The Student's Elements of Geology Part 35

We now come to the Lower Cretaceous Formation which was formerly called Lower Greensand, and for which it will be useful for reasons before explained (Chapter 17) to use the term "Neocomian."

TABLE 18.1. LOWER CRETACEOUS OR NEOCOMIAN GROUP.

COLUMN 1: MARINE.

COLUMN 2: FRESH-WATER.

1. Upper Neocomian-- Greensand of Folkestone, Sandgate, and Hythe, Atherfield clay, upper part of Speeton clay: Part of Wealden beds of Kent, Surrey, Suss.e.x, Hants, and Dorset.

2. Middle Neocomian-- Punfield Marine bed, Tealby beds, middle part of Speeton clay: Part of Wealden beds of Kent, Surrey, Suss.e.x, Hants, and Dorset.

3. Lower Neocomian-- Lower part of Speeton clay: Part of Wealden beds of Kent, Surrey, Suss.e.x, Hants, and Dorset.

In Western France, the Alps, the Carpathians, Northern Italy, and the Apennines, an extensive series of rocks has been described by Continental geologists under the name of t.i.thonian. These beds, which are without any marine equivalent in this country, appear completely to bridge over the interval between the Neocomian and the Oolites. They may, perhaps, as suggested by Mr. Judd, be of the same age as part of the Wealden series.

UPPER NEOCOMIAN.

FOLKSTONE AND HYTHE BEDS.

(FIGURE 277. Nautilus plicatus, Sowerby, in Fitton's Monog.)

(FIGURE 278. Ancyloceras gigas, d'Orbigny.)

(FIGURE 279. Gervillia anceps, Desh. Upper Neocomian, Surrey.)

(FIGURE 280. Trigonia caudata, Aga.s.siz. Upper Neocomian.)

(FIGURE 281. Terebratula sella, Sowerby. Upper Neocomian, Hythe.)

(FIGURE 282. Diceras Lonsdalii. Upper Neocomian, Wilts.

a. The bivalve sh.e.l.l.

b. Cast of one of the valves enlarged.)

The sands which crop out beneath the Gault in Wilts.h.i.+re, Surrey, and Suss.e.x are sometimes in the uppermost part pure white, at others of a yellow and ferruginous colour, and some of the beds contain much green matter. At Folkestone they contain layers of calcareous matter and chert, and at Hythe, in the neighbourhood, as also at Maidstone and other parts of Kent, the limestone called Kentish Rag is intercalated. This somewhat clayey and calcareous stone forms strata two feet thick, alternating with quartzose sand. The total thickness of these Folkestone and Hythe beds is less than 300 feet, and they are seen to rest immediately on a grey clay, to which we shall presently allude as the Atherfield clay. Among the fossils of the Folkestone and Hythe beds we may mention Nautilus plicatus (Figure 277), Ancyloceras (Scaphites) gigas (Figure 278), which has been aptly described as an Ammonite more or less uncoiled; Trigonia caudata (Figure 280), Gervillia anceps (Figure 279), a bivalve genus allied to Avicula, and Terebratula sella (Figure 281). In ferruginous beds of the same age in Wilts.h.i.+re is found a remarkable sh.e.l.l called Diceras Lonsdalii (Figure 282), which abounds in the Upper and Middle Neocomian of Southern Europe. This genus is closely allied to Chama, and the cast of the interior has been compared to the horns of a goat.

ATHERFIELD CLAY.

We mentioned before that the Folkstone and Hythe series rest on a grey clay.

This clay is only of slight thickness in Kent and Surrey, but acquires great dimensions at Atherfield, in the Isle of Wight. The difference, indeed, in mineral character and thickness of the Upper Neocomian formation near Folkestone, and the corresponding beds in the south of the Isle of Wight, about 100 miles distant, is truly remarkable. In the latter place we find no limestone answering to the Kentish Rag, and the entire thickness from the bottom of the Atherfield clay to the top of the Neocomian, instead of being less than 300 feet as in Kent, is given by the late Professor E. Forbes as 843 feet, which he divides into sixty-three strata, forming three groups. The uppermost of these consists of ferruginous sands, the second of sands and clay, and the third or lowest of a brown clay, abounding in fossils.

Pebbles of quartzose sandstone, jasper, and flinty slate, together with grains of chlorite and mica, and, as Mr. G.o.dwin-Austen has shown, fragments and water- worn fossils of the oolitic rocks, speak plainly of the nature of the pre- existing formations, by the wearing down of which the Neocomian beds were formed. The land, consisting of such rocks, was doubtless submerged before the origin of the white chalk, a deposit which was formed in a more open sea, and in clearer waters.

(FIGURE 283. Perna Mulleti, Desh. One-eighth natural size.

a. Exterior.

b. Part of hinge-line of upper or right valve.)

Among the sh.e.l.ls of the Atherfield clay the biggest and most abundant sh.e.l.l is the large Perna Mulleti, of which a reduced figure is given in Figure 283.

SIMILARITY OF CONDITIONS CAUSING REAPPEARANCE OF SPECIES.

Some species of mollusca and other fossils range through the whole series, while others are confined to particular subdivisions, and Forbes laid down a law which has since been found of very general application in regard to estimating the chronological relations of consecutive strata. Whenever similar conditions, he says, are repeated, the same species reappear, provided too great a lapse of time has not intervened; whereas if the length of the interval has been geologically great, the same genera will reappear represented by distinct species. Changes of depth, or of the mineral nature of the sea-bottom, the presence or absence of lime or of peroxide of iron, the occurrence of a muddy, or a sandy, or a gravelly bottom, are marked by the banishment of certain species and the predominance of others. But these differences of conditions being mineral, chemical, and local in their nature, have no necessary connection with the extinction, throughout a large area, of certain animals or plants. When the forms proper to loose sand or soft clay, or to perfectly clear water, or to a sea of moderate or great depth, recur with all the same species, we may infer that the interval of time has been, geologically speaking, small, however dense the ma.s.s of matter acc.u.mulated. But if, the genera remaining the same, the species are changed, we have entered upon a new period; and no similarity of climate, or of geographical and local conditions, can then recall the old species which a long series of destructive causes in the animate and inanimate world has gradually annihilated.

SPEETON CLAY, UPPER DIVISION.

(FIGURE 284. Ammonites Deshayesii, Leym. Upper Neocomian.)

On the coast, beneath the white chalk of Flamborough Head, in Yorks.h.i.+re, an argillaceous formation crops out, called the Speeton clay, several hundred feet in thickness, the palaeontological relations of which have been ably worked out by Mr. John W. Judd, and he has shown that it is separable into three divisions, the uppermost of which, 150 feet thick, and containing 87 species of mollusca, decidedly belongs to the Atherfield clay and a.s.sociated strata of Hythe and Folkestone, already described. (Judd, Speeton clay, Quarterly Geological Journal volume 24 1868 page 218.) It is characterised by the Perna Mulleti (Figure 283) and Terebratula sella (Figure 281), and by Ammonites Deshayesii (Figure 284), a well-known Hythe fossil. Fine skeletons of reptiles of the genera Pliosaurus and Teleosaurus have been obtained from this clay. At the base of this upper division of the Speeton clay there occurs a layer of large Septaria, formerly worked for the manufacture of cement. This bed is crowded with fossils, especially Ammonites, one species of which, three feet in diameter, was observed by Mr. Judd.

MIDDLE NEOCOMIAN.

TEALBY SERIES.

(FIGURE 285. Pecten cinctus, Sowerby. (P. cra.s.sitesta, Rom.) Middle Neocomian, England; Middle and Lower Neocomian, Germany. One-fifth natural size.)

(FIGURE 286. Ancyloceras (Crioceras) Duvallei, Leveille. Middle and Lower Neocomian. One-fifth natural size.)

At Tealby, a village in the Lincolns.h.i.+re Wolds, there crop out beneath the white chalk some non-fossiliferous ferruginous sands about twenty-feet thick, beneath which are beds of clay and limestone, about fifty feet thick, with an interesting suite of fossils, among which are Pecten cinctus (Figure 285), from 9 to 12 inches in diameter, Ancyloceras Duvallei (Figure 286), and some forty other sh.e.l.ls, many of them common to the Middle Speeton clay, about to be mentioned. Mr. Judd remarks that as Ammonites clypei-formis and Terebratula hippopus characterise the Middle Neocomian of the Continent, it is to this stage that the Tealby series containing the same fossils may be a.s.signed. (Judd Quarterly Geological Journal 1867 volume 23 page 249.)

The middle division of the Speeton clay, occurring at Speeton below the cement- bed, before alluded to, is 150 feet thick, and contains about 39 species of mollusca, half of which are common to the overlying clay. Among the peculiar sh.e.l.ls, Pecten cinctus (Figure 285) and Ancyloceras (Crioceras) Duvallei (Figure 286) occur.

LOWER NEOCOMIAN.

(FIGURE 287. Ammonites Noricus, Schloth. Lower Neocomian, Speeton.)

In the lower division of the Speeton clay, 200 feet thick, 46 species of mollusca have been found, and three divisions, each characterised by its peculiar ammonite, have been noticed by Mr. Judd. The central zone is marked by Ammonites Noricus (see Figure 287). On the Continent these beds are well-known by their corresponding fossils, the Hils clay and conglomerate of the north of Germany agreeing with the Middle and Lower Speeton, the latter of which, with the same mineral characters and fossils as in Yorks.h.i.+re, is also found in the little island of Heligoland. Yellow limestone, which I have myself seen near Neuchatel, in Switzerland, represents the Lower Neocomian at Speeton.

WEALDEN FORMATION.

Beneath the Atherfield clay or Upper Neocomian of the S.E. of England, a fresh- water formation is found, called the Wealden, which, although it occupies a small horizontal area in Europe, as compared to the White Chalk and the marine Neocomian beds, is nevertheless of great geological interest, since the imbedded remains give us some insight into the nature of the terrestrial fauna and flora of the Lower Cretaceous epoch. The name of Wealden was given to this group because it was first studied in parts of Kent, Surrey, and Suss.e.x, called the Weald; and we are indebted to Dr. Mantell for having shown, in 1822, in his "Geology of Suss.e.x," that the whole group was of fluviatile origin. In proof of this he called attention to the entire absence of Ammonites, Belemnites, Brachiopoda, Echinodermata, Corals, and other marine fossils, so characteristic of the Cretaceous rocks above, and of the Oolitic strata below, and to the presence in the Weald of Paludinae, Melaniae, Cyrenae, and various fluviatile sh.e.l.ls, as well as the bones of terrestrial reptiles and the trunks and leaves of land-plants.

(FIGURE 288. Section from (left) W.S.W. through Brixton bay, Isle of Wight, Solent and South Downs to E.N.E. (right).

1. Tertiary.

2. Chalk and Gault.

3. Upper Neocomian (or Lower Greensand).

4. Wealden (Weald Clay and Hastings Sands).)

The evidence of so unexpected a fact as that of a dense ma.s.s of purely fresh- water origin underlying a deep-sea deposit (a phenomenon with which we have since become familiar) was received, at first, with no small doubt and incredulity. But the relative position of the beds is unequivocal; the Weald Clay being distinctly seen to pa.s.s beneath the Atherfield Clay in various parts of Surrey, Kent, and Suss.e.x, and to reappear in the Isle of Wight at the base of the Cretaceous series, being, no doubt, continuous far beneath the surface, as indicated by the dotted lines in Figure 288. They are also found occupying the same relative position below the chalk in the peninsula of Purbeck, Dorsets.h.i.+re, where, as we shall see in the sequel, they repose on strata referable to the Upper Oolite.

WEALD CLAY.

The Upper division, or Weald Clay, is, in great part, of fresh-water origin, but in its highest portion contains beds of oysters and other marine sh.e.l.ls which indicate fluvio-marine conditions. The uppermost beds are not only conformable, as Dr. Fitton observes, to the inferior strata of the overlying Neocomian, but of similar mineral composition. To explain this, we may suppose that, as the delta of a great river was tranquilly subsiding, so as to allow the sea to encroach upon the s.p.a.ce previously occupied by fresh-water, the river still continued to carry down the same sediment into the sea. In confirmation of this view it may be stated that the remains of the Iguanodon Mantelli, a gigantic terrestrial reptile, very characteristic of the Wealden, has been discovered near Maidstone, in the overlying Kentish Rag, or marine limestone of the Upper Neocomian. Hence we may infer that some of the saurians which inhabited the country of the great river continued to live when part of the district had become submerged beneath the sea. Thus, in our own times, we may suppose the bones of large alligators to be frequently entombed in recent fresh-water strata in the delta of the Ganges. But if part of that delta should sink down so as to be covered by the sea, marine formations might begin to acc.u.mulate in the same s.p.a.ce where fresh-water beds had previously been formed; and yet the Ganges might still pour down its turbid waters in the same direction, and carry seaward the carca.s.ses of the same species of alligator, in which case their bones might be included in marine as well as in subjacent fresh-water strata.

(FIGURES 289 AND 290. Tooth of Iguanodon Mantelli.

(FIGURE 289. a, and b.)

(FIGURE 290. A. Partially worn tooth of young individual of the same.

b. Crown of tooth in adult worn down. (Mantell.)))