A Manual of Elementary Geology Part 31

[192-A] Cuvier, Oss. Foss., tom. iii. p. 255.

[194-A] This species is found both in the Paris and London basins.

[197-A] Coquilles caracterist. des Terrains, 1831.

[197-B] Quarterly Geol. Journal, vol. iii. p. 353.

[199-A] Prestwich, Quart. Geol. Journ. vol. iii. p. 386.

[199-B] Palaeont. Soc. Monograph. Rept. pt. ii. p. 61.

[202-A] For description of Eocene Cephalopoda, see Monograph by F. E.

Edwards, Palaeontograph. Soc. 1849.

[203-A] Annals of Nat. Hist. vol. iv. No. 23. Nov. 1839.

[206-A] Murchison, Quart. Journ. of Geol. Soc. vol. v., and Lyell, vol. vi.

1850. Anniversary Address.

[206-B] See paper by the author, Quart. Journ. Geol. Soc. vol. iv, p. 12.; and Second Visit to the U. S. vol. ii. p. 59.

[206-C] Quart. Journ. Geol Soc. vol. vi. p. 32.

[207-A] See Memoir by R. W. Gibbes, Journ. of Acad. Nat. Sci. Philad.

vol. i. 1847.

[208-A] Lyell, Quart. Journ. Geol. Soc. 1847, vol. iv. p. 15.

CHAPTER XVII.

CRETACEOUS GROUP.

Divisions of the cretaceous series in North-Western Europe--Upper cretaceous strata--Maestricht beds--Chalk of Faxoe--White chalk--Characteristic fossils--Extinct cephalopoda--Sponges and corals of the chalk--Signs of open and deep sea--Wide area of white chalk--Its origin from corals and sh.e.l.ls--Single pebbles in chalk--Siliceous sandstone in Germany contemporaneous with white chalk--Upper greensand and gault--Lower cretaceous strata--Atherfield section, Isle of Wight--Chalk of South of Europe--Hippurite limestone--Cretaceous Flora--Chalk of United States.

Having treated in the preceding chapters of the tertiary strata, we have next to speak of the uppermost of the secondary groups, called the Chalk or Cretaceous (No. 6. Table, p. 103.), because in those parts of Europe where it was first studied its upper members are formed of that remarkable white earthy limestone, termed chalk (_creta_). The inferior division consists, for the most part, of clays and sands, called Greensand, because some of the sands derive a bright green colour from intermixed grains of chloritic matter. The cretaceous strata in the north-west of Europe may be thus divided[209-A]:

_Upper Cretaceous._

1. Maestricht beds and Faxoe limestone.

2. Upper white chalk, with flints.

3. Lower white chalk, without flints, pa.s.sing downwards into chalk marl, which is slightly argillaceous.

4. Upper greensand.

5. Gault.

_Lower Cretaceous._

6. Lower greensand--Ironsand, clay, and occasional beds of limestone (Kentish rag).

_Maestricht Beds._--On the banks of the Meuse, at Maestricht, reposing on ordinary white chalk with flints, we find an upper calcareous formation about 100 feet thick, the fossils of which are, on the whole, very peculiar, and all distinct from tertiary species. Some few are of species common to the inferior white chalk, among which may be mentioned _Belemnites mucronatus_ (see fig. 197.) and _Pecten quadricostatus_.

Besides the Belemnite there are other _genera_, such as Ammonite, Baculite, and Hamite, never found in strata newer than the cretaceous, but frequently met with in these Maestricht beds. On the other hand, Volutes and other genera of univalve sh.e.l.ls, usually met with only in tertiary strata, occur.

The upper part of the rock, about 20 feet thick, as seen in St. Peter's Mount, in the suburbs of Maestricht, abounds in corals, often detachable from the matrix; and these beds are succeeded by a soft yellowish limestone 50 feet thick, extensively quarried from time immemorial for building. The stone below is whiter, and contains occasional nodules of grey chert or chalcedony.

M. Bosquet, with whom I lately examined this formation (August, 1850), pointed out to me a layer of chalk from 2 to 4 inches thick, containing green earth and numerous encrinital stems, which forms the line of demarcation between the strata containing the fossils peculiar to Maestricht and the white chalk below. The latter is distinguished by regular layers of black flint in nodules, and by several sh.e.l.ls, such as _Terebratula carnea_ (see fig. 201.), wholly wanting in beds higher than the green band. Some of the organic remains, however, for which St. Peter's Mount is celebrated, occur both above and below that parting layer, and, among others, the great marine reptile, called _Mosasaurus_, a saurian supposed to have been 24 feet in length, of which the entire skull and a great part of the skeleton have been found. Such remains are chiefly met with in the soft freestone, the princ.i.p.al member of the Maestricht beds.

_Chalk of Faxoe._--In the island of Seeland, in Denmark, the newest member of the chalk series, seen in the sea-cliffs at Stevens Klint resting on white chalk with flints, is a yellow limestone, a portion of which, at Faxoe, where it is used as a building-stone, is composed of corals, even more conspicuously than is usually observed in recent coral reefs. It has been quarried to the depth of more than 40 feet, but its thickness is unknown. The imbedded sh.e.l.ls are chiefly casts, many of them of univalve mollusca, which, as they strictly belong to the Cretaceous era, are worthy of notice, since such forms, whether spiral or patelliform, are wanting in the white chalk of Europe generally. Thus, there are two species of _Cypraea_, one of _Oliva_, two of _Mitra_, four of the genus _Cerithium_, six of _Fusus_, two of _Trochus_, one _Patella_, one _Emarginula_, &c., on the whole, more than thirty univalves, spiral or patelliform, not one of which is common to the white chalk. At the same time, a large proportion of the accompanying bivalve sh.e.l.ls, echinoderms, and zoophytes, are specifically identical with fossils of older parts of the Cretaceous series. Among the cephalopoda of Faxoe, may be mentioned _Baculites Faujasii_ and _Belemnites mucronatus_, sh.e.l.ls of the white chalk.

The claws and entire sh.e.l.l of a small crab, _Brachyurus rugosus_ (Schlotheim), are scattered through the Faxoe stone, reminding us of similar crustaceans enclosed in the rocks of many modern coral reefs.[211-A] Some small portions of this coralline formation consist of white earthy chalk; it is, therefore, clear that this substance must have been produced simultaneously, a fact of some importance, as bearing on the theory of the origin of white chalk; for the decomposition of such corals as we see at Faxoe is capable, we know, of forming white mud, undistinguishable from chalk, and which we may suppose to have been dispersed far and wide through the ocean, in which such reefs as that of Faxoe grew.

[Ill.u.s.tration: Fig. 193. Section from Hertfords.h.i.+re, in England, to Sena, in France.]

_White Chalk_ (2. and 3. Tab. p. 209.).--The highest beds of chalk in England and France consist of a pure, white, calcareous ma.s.s, usually too soft for a building stone, but sometimes pa.s.sing into a more solid state. It consists, almost purely, of carbonate of lime; the stratification is often obscure, except where rendered distinct by interstratified layers of flint, a few inches thick, occasionally in continuous beds, but oftener in nodules, and recurring at intervals from 2 to 4 feet distant from each other.

This upper chalk is usually succeeded, in the descending order, by a great ma.s.s of white chalk without flints, below which comes the chalk marl, in which there is a slight admixture of argillaceous matter. The united thickness of the three divisions in the south of England equals, in some places, 1000 feet.[211-B]

The annexed section, fig. 193., will show the manner in which the white chalk extends from England into France, covered by the tertiary strata described in former chapters, and reposing on lower cretaceous beds.

Among the conspicuous forms of mollusca wholly foreign to the tertiary and recent periods, and which we meet with in the white chalk, are the Belemnite, Ammonite, Baculite, and Turrilite, all genera of _Cephalopoda_, a family to which the living cuttle-fish and nautilus belong.

[Ill.u.s.tration: Fig. 194. Portion of _Baculites Faujasii_. Maestricht and Faxoe beds and white chalk.]

[Ill.u.s.tration: Fig. 195. Portion of _Baculites anceps_. Maestricht and Faxoe beds and white chalk.]

[Ill.u.s.tration: Fig. 196. Turrilites.

_a._ _Turrilites costatus._ Chalk marl.

_b._ Same, showing the indented border of the part.i.tion of the chambers.]

[Ill.u.s.tration: Fig. 197. Belemnites.

_a._ _Belemnites mucronatus._ _b._ Same, showing internal structure.

Maestricht, Faxoe, and white chalk.]

Among the brachiopoda in the white chalk, the _Terebratulae_ are very abundant. These sh.e.l.ls are known to live at the bottom of the sea, where the water is tranquil and of some depth (see figs. 198, 199, 200, 201.).

With these are a.s.sociated some forms of oyster (see figs. 202. and 204.), and other bivalves (figs. 203, 205, 206, 207, 208.).

[Ill.u.s.tration: Fig. 198. _Terebratula plicatilis_, dorsal view.

Upper white chalk.]

[Ill.u.s.tration: Fig. 199. _Terebratula plicatilis_, side view.]

[Ill.u.s.tration: Fig. 200. _Terebratula pumilus._ (_Magas pumilus_, Sow.) Upper white chalk.]

[Ill.u.s.tration: Fig. 201. _Terebratula carnea._ Upper white chalk.]