The Student's Elements of Geology Part 28

Many of the marine sh.e.l.ls of the brackish-water beds of the above series, both in the Isle of Wight and Hordwell Cliff, are common to the underlying Barton Clay: and, on the other hand, there are some fresh-water sh.e.l.ls, such as Cyrena obovata, which are common to the Bembridge beds, notwithstanding the intervention of the St. Helen's series. The white and green marls of the Headon series, and some of the accompanying limestones, often resemble the Eocene strata of France in mineral character and colour in so striking a manner as to suggest the idea that the sediment was derived from the same region or produced contemporaneously under very similar geographical circ.u.mstances.

(FIGURE 180. Solenastraea cellulosa, Duncan. Brockenhurst.)

At Brockenhurst, near Lyndhurst, in the New Forest, marine strata have recently been found containing fifty-nine sh.e.l.ls, of which many have been described by Mr. Edwards. These beds rest on the Lower Headon, and are considered as the equivalent of the middle part of the Headon series, many of the sh.e.l.ls being common to the brackish-water or Middle Headon beds of Colwell and Whitecliff Bays, such as Cancellaria muricata, Sowerby, Fusus l.a.b.i.atus, Sowerby, etc. In these beds at Brockenhurst, corals, ably described by Dr. Duncan, have recently been found in abundance and perfection; see Figure 180, Solenastraea cellulosa.

Baron von Konen has pointed out that no less than forty-six out of the fifty- nine Brockenhurst sh.e.l.ls, or a proportion of 78 per cent, agree with species occurring in Dumont's Lower Tongrian formation in Belgium. (Quarterly Geological Journal volume 20 page 97 1864.) This being the case, we might fairly expect that if we had a marine equivalent of the Bembridge series or of the contemporaneous Paris gypsum, we should find it to contain a still greater number of sh.e.l.ls common to the Tongrian beds of Belgium, but the exact correlation of these fresh-water groups of France, Belgium, and Britain has not yet been fully made out. It is possible that the Tongrian of Dumont may be newer than the Bembridge series, and therefore referable to the Lower Miocene. If ever the whole series should be complete, we must be prepared to find the marine equivalent of the Bembridge beds, or the uppermost Eocene, pa.s.sing by imperceptible shades into the inferior beds of the overlying Miocene strata.

Among the fossils found in the Middle Headon are Cytherea incra.s.sata and Cerithium plicatum (Figure 160 Chapter 15). These sh.e.l.ls, especially the latter, are very characteristic of the Lower Miocene, and their occurrence in the Headon series has been cited as an objection to the line proposed to be drawn between Miocene and Eocene. But if we were to attach importance to such occasional pa.s.sages, we should soon find that no lines of division could be drawn anywhere, for in the present state of our knowledge of the Tertiary series there will always be species common to beds above and below our boundary-lines.

BARTON SERIES (SANDS AND CLAYS), A.4 TABLE 16.1.)

(FIGURE 181. Chama squamosa, Eichw. Barton.)

Both in the Isle of Wight, and in Hordwell Cliff, Hants, the Headon beds, above- mentioned, rest on white sands usually devoid of fossils, and used in the Isle of Wight for making gla.s.s. In one of these sands Dr. Wright found Chama squamosa, a Barton Clay sh.e.l.l, in great plenty, and certain impressions of marine sh.e.l.ls have been found in sands supposed to be of the same age in Whitecliff Bay. These sands have been called Upper Bagshot in the maps of our Government Survey, but this identification of a fossiliferous series in the Isle of Wight with an unfossiliferous formation in the London Basin can scarcely be depended upon. The Barton Clay, which immediately underlies these sands, is seen vertical in Alum Bay, Isle of Wight, and nearly horizontal in the cliffs of the mainland near Lymington. This clay, together with the Bracklesham beds, presently to be described, has been termed Middle Bagshot by the Survey. In Barton Cliff, where it attains a thickness of about 300 feet, it is rich in marine fossils.

It was formerly confounded with the London Clay, an older Eocene deposit of very similar mineral character, to be mentioned below, which contains many sh.e.l.ls in common, but not more than one-fourth of the whole. In other words, there are known at present 247 species in the London Clay and 321 in that of Barton, and only 70 common to the two formations. Fifty-six of these have been found in the intermediate Bracklesham beds, and the reappearance of the other 14 may imply a return of similar conditions, whether of temperature or depth or of a muddy argillaceous bottom, common to the two periods of the London and Barton Clays.

According to M. Hebert, the most characteristic Barton Clay fossils correspond to those of the Gres de Beauchamp, or Sables Moyens, of the Paris Basin, but it also contains many common to the older Calcaire Grossier.

Sh.e.l.lS OF THE BARTON CLAY.

(FIGURE 182. Mitra scabra, Sowerby.)

(FIGURE 183. Voluta ambigua, Sol.)

(FIGURE 184. Typhis pungens, Brand.)

(FIGURE 185. Voluta athleta, Sol. Barton and Bracklesham.)

(FIGURE 186. Terebellum fusiforme, Lam. Barton and Bracklesham.)

(FIGURE 187. Terebellum sopita, Brand.)

(FIGURE 188. Cardita sulcata, Brand. Barton.)

(FIGURE 189. Cra.s.satella sulcata, Sowerby. Bracklesham and Barton.)

(FIGURE 190. Nummulites variolaria, Lam. Var. of Nummulites radiata, Sowerby.

Middle Eocene, Bracklesham Bay.

a. Natural size.

b. Magnified.)

Certain foraminifera called Nummulites begin, when we study the Tertiary formations in a descending order, to make their first appearance in these beds.

A small species called Nummulites variolaria, Figure 190, is found both on the Hamps.h.i.+re coast and in beds of the same age in Whitecliff Bay, in the Isle of Wight. Several marine sh.e.l.ls, such as Corbula pisum (Figure 158), are common to the Barton beds and the Hempstead or Lower Miocene series, and a still greater number, as before stated, are common to the Headon series.

MIDDLE EOCENE, ENGLAND.

BRACKLESHAM BEDS AND BAGSHOT SANDS (B.1, TABLE 16.1).

(FIGURE 191. Cardita (Venericardia) planicosta, Lam.)

(FIGURE 192. Nummulites (Nummularia) laevigata. Bracklesham. Dixon's Fossils of Suss.e.x, Plate 8.

a. Section of nummulite.

b. Group, with an individual showing the exterior of the sh.e.l.l.)

Beneath the Barton Clay we find in the north of the Isle of Wight, both in Alum and Whitecliff Bays, a great series of various coloured sands and clays for the most part unfossiliferous, and probably of estuarine origin. As some of these beds contain Cardita planicosta (Figure 191) they have been identified with the marine beds much richer in fossils seen in the coast section in Bracklesham Bay near Chichester in Suss.e.x, where the strata consist chiefly of green clayey sands with some lignite. Among the Bracklesham fossils besides the Cardita, the huge Cerithium giganteum is seen, so conspicuous in the Calcaire Grossier of Paris, where it is sometimes two feet in length. The Nummulites laevigata (see Figure 192), so characteristic of the lower beds of the Calcaire Grossier in France, where it sometimes forms stony layers, as near Compiegne, is very common in these beds, together with Nummulites scabra and Nummulites variolaria. Out of 193 species of testacea procured from the Bagshot and Bracklesham beds in England, 126 occur in the Calcaire Grossier in France. It was clearly, therefore, coeval with that part of the Parisian series more nearly than with any other.

(FIGURE 193. Palaeophis typhoeus, Owen; an Eocene sea-serpent. Bracklesham.

a, b. Vertebra, with long neural spine preserved.

c. Two vertebrae articulated together.)

(FIGURE 194. Defensive spine of Ostracion. Bracklesham.)

(FIGURE 195. Dental plates of Myliobates Edwardsi. Bracklesham Bay.

Dixon's Fossils of Suss.e.x, Plate 8.)

According to tables compiled from the best authorities by Mr. Etheridge, the number of mollusca now known from the Bracklesham beds in Great Britain is 393, of which no less than 240 are peculiar to this subdivision of the British Eocene series, while 70 are common to the Older London Clay, and 140 to the Newer Barton Clay. The volutes and cowries of this formation, as well as the lunulites and corals, favour the idea of a warm climate having prevailed, which is borne out by the discovery of a serpent, Palaeophis typhoeus (see Figure 193), exceeding, according to Professor Owen, twenty feet in length, and allied in its osteology to the Boa, Python, Coluber, and Hydrus. The compressed form and diminutive size of certain caudal vertebrae indicate so much a.n.a.logy with Hydrus as to induce Professor Owen to p.r.o.nounce this extinct ophidian to have been marine. (Palaeontological Society Monograph Reptiles part 2 page 61.) Among the companions of the sea-snake of Bracklesham was an extinct crocodile (Gavialis Dixoni, Owen), and numerous fish, such as now frequent the seas of warm lat.i.tudes, as the Ostracion of the family Balistidae, of which a dorsal spine is figured (see Figure 194), and gigantic rays of the genus Myliobates (see Figure 195).

(FIGURE 196. Carcharodon angustidens, Aga.s.siz.)

(FIGURE 197. Otodus obliquus, Aga.s.siz.)

(FIGURE 198. Lamna elegans, Aga.s.siz.)

(FIGURE 199. Galeocerdo latidens, Aga.s.siz.)

The teeth of sharks also, of the genera Carcharodon, Otodus, Lamna, Galeocerdo, and others, are abundant. (See Figures 196, 197, 198, 199.)

MARINE Sh.e.l.lS OF BRACKLESHAM BEDS.

ALUM BAY AND BOURNEMOUTH BEDS. (LOWER BAGSHOT OF ENGLISH SURVEY), B.2, TABLE 16.1.)

(FIGURE 200. Pleurotoma attenuata, Sowerby.)

(FIGURE 201. Voluta Selseiensis, Edwards.)

(FIGURE 202. Turritella multisulcata, Lam.)

(FIGURE 203. Lucina serrata, Sowerby. Magnified.)

(FIGURE 204. Conus deperditus, Brug.)

To that great series of sands and clays which intervene between the equivalents of the Bracklesham Beds and the London Clay or Lower Eocene, our Government Survey has given the name of the Lower Bagshot sands, for they are supposed to agree in age with the inferior unfossiliferous sands of the country round Bagshot in the London Basin. This part of the series is finely exposed in the vertical beds of Alum bay, in the Isle of Wight, and east and west of Bournemouth, on the south coast of Hamps.h.i.+re. In some of the close and white compact clays of this locality, there are not only dicotyledonous leaves, but numerous fronds of ferns allied to Gleichenia which are well preserved with their fruit.

None of the beds are of great horizontal extent, and there is much cross- stratification in the sands, and in some places black carbonaceous seams and lignite. In the midst of these leaf-beds in Studland Bay, Purbeck sh.e.l.ls of the genus Unio attest the fresh-water origin of the white clay.

No less than forty species of plants are mentioned by MM. de la Harpe and Gaudin from this formation in Hamps.h.i.+re, among which the Proteaceae (Dryandra, etc.) and the fig tribe are abundant, as well as the cinnamon and several other laurineae, with some papilionaceous plants. On the whole, they remind the botanist of the types of subtropical India and Australia. (Heer Climat et Vegetation du Pays Tertiaire page 172.)

Heer has mentioned several species which are common to this Alum Bay flora and that of Monte Bolca, near Verona, so celebrated for its fossil fish, and where the strata contain nummulites and other Middle Eocene fossils. He has particularly alluded to Aralia primigenia (of which genus a fruit has since been found by Mr. Mitch.e.l.l at Bournemouth), Daphnogene Veronensis, and Ficus granadilla, as among the species common to and characteristic of the Isle of Wight and Italian Eocene beds; and he observes that in the flora of this period these forms of a temperate climate which const.i.tute a marked feature in the European Miocene formations, such as the willow, poplar, birch, alder, elm, hornbeam, oak, fir, and pine, are wanting. The American types are also absent, or much more feebly represented than in the Miocene period, although fine specimens of the fan-palm (Sabal) have been found in these Eocene clays at Studland. The number of exotic forms which are common to the Eocene and Miocene strata of Europe, like those to be alluded to in the sequel which are common to the Eocene and Cretaceous fauna, demonstrate the remoteness of the times in which the geographical distribution of living plants originated. A great majority of the Eocene genera have disappeared from our temperate climates, but not the whole of them; and they must all have exerted some influence on the a.s.semblages of species which succeeded them. Many of these last occurring in the Upper Miocene are indeed so closely allied to the flora now surviving as to make it questionable, even in the opinion of naturalists opposed to the doctrine of trans.m.u.tation, whether they are not genealogically related the one to the other.