The Student's Elements of Geology Part 27

COLUMN 1: NAME OF STRATA.

COLUMN 2: ENGLISH SUBDIVISIONS.

COLUMN 3: FRENCH EQUIVALENTS.

UPPER EOCENE.

A.1: Bembridge series, Isle of Wight: Gypseous series of Montmartre.

A.2: Osborne or St. Helen's series, Isle of Wight: Calcaire siliceux, or Travertin Inferieur.

A.3: Headon series, Isle of Wight: Calcaire siliceux, or Travertin Inferieur.

A.4: Barton series. Sands and clays of Barton Cliff, Hants: Gres de Beauchamp, or Sables Moyens.

MIDDLE EOCENE.

B.1: Bracklesham series: Calcaire Grossier.

B.2: Alum Bay and Bournemouth beds: Wanting in France?

B.2: Wanting in England?: Soissonnais Sands, or Lits Coquilliers.

LOWER EOCENE.

C.1: London Clay: Argile de Londres, Ca.s.sel, near Dunkirk.

C.2: Woolwich and Reading series: Argile plastique and lignite.

C.3: Thanet sands: Sables de Bracheux.

It is in the northern part of the Isle of Wight that we have the uppermost beds of the true Eocene best exhibited-- namely, those which correspond in their fossils with the celebrated gypsum of the Paris basin before alluded to in Chapter 15 (see Table 16.1). That gypsum has been selected by almost all Continental geologists as affording the best line of demarkation between the Middle and Lower Tertiary, or, in other words, between the Lower Miocene and Eocene formations.

In reference to Table 16.1 I may observe, that the correlation of the French and English subdivisions here laid down is often a matter of great doubt and difficulty, notwithstanding their geographical proximity. This arises from various circ.u.mstances, partly from the former prevalence of marine conditions in one basin simultaneously with fluviatile or lacustrine in the other, and sometimes from the existence of land in one area causing a break or absence of all records during a period when deposits may have been in progress in the other basin. As bearing on this subject, it may be stated that we have unquestionable evidence of oscillations of level shown by the superposition of salt or brackish-water strata to fluviatile beds; and those of deep-sea origin to strata formed in shallow water. Even if the upward and downward movements were uniform in amount and direction, which is very improbable, their effect in producing the conversion of sea into land or land into sea would be different, according to the previous shape and varying elevation of the land and bottom of the sea.

Lastly, denudation, marine and subaerial, has frequently caused the absence of deposits in one basin of corresponding age to those in the other, and this destructive agency has been more than ordinarily effective on account of the loose and unconsolidated nature of the sands and clays.

UPPER EOCENE OF ENGLAND.

BEMBRIDGE SERIES, A.1.

These beds are about 120 feet thick, and, as stated in Chapter 15, lie immediately under the Hempstead beds, near Yarmouth, in the Isle of Wight, being conformable with those Lower Miocene strata. They consist of marls, clays, and limestones of fresh-water, brackish, and marine origin. Some of the most abundant sh.e.l.ls, as Cyrena semistriata var., and Paludina lenta, Figure 163 Chapter 15, are common to this and to the overlying Hempstead series; but the majority of the species are distinct. The following are the subdivisions described by the late Professor Forbes:

(FIGURE 165. Melania turritissima, Forbes. Bembridge.)

a. Upper marls, distinguished by the abundance of Melania turritissima, Forbes (Figure 165).

(FIGURE 166. Fragment of carapace of Trionyx. Bembridge Beds, Isle of Wight.)

b. Lower marls, characterised by Cerithium mutabile, Cyrena pulchra, etc., and by the remains of Trionyx (see Figure 166).

c. Green marls, often abounding in a peculiar species of oyster, and accompanied by Cerithium, Mytilus, Arca, nucula, etc.)

(FIGURE 167. Bulimus ellipticus, Sowerby. Bembridge Limestone. 1/2 natural size.)

(FIGURE 168. Helix occlusa, Edwards. Bembridge Limestone, Isle of Wight.)

(FIGURE 169. Paludina orbicularis. Bembridge.)

(FIGURE 170. Planorbis discus, Edwards. Bembridge. 1/2 diameter.)

(FIGURE 171. Lymnea longiscata, Brand. Natural size.)

(FIGURE 172. Chara tuberculata, seed-vessel. Bembridge Limestone, Isle of Wight.)

d. Bembridge limestones, compact cream-coloured limestones alternating with shales and marls, in all of which land-sh.e.l.ls are common, especially at Sconce, near Yarmouth, as described by Mr. F. Edwards. The Bulimus ellipticus, Figure 167, and Helix occlusa, Figure 168, are among its best known land-sh.e.l.ls.

Paludina orbicularis, Figure 169, is also of frequent occurrence. One of the bands is filled with a little globular Paludina. Among the fresh-water pulmonifera, Lymnea longiscata (Figure 171) and Planorbis discus (Figure 170) are the most generally distributed: the latter represents or takes the place of the Planorbis euomphalus (see Figure 175) of the more ancient Headon series.

Chara tuberculata (Figure 172) is the characteristic Bembridge gyrogonite or seed-vessel.

(FIGURE 173. Anoplotherium commune. Binstead, Isle of Wight.

Lower molar tooth, natural size.)

(FIGURE 174. Palaeotherium magnum, Cuvier.)

(FIGURE 175. Planorbis euomphalus, Sowerby. Headon Hill. 1/2 diameter.)

From this formation on the sh.o.r.es of Whitecliff Bay, Dr. Mantell obtained a fine specimen of a fan palm, Flabellaria Lamanonis, Brong., a plant first obtained from beds of corresponding age in the suburbs of Paris. The well-known building- stone of Binstead, near Ryde, a limestone with numerous hollows caused by Cyrenae which have disappeared and left the moulds of their sh.e.l.ls, belongs to this subdivision of the Bembridge series. In the same Binstead stone Mr. Pratt and the Reverend Darwin Fox first discovered the remains of mammalia characteristic of the gypseous series of Paris, as Palaeotherium magnum (Figure 174), Palaeotherium medium, Palaeotherium minus, Palaeotherium minimum, Palaeotherium curtum, Palaeotherium cra.s.sum; also Anoplotherium commune (Figure 173), Anoplotherium secundarium, Dichobune cervinum, and Chaeropotamus Cuvieri.

The Palaeothere above alluded to resembled the living tapir in the form of the head, and in having a short proboscis, but its molar teeth were more like those of the rhinoceros. Palaeotherium magnum was of the size of a horse, three or four feet high. The woodcut, Figure 174, is one of the restorations which Cuvier attempted of the outline of the living animal, derived from the study of the entire skeleton. As the vertical range of particular species of quadrupeds, so far as our knowledge extends, is far more limited than that of the testacea, the occurrence of so many species at Binstead, agreeing with fossils of the Paris gypsum, strengthens the evidence derived from sh.e.l.ls and plants of the synchronism of the two formations.

OSBORNE OR ST. HELEN'S SERIES, A.2.

This group is of fresh and brackish-water origin, and very variable in mineral character and thickness. Near Ryde, it supplies a freestone much used for building, and called by Professor Forbes the Nettlestone grit. In one part ripple-marked flagstones occur, and rocks with fucoidal markings. The Osborne beds are distinguished by peculiar species of Paludina, Melania, and Melanopsis, as also of Cypris and the seeds of Chara.

HEADON SERIES A.3.

These beds are seen both in Whitecliff Bay, Headon Hill, and Alum Bay, or at the east and west extremities of the Isle of Wight. The upper and lower portions are fresh-water, and the middle of mixed origin, sometimes brackish and marine.

Everywhere Planorbis euomphalus, Figure 175, characterises the fresh-water deposits, just as the allied form, Planorbis discus, Figure 170, does the Bembridge limestone. The brackish-water beds contain Potamomya plana, Cerithium mutabile, and Potamides cinctus (Figure 37 Chapter 3), and the marine beds Venus (or Cytherea) incra.s.sata, a species common to the Limburg beds and Gres de Fontainebleau, or the Lower Miocene series. The prevalence of salt-water remains is most conspicuous in some of the central parts of the formation.

(FIGURE 176. Helix labyrinthica, Say. Headon Hill, Isle of Wight; and Hordwell Cliff, Hants-- also recent.)

(FIGURE 177. Neritina concava, Sowerby. Headon series.)

(FIGURE 178. Lymnea caudata, Edw. Headon series.)

(FIGURE 179. Cerithium concavum, Sowerby. Headon series.)

Among the sh.e.l.ls which are widely distributed through the Headon series are Neritina concava (Figure 177), Lymnea caudata (Figure 178), and Cerithium concavum (Figure 179). Helix labyrinthica, Say (Figure 176), a land-sh.e.l.l now inhabiting the United States, was discovered in this series by Mr. Searles Wood in Hordwell Cliff. It is also met with in Headon Hill, in the same beds. At Sconce, in the Isle of Wight, it occurs in the Bembridge series, and affords a rare example of an Eocene fossil of a species still living, though, as usual in such cases, having no local connection with the actual geographical range of the species. The lower and middle portion of the Headon series is also met with in Hordwell Cliff (or Hordle, as it is often spelt), near Lymington, Hants. Among the sh.e.l.ls which abound in this cliff are Paludina lenta and various species of Lymnea, Planorbis, Melania, Cyclas, Unio, Potamomya, Dreissena, etc.

Among the chelonians we find a species of Emys, and no less than six species of Trionyx; among the saurians an alligator and a crocodile; among the ophidians two species of land-snakes (Paleryx, Owen); and among the fish Sir P. Egerton and Mr. Wood have found the jaws, teeth, and hard s.h.i.+ning scales of the genus Lepidosteus, or bony pike of the American rivers. This same genus of fresh-water ganoids has also been met with in the Hempstead beds in the Isle of Wight. The bones of several birds have been obtained from Hordwell, and the remains of quadrupeds of the genera Palaeotherium (Palaeotherium minus), Anoplotherium, Anthracotherium, Dichodon, Dichobune, Spalacodon, and Hyaenodon. The latter offers, I believe, the oldest known example of a true carnivorous animal in the series of British fossils, although I attach very little theoretical importance to the fact, because herbivorous species are those most easily met with in a fossil state in all save cavern deposits. In another point of view, however, this fauna deserves notice. Its geological position is considerably lower than that of the Bembridge or Montmartre beds, from which it differs almost as much in species as it does from the still more ancient fauna of the Lower Eocene beds to be mentioned in the sequel. It therefore teaches us what a grand succession of distinct a.s.semblages of mammalia flourished on the earth during the Eocene period.