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 shells — 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, passing 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 shells, 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 shells, 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 principal 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 shells 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 Cypræa, 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 shells, 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, shells of the white chalk.