing mountain groups of Britain are perhaps merely fragments. The marine type of the Lower Cretaceous rocks is now commonly termed Neocomian. from N eufchatel(Neocomum) , where it is well developed. In the south of England only the upper division appears, overlying coiiformably the Wealden series, and showing the gradual depression of the old delta and the advance of the sea. In Yorkshire, how- ever, a thick deposit known as the Speeton Clay has been ascertained by Mr Judd to pass down into the Jurassic system, and to contain a representation of the upper parts of the Neocomian of the Continent. The lower division of the Speeton Clay contains, among other fossils, Ammonilcs Noricus. The central zone is marked by Pectm cinctus, Ancyloccras Dura-llii, and xllcycria ornata. The upper di- vision is characterized by Pcrna Mulleti, Ammonilcs Deshayesii, I’cctcnorbicula.r2's. It is the fossils of this upper division which occur in the Lower Greensand of Kent. They amount to about 300 species, of which only 18 or 20 per cent. pass up into the Upper Cretaceous. This marked palzeoutological break, taken in connexion with traces of unconformability between the Lower Greensand and the Gault, shows that a deﬁnite geological boiindary-line can be drawn between the lower and upper parts of the Cretaceous system. Upper Orr-tr1ceous.—.-'&t the base of this series lies the Gault—a dark blue stiff clay or marl, sometimes sandy and calcareous. It overlaps the older parts of the Cretaceous series, and in Wiltshire lies on Kiiiieridge Clay. Among the characteristics fossils of this division are C3/clocg,/atlezzs Fittom, Car;/op/n'I2'a Bowerbcnzl-2727, .Vucula peclinala, Ino- ceramus sulcalus, .Vatz'ca Gaulthza, It’0steIla7'z'a caruzala, A mmoniles clentatzts, and Ilamites allemmtus. In all, about 200 species of fossils occur, of which about 46 per cent. pass into up into the Upper Greensaiid. Lower Cretaceous or Neocomian.