The Student's Elements of Geology Part 31

CHAPTER XVII.

UPPER CRETACEOUS GROUP.

Lapse of Time between Cretaceous and Eocene Periods.

Table of successive Cretaceous Formations.

Maestricht Beds.

Pisolitic Limestone of France.

Chalk of Faxoe.

Geographical Extent and Origin of the White Chalk.

Chalky Matter now forming in the Bed of the Atlantic.

Marked Difference between the Cretaceous and existing Fauna.

Chalk-flints.

Pot-stones of Horstead.

Vitreous Sponges in the Chalk.

Isolated Blocks of Foreign Rocks in the White Chalk supposed to be ice-borne.

Distinctness of Mineral Character in contemporaneous Rocks of the Cretaceous Epoch.

Fossils of the White Chalk.

Lower White Chalk without Flints.

Chalk Marl and its Fossils.

Chloritic Series or Upper Greensand.

Coprolite Bed near Cambridge.

Fossils of the Chloritic Series.

Gault.

Connection between Upper and Lower Cretaceous Strata.

Blackdown Beds.

Flora of the Upper Cretaceous Period.

Hippurite Limestone.

Cretaceous Rocks in the United States.

We have treated in the preceding chapters of the Tertiary or Cainozoic strata, and have next to speak of the Secondary or Mesozoic formations. The uppermost of these last is commonly called the chalk or the cretaceous formation, from creta, the latin name for that remarkable white earthy limestone, which const.i.tutes an upper member of the group in those parts of Europe where it was first studied.

The marked discordance in the fossils of the tertiary, as compared with the cretaceous formations, has long induced many geologists to suspect that an indefinite series of ages elapsed between the respective periods of their origin. Measured, indeed, by such a standard, that is to say, by the amount of change in the Fauna and Flora of the earth effected in the interval, the time between the Cretaceous and Eocene may have been as great as that between the Eocene and Recent periods, to the history of which the last seven chapters have been devoted. Several deposits have been met with here and there, in the course of the last half century, of an age intermediate between the white chalk and the plastic clays and sands of the Paris and London districts, monuments which have the same kind of interest to a geologist which certain medieval records excite when we study the history of nations. For both of them throw light on ages of darkness, preceded and followed by others of which the annals are comparatively well-known to us. But these newly-discovered records do not fill up the wide gap, some of them being closely allied to the Eocene, and others to the Cretaceous type, while none appear as yet to possess so distinct and characteristic a fauna as may ent.i.tle them to hold an independent place in the great chronological series.

Among the formations alluded to, the Thanet Sands of Prestwich have been sufficiently described in the last chapter, and cla.s.sed as Lower Eocene. To the same tertiary series belong the Belgian formations, called by Professor Dumont, Landenian. On the other hand, the Maestricht and Faxoe limestones are very closely connected with the chalk, to which also the Pisolitic limestone of France is referable.

CLa.s.sIFICATION OF THE CRETACEOUS ROCKS.

TABLE 17.1.

UPPER CRETACEOUS OR CHALK PERIOD.

1. Maestricht Beds and Faxoe Limestone.

2. Upper White Chalk, with flints.

3. Lower White Chalk, without flints.

4. Chalk Marl.

5. Chloritic series (or Upper Greensand).

6. Gault.

LOWER CRETACEOUS OR NEOCOMIAN.

1. Marine: Upper Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper part).

2. Marine: Middle Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper part).

3. Marine: Lower Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper part).

The cretaceous group has generally been divided into an Upper and a Lower series, the Upper called familiarly THE CHALK, and the Lower THE GREENSAND; the one deriving its name from the predominance of white earthy limestone and marl, of which it consists in a great part of France and England, the other or lower series from the plentiful mixture of green or chloritic grains contained in some of the sands and cherts of which it largely consists in the same countries. But these mineral characters often fail, even when we attempt to follow out the same continuous subdivisions throughout a small portion of the north of Europe, and are worse than valueless when we desire to apply them to more distant regions.

It is only by aid of the organic remains which characterise the successive marine subdivisions of the formation that we are able to recognise in remote countries, such as the south of Europe or North America, the formations which were there contemporaneously in progress. To the English student of geology it will be sufficient to begin by enumerating those groups which characterise the series in this country and others immediately contiguous, alluding but slightly to those of more distant regions. In Table 17.1 it will be seen that I have used the term Neocomian for that commonly called "Lower Greensand;" as this latter term is peculiarly objectionable, since the green grains are an exception to the rule in many of the members of this group even in districts where it was first studied and named.

MAESTRICHT BEDS.

(FIGURE 226. Belemnitella mucronata, Maestricht, Faxoe, and White Chalk.

a. Entire specimen, showing vascular impression on outer surface, and characteristic slit.

b. Section of same, showing place of phragmocone. (For particulars of structure see Chapter 18.))

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 Belemnitella mucronata (Figure 226) and Pecten quadricostatus, a sh.e.l.l regarded by many as a mere variety of Pecten quinquecostatus (see Figure 270). Besides the Belemnite there are other genera, such as Baculites and Hamites, never found in strata newer than the cretaceous, but frequently met with in these Maestricht beds. On the other hand, Voluta, Fasciolaria, 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 and Bryozoa, 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.

(FIGURE 227. Mosasaurus Camperi. Original more than three feet long.)

(FIGURE 228. Hemipneustes radiatus, Ag. Spatangus radiatus, Lam.

Chalk of Maestricht and white chalk.)

M. Bosquet, with whom I examined this formation (August, 1850), pointed out to me a layer of chalk from two to four inches thick, containing green earth and numerous encrinital stems, which forms the line of demarkation 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 Figure 246), 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 (see Figure 227), 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. Among the fossils common to the Maestricht and white chalk may be instanced the echinoderm, Figure 228.

I saw proofs of the previous denudation of the white chalk exhibited in the lower bed of the Maestricht formation in Belgium, about 30 miles S.W. of Maestricht, at the village of Jendrain, where the base of the newer deposit consisted chiefly of a layer of well-rolled, black chalk-flint pebbles, in the midst of which perfect specimens of Thecidea papillata and Belemnitella mucronata are imbedded. To a geologist accustomed in England to regard rolled pebbles of chalk-flint as a common and distinctive feature of tertiary beds of different ages, it is a new and surprising phenomenon to behold strata made up of such materials, and yet to feel no doubt that they were acc.u.mulated in a sea in which the belemnite and other cretaceous mollusca flourished.

PISOLITIC LIMESTONE OF FRANCE.

Geologists were for many years at variance respecting the chronological relations of this rock, which is met with in the neighbourhood of Paris, and at places north, south, east, and west of that metropolis, as between Vertus and Laversines, Meudon and Montereau. By many able palaeontologists the species of fossils, more than fifty in number, were declared to be more Eocene in their appearance than Cretaceous. But M. Hebert found in this formation at Montereau, near Paris, the Pecten quadricostatus, a well-known Cretaceous species, together with some other fossils common to the Maestricht chalk and to the Baculite limestone of the Cotentin, in Normandy. He therefore, as well as M. Alcide d'Orbigny, who had carefully studied the fossils, came to the opinion that it was an upper member of the Cretaceous group. It is usually in the form of a coa.r.s.e yellowish or whitish limestone, and the total thickness of the series of beds already known is about 100 feet. Its geographical range, according to M.

Hebert, is not less than 45 leagues from east to west, and 35 from north to south. Within these limits it occurs in small patches only, resting unconformably on the white chalk.

(FIGURE 229. Portion of Baculites Faujasii.

Maestricht and Faxoe beds and white chalk.)

(FIGURE 230. Nautilus Danicus, Schl. Faxoe, Denmark.)

The Nautilus Danicus, Figure 230, and two or three other species found in this rock, are frequent in that of Faxoe, in Denmark, but as yet no Ammonites, Hamites, Scaphites, Turrilites, Baculites, or Hippurites have been met with. The proportion of peculiar species, many of them of tertiary aspect, is confessedly large; and great aqueous erosion suffered by the white chalk, before the pisolitic limestone was formed, affords an additional indication of the two deposits being widely separated in time. The pisolitic formation, therefore, may eventually prove to be somewhat more intermediate in date between the secondary and tertiary epochs than the Maestricht rock.

CHALK OF FAXOE.

In the island of Seeland, in Denmark, the newest member of the chalk series, seen in the sea-cliffs at Stevensklint 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 are usually very rare in the white chalk of Europe. 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 of Patella, one of Emarginula, etc.; on the whole, more than thirty univalves, spiral or patelliform. At the same time, some of the accompanying bivalve sh.e.l.ls, echinoderms, and zoophytes, are specifically identical with fossils of the true Cretaceous series. Among the cephalopoda of Faxoe may be mentioned Baculites Faujasii (Figure 229), and Belemnitella mucronata (Figure 226), sh.e.l.ls of the white chalk. The Nautilus Danicus (see Figure 230) is characteristic of this formation; and it also occurs in France in the calcaire pisolitique of Laversin (Department of Oise). The claws and entire skull of a small crab, Brachyurus rugosus (Schlott.), are scattered through the Faxoe stone, reminding us of similar crustaceans inclosed in the rocks of modern coral reefs. Some small portions of this coralline formation consist of white earthy chalk.

COMPOSITION, EXTENT AND ORIGIN OF THE WHITE CHALK.

(FIGURE 231. Diagrammatic section from Hertfords.h.i.+re, in England, to Sens, in France.

Through London (left), Hythe, Boulogne, Valley of Bray, Paris and Sens (right).)