The Student's Elements of Geology Part 33

(Figures 253 to 256. Radiolites Mortoni. Mantell. Houghton, Suss.e.x. White chalk.

Diameter one-seventh natural size. On the side where the sh.e.l.l is thinnest, there is one external furrow and corresponding internal ridge, a, b, Figures 253, 254; but they are usually less prominent than in these figures. The upper or opercular valve is wanting.

(FIGURE 253. Two individuals deprived of their upper valves, adhering together.)

(FIGURE 254. Same seen from above.)

(FIGURE 255. Transverse section of part of the wall of the sh.e.l.l, magnified to show the structure.)

(FIGURE 256. Vertical section of the same.))

Of the singular family called Rudistes by Lamarck, hereafter to be mentioned as extremely characteristic of the chalk of southern Europe, a single representative only (Figure 253) has been discovered in the white chalk of England.

(FIGURE 257. Eschara disticha. White chalk.

a. Natural size.

b. Portion magnified.)

(FIGURE 258. Escharina oceani.

a. Natural size.

b. Part of the same magnified.

White chalk.)

(FIGURE 259. A branching sponge in a flint, from the white chalk. From the collection of Mr. Bowerbank.)

The general absence of univalve mollusca in the white chalk is very marked. Of bryozoa there is an abundance, such as Eschara and Escharina (Figures 257, 258).

These and other organic bodies, especially sponges, such as Ventriculites (Figure 238), are dispersed indifferently through the soft chalk and hard flint, and some of the flinty nodules owe their irregular forms to inclosed sponges, such as Figure 259, a, where the hollows in the exterior are caused by the branches of a sponge (Figure 259, b), seen on breaking open the flint.

(FIGURE 260. Palatal tooth of Ptychodus decurrens. Lower white chalk.

Maidstone.)

(FIGURE 261. Cestracion Phillippi; recent. Port Jackson. Buckland, Bridgewater Treatise Plate 27 d.))

The remains of fishes of the Upper Cretaceous formations consist chiefly of teeth belonging to the shark family. Some of the genera are common to the Tertiary formations, and some are distinct. To the latter belongs the genus Ptychodus (Figure 260), which is allied to the living Port Jackson shark, Cestracion Phillippi, the anterior teeth of which (see Figure 261, a) are sharp and cutting, while the posterior or palatal teeth (b) are flat (Figure 260). But we meet with no bones of land-animals, nor any terrestrial or fluviatile sh.e.l.ls, nor any plants, except sea-weeds, and here and there a piece of drift-wood. All the appearances concur in leading us to conclude that the white chalk was the product of an open sea of considerable depth.

The existence of turtles and oviparous saurians, and of a Pterodactyl or winged lizard, found in the white chalk of Maidstone, implies, no doubt, some neighbouring land; but a few small islets in mid-ocean, like Ascension, formerly so much frequented by migratory droves of turtle, might perhaps have afforded the required retreat where these creatures laid their eggs in the sand, or from which the flying species may have been blown out to sea. Of the vegetation of such islands we have scarcely any indication, but it consisted partly of cycadaceous plants; for a fragment of one of these was found by Captain Ibbetson in the Chalk Marl of the Isle of Wight, and is referred by A. Brongniart to Clathraria Lyellii, Mantell, a species common to the antecedent Wealden period.

The fossil plants, however, of beds corresponding in age to the white chalk at Aix-la-Chapelle, presently to be described, like the sandy beds of Saxony, before alluded to, afford such evidence of land as to prove how vague must be any efforts of ours to restore the geography of that period.

The Pterodactyl of the Kentish chalk, above alluded to, was of gigantic dimensions, measuring 16 feet 6 inches from tip to tip of its outstretched wings. Some of its elongated bones were at first mistaken by able anatomists for those of birds; of which cla.s.s no osseous remains have as yet been derived from the white chalk, although they have been found (as will be seen) in the Chloritic sand.

(FIGURE 262. Coprolites of fish, from the chalk.)

The collector of fossils from the white chalk was formerly puzzled by meeting with certain bodies which they call larch-cones, which were afterwards recognised by Dr. Buckland to be the excrement of fish (see Figure 262). They are composed in great part of phosphate of lime.

LOWER WHITE CHALK.

(FIGURE 263. Baculites anceps, Lam. Lower chalk.)

The Lower White Chalk, which is several hundred feet thick, without flints, has yielded 25 species of Ammonites, of which half are peculiar to it. The genera Baculite, Hamite, Scaphite, Turrilite, Nautilus, Belemnite, and Belemnitella, are also represented.

CHALK MARL.

(FIGURE 264. Ammonites Rhotomagensis. Chalk marl. Back and side view.)

(FIGURE 265. Turrilites costatus, Lam. Lower chalk and chalk marl.

a. Section, showing the foliated border of the sutures of the chambers.)

(FIGURE 266. Scaphites aequalis. Chloritic marl and sand, Dorsets.h.i.+re.)

The lower chalk without flints pa.s.ses gradually downward, in the south of England, into an argillaceous limestone, "the chalk marl," already alluded to.

It contains 32 species of Ammonites, seven of which are peculiar to it, while eleven pa.s.s up into the overlying lower white chalk. A. Rhotomagensis is characteristic of this formation. Among the British cephalopods of other genera may be mentioned Scaphites aequalis (Figure 266) and Turrilites costatus (Figure 265).

CHLORITIC SERIES (OR UPPER GREENSAND).

According to the old nomenclature, this subdivision of the chalk was called Upper Greensand, in order to distinguish it from those members of the Neocomian or Lower Cretaceous series below the Gault to which the name of Greensand had been applied. Besides the reasons before given for abandoning this nomenclature, it is objectionable in this instance as leading the uninitiated to suppose that the divisions thus named Upper and Lower Greensand are of co-ordinate value, instead of which the chloritic sand is quite a subordinate member of the Upper Cretaceous group, and the term Greensand has very commonly been used for the whole of the Lower Cretaceous rocks, which are almost comparable in importance to the entire Upper Cretaceous series. The higher portion of the Chloritic series in some districts has been called chloritic marl, from its consisting of a chalky marl with chloritic grains. In parts of Surrey, where calcareous matter is largely intermixed with sand, it forms a stone called malm-rock or firestone.

In the cliffs of the southern coast of the Isle of Wight it contains bands of calcareous limestone with nodules of chert.

COPROLITE BED.

The so-called coprolite bed, found near Farnham, in Surrey, and near Cambridge, contains nodules of phosphate of lime in such abundance as to be largely worked for the manufacture of artificial manure. It belongs to the upper part of the Chloritic series, and is doubtless chiefly of animal origin, and may perhaps be partly coprolitic, derived from the excrement of fish and reptiles. The late Mr.

Barrett discovered in it, near Cambridge, in 1858, the remains of a bird, which was rather larger than the common pigeon, and probably of the order Natatores, and which, like most of the Gull tribe, had well-developed wings. Portions of the metacarpus, metatarsus, tibia, and femur have been detected, and the determinations of Mr. Barrett have been confirmed by Professor Owen.

This phosphatic bed in the suburbs of Cambridge must have been formed partly by the denudation of pre-existing rocks, mostly of Cretaceous age. The fossil sh.e.l.ls and bones of animals washed out of these denuded strata, now forming a layer only a few feet thick, have yielded a rich harvest to the collector. A large Rudist of the genus Radiolite, no less than two feet in height, may be seen in the Cambridge Museum, obtained from this bed. The number of reptilian remains, all apparently of Cretaceous age, is truly surprising; more than ten species of Pterodactyl, five or six of Ichthyosaurus, one of Pliosaurus, one of Dinosaurus, eight of Chelonians, besides other forms, having been recognised.

The chloritic sand is regarded by many geologists as a littoral deposit of the Chalk Ocean, and therefore contemporaneous with part of the chalk marl, and even, perhaps, with some part of the white chalk. For, as the land went on sinking, and the cretaceous sea widened its area, white mud and chloritic sand were always forming somewhere, but the line of sea-sh.o.r.e was perpetually s.h.i.+fting its position. Hence, though both sand and mud originated simultaneously, the one near the land, the other far from it, the sands in every locality where a sh.o.r.e became submerged might const.i.tute the underlying deposit.

(FIGURE 267. Ostrea columba. Syn. Gryphaea columba. Chloritic sand.)

(FIGURE 268. Ostrea carinata. Chalk marl and chloritic sand. Neocomian.)

(FIGURE 269. Terebrirostra lyra, Sowerby. Chloritic sand.)

(FIGURE 270. Pecten 5-costatus. White chalk and chloritic sand. Neocomian.)

(FIGURE 271. Plagiostoma Hoperi, Sowerby. Syn. Lima Hoperi. White chalk and chloritic sand.)

Among the characteristic mollusca of the chloritic sand may be mentioned Terebrirostra lyra (Figure 269), Plagiostoma Hoperi (Figure 271), Pecten quinque-costatus (Figure 270), and Ostrea columba (Figure 267).

The Cephalopoda are abundant, among which 40 species of Ammonites are now known, 10 being peculiar to this subdivision, and the rest common to the beds immediately above or below.

GAULT.

(FIGURE 272. Ancyloceras spinigerum, d'Orb. Syn. Hamites spiniger, Sowerby. Near Folkestone. Gault.)

The lowest member of the Upper Cretaceous group, usually about 100 feet thick in the S.E. of England, is provincially termed Gault. It consists of a dark blue marl, sometimes intermixed with green sand. Many peculiar forms of cephalopoda, such as the Hamite (Figure 272), and Scaphite, with other fossils, characterise this formation, which, small as is its thickness, can be traced by its organic remains to distant parts of Europe, as, for example, to the Alps.

Twenty-one species of British Ammonites are recorded as found in the Gault, of which only eight are peculiar to it, ten being common to the overlying Chloritic series.

CONNECTION BETWEEN UPPER AND LOWER CRETACEOUS STRATA.-- BLACKDOWN BEDS.