Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/372

Rh 358 The Gault is overlaid by a group of sands and sandstones often of a greenish tint from the presence of glauconitc grains. Hence the name Upper Greensand which is applied to them. These strata can be traced westwards into Dcvonshire, and eastwards to the headlands of Kent, but they die out towards the north. Their mineralogical characters and variable thickness seem to point to them as deposits of the shore of the sea in which the chalk was subsequently laid down upon them. Among their characteristic fossils are the sponges .S'ipIzon2'a pyri- formis and S. costata; urchins of the genera Cidaris, Echinus and Salcnia; numerous Tcrcbratula: and Iﬂzynchoncllw; many ]amcl- libranchs, particularly of the genera Exogym, Ostrca, Gryphwa, Lima, Pcctcn, and Trigonia ; and gasteropods of the genera Natica, Turritclla, and others. The cephalopods abound and comprise many forms of Ammomtcs (40 species), Hamitcs, Scaphilcs, b'aculitcs, Nautilus, and Bclcmnites. (7/ca ll-.—This conspicuousmember of theCretaceous system has at its base a white or pale yellow marl with green grains -of glauconite, phosphatic nodules, and_irou pyrites (Chloritic Marl), which is succeeded sometimes by a kind of argil- laceous chalk (Chalk M-.11-1) forming the base of the true Chalk. It can be traced from Flamborough Head in York- shire across the south-eastern counties to the coast of Dorset. ‘Throughout this long course its western edge usually rises somewhat abruptly from the plains as a long winding escarpment, which fro111 a distance often reminds one of an old coast-line. The upper half of the Chalk is generally distinguished by the presence of many nodular layers of ﬂint. With the exception of these enclosures, however, the whole formation is a remarkably pure white pulverulent -dull limestone, meagre to the touch, and soiling the ﬁngers. It is composed mainly of crumbled foraininifera, with the mingled debris of urchins, corals, and mollusks. It must have been accumulated in a sea of some depth and tolerabl y free from sediment, like some of the foraminiferal ooze of the existing sea-bed. There is, however, no evidence that the depth of the water at all approached that of the abysses in which the present Atlantic globigerina-ooze is being laid down. Indeed, the character of the foraminifera, and the variety and association of the other organic remains, are not like those which have been found to obtain now on the deep floor of the Atlantic. Somewhere about 800 species of fossils are known from the Eng- lish Chalk. Occasional rare fragments of terrestrial wood occur, perforated by the tercdo, and telling of 9. transport of some dis- tance from land. Sponges are numerous. They have usually been siliciﬁed and preserved in the tlint nodules. Among “the more characteristic genera are Clzoanitcs, Cliona, Ventri- -culitos, Brachiolitcs. Spongia, and Szjphonia. Careful preparation of a fragment of chalk usually brings to light remains, sometimes well preserved, of foraminifera.(Ilotalina ornata, C’)-islcllav-2'11 rotulata, (}’lobigcrina bulloides). Corals are represented by about 15 species (l’/zrt1.smz'lia, C’(clos1nilz'a, 0ar_1/(yllzyllia, &c.). The cclzini form one of the most conspicuous features among the Chalk fossils, from their individual numbers and their variety of forms. Among the more common genera the following may be nanied—Ammclzytrs, Echiiwconus (Galcritcs), Cardiaster, zlficrastcr, 03/plzosoma, Cidaris, Pscmiodiadcma, Discoidca, and Salcnia. Among other star—fishcs the genus Goniastcr occurs in numerous species in the upper division of the Chalk. The crinoids were represented in the sea of the period by a Comalula, one or two Pcntrwrinitcs, Jllarsupitcs, and Bourguctz'- rrimls. Polyzoa abound in the Upper (‘halk (llomcrosolcn, Pus- tulopora, Ilolostoma, &c.). The braehiopods appear in the form of great numbers of Rhynchontzlla, Tcrcbratula, and Tcrcbralulina, with Urania, Tlzccidca, and Kingcna. Among the lamcllibranchs the genera Ostrea, Pcctcn, Inoccramus, and Lima are particularly fre- quent. Gastcropods are comaprativcly few, Plezzrotomm-ia pcrs72cc- tirva being one of the few forms found both in the lower and upper division of the Chalk. Cephalopods, however, abound ; character- istic speeies are Bclcmmtcllaplczuz, B. mucronata, Nautilus DcsImzg- (‘hI1.71?]78ill1llL9,A77?‘m01lilc$7lf‘(1:iClIf(I7'i8, A. mrians, A. Ilotlzornngrnsis, 7'1IrriI1'las costatus, Brzculites baculoirlcs. Sc/zplzitcs azqmzlis, and Ilnmiws armatus. Upwards of 80 species of ﬁsh have been dis- covered. These include chimaeroids (E/Japhodmr, Is-chyodus), sharks (l[_:/bmlus, Plychodus, Lanma, Olorlus), ganoids Ulacrapoma, 1’y - 720.4113), and teleostcous or bony ﬁshes (Berg/.r, 1:‘-nchorlus, Saurocc- pha./us). Numerous reptilian remains have been found, more par- GEOLOGY '[vr. STR.-‘LTIGRAPHICAL ticularly in a bed about 1 foot thick lying at the base of the Chalk of Cambridge, and largely worked for phosphate of lime derived from reptilian coprolites and bones. Among the known forms are several ehelonians, the great dcinosaur Acantlmpholis, several species of I’Ics1'os(un'us, 5 or 6 species of IchtIzyosam'1(s, 10 species of Plcrodaclylus from the size of a pigcon upwards, one of them having a spread of wing amounting perhaps to 25 feet, 3 species of .l[osasmn'm-. a croeodilian (I’uIypt_1/citation), and some others. At Cznnbridge also the bones of one or two species of birds have been found, probably belonging to Na/atorcs allied to the living gulls. CONTINENTAL EUROPI-I.-—The Cretaceous system in many detached areas covers a large extent of the Continent. From the south of England it spreads southward across the north of France up to the base of the ancient central plateau of that country. Eastwards it ranges beneath the 'l‘crtiary and post—Tertiary deposits of the great plain, appearing on the north side at the southern end of Scandinavia am] in Denmark, on the south side in Belgium and ll-anovcr, round the ﬂanks of the Harz, in Bohemia and Poland, eastwards into Russia, where it covers many thousand square miles up to the southern end of the Ural chain. To the south of the central axis in France, it underlies the great basin of the Garonne, ﬂanks the chain of the Pyrenees on both sides, spreads out largely over the eastern side of the Spanish table-land, and reappears on the west side of the crystalline axis of that region along the coast of Portugal. It is seen at intervals along the north and south fronts of the Alps, extending down the valley'of the llhone to the Mediterranean, ranging along the chain of the Apennines into Sicily and the north of Africa, and widening out from the eastern shores of the Adriatic through Greece, and along the northern base of the Balkans to the Black Sea, round the southern shores of which it ranges in its progress into Asia, where it again covers an enormous area. A scrics of rocks covering so vast an extent of surface must In-(‘-«ls present many differences of type, alike in their lithological charac- ters and in their organic contents. They bring before us the records of atime when one continuous sea stretched over all the centre with most of the south of Europe, covered thc north of Africa, and swept eastwards to the far east of Asia. There were doubtless many islands aml ridges in this wide expanse of water, whcreb_v its areas of deposit and biological provinces must have been more ll‘ less sharply defined. Some of these barriers can still be traced, as will be immediately pointed out. The accompanying table contains the subdivisions of the Cretaceous system which have been adopted in a. few of the more important areas of Contincntal Euro )0. It will be seen from this table that while there is suflieient palrcontological similarity to allow a general parallelism to be drawn among the Cretaceous rocks of western Europe, there are yet strongly marked differences pointing to very distinct conditions of life, and probably, in many cases, to disconnected areas of deposit. l'owhcrc can these contrasts be more strikingly seen than in crossing from the Cretaceous basin of the Loire to that of the Garonne. In the north of France the Upper Cretaceous beds are precisely like those of England, the soft white Chalk forming a conspicuous feature in both countries ; but, on the south side of the great axis of crystalline rocks, the soft chalk is replaced by hard limcstoncs, T here is a prevalence of calcareous matter, often sparry, throughout the. whole series of formations, with compara- tively few sandy or clayey beds. This mass of limestone attains its greatest development in the southern part of the department of the Dordognc, where it is said to be about 800 feet thick. llnt the lithological diflcrcnees are not greater than those of the fossils. l n the north of France, Belgium, and England, the sinmilar inolluscan family of the H1']y7ur2'tz'a.'(L' or It’udz'stc.9 appears only occasionally and sporadically in the Cretaceous rocks, as if a stray individual had from time to time found its way into the region, but without being able to establish a colony there. In the south of Franec, however, the hippurites occur in prodigious quantity. They often mainl_v compose the limcstoncs, hence called hippuritc linn-stones (ltudisten-Kalke). They attained a great size, and seem to have grown on immense banks like our modern oyster. They appear in successive species on the different stages of the Cretaceous system, and can be used for marking palat-ontological horizons, as the cephalopods are elsewhere. But while these lamellibranebs played so important a part throughout the Cretaceous period in the south of France, the numerous ammonites and beleinnites, so characteristic of the Chalk in lingland, were absent from that region. This very distinctive type of hippuritc limestone has so