The Student's Elements of Geology Part 37

Great Plants of the Oolite.

Oolite and Bradford Clay.

Stonesfield Slate.

Fossil Mammalia.

Fuller's Earth.

Inferior Oolite and Fossils.

Northamptonshire Slates.

Yorkshire Oolitic Coal-field.

Brora Coal.

Palaeontological Relations of the several Subdivisions of the Oolitic group.

CLa.s.sIFICATION OF THE OOLITE.

Immediately below the Hastings Sands we find in Dorsetshire another remarkable fresh-water formation, called THE PURBECK, because it was first studied in the sea-cliffs of the peninsula of Purbeck in that county. These beds are for the most part of fresh-water origin, but the organic remains of some few intercalated beds are marine, and show that the Purbeck series has a closer affinity to the Oolitic group, of which it may be considered as the newest or uppermost member.

In England generally, and in the greater part of Europe, both the Wealden and Purbeck beds are wanting, and the marine cretaceous group is followed immediately, in the descending order, by another series called the Jura.s.sic. In this term, the formations commonly designated as "the Oolite and Lias" are included, both being found in the Jura Mountains. The Oolite was so named because in the countries where it was first examined the limestones belonging to it had an Oolitic structure (see Chapter 3). These rocks occupy in England a zone nearly thirty miles in average breadth, which extends across the island, from Yorkshire in the north-east, to Dorsetshire in the south-west. Their mineral characters are not uniform throughout this region; but the following are the names of the princ.i.p.al subdivisions observed in the central and south- eastern parts of England.

TABLE 19.1. OOLITE.

UPPER OOLITE: a. Purbeck beds.

b. Portland stone and sand.

c. Kimmeridge clay.

MIDDLE OOLITE: d. Coral rag.

e. Oxford clay, and Kelloway rock.

LOWER OOLITE: f. Cornbrash and Forest marble.

g. Great Oolite and Stonesfield slate.

h. Fuller's earth.

i. Inferior Oolite.

The Upper Oolitic system of the Table 19.1 has usually the Kimmeridge clay for its base; the Middle Oolitic system, the Oxford clay. The Lower system reposes on the Lias, an argillo-calcareous formation, which some include in the Lower Oolite, but which will be treated of separately in the next chapter. Many of these subdivisions are distinguished by peculiar organic remains; and, though varying in thickness, may be traced in certain directions for great distances, especially if we compare the part of England to which the above-mentioned type refers with the north-east of France and the Jura Mountains adjoining. In that country, distant above 400 geographical miles, the a.n.a.logy to the accepted English type, notwithstanding the thinness or occasional absence of the clays, is more perfect than in Yorkshire or Normandy.

PHYSICAL GEOGRAPHY.

The alternation, on a grand scale, of distinct formations of clay and limestone has caused the oolitic and lia.s.sic series to give rise to some marked features in the physical outline of parts of England and France. Wide valleys can usually be traced throughout the long bands of country where the argillaceous strata crop out; and between these valleys the limestones are observed, forming ranges of hills or more elevated grounds. These ranges terminate abruptly on the side on which the several clays rise up from beneath the calcareous strata.

(FIGURE 298. Section through Lias (left), Lower Oolite, Oxford Clay, Middle Oolite, Kim. Clay. Upper Oolite. Gault, Chalk and London Clay (right).)

Figure 298 will give the reader an idea of the configuration of the surface now alluded to, such as may be seen in pa.s.sing from London to Cheltenham, or in other parallel lines, from east to west, in the southern part of England. It has been necessary, however, in this drawing, greatly to exaggerate the inclination of the beds, and the height of the several formations, as compared to their horizontal extent. It will be remarked, that the lines of steep slope, or escarpment, face towards the west in the great calcareous eminences formed by the chalk and the Upper, Middle, and Lower Oolites; and at the base of which we have respectively the Gault, Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a broad vale at the foot of the escarpment of inferior Oolite, but where it acquires considerable thickness, and contains solid beds of marlstone, it occupies the lower part of the escarpment.

The external outline of the country which the geologist observes in travelling eastward from Paris to Metz, is precisely a.n.a.logous, and is caused by a similar succession of rocks intervening between the tertiary strata and the Lias; with this difference, however, that the escarpments of Chalk, Upper, Middle, and Lower Oolites face towards the east instead of the west. It is evident, therefore, that the denuding causes (see Chapter 6) have acted similarly over an area several hundred miles in diameter, removing the softer clays more extensively than the limestones, and causing these last to form steep slopes or escarpments wherever the harder calcareous rock was based upon a more yielding and destructible formation.

UPPER OOLITE.

PURBECK BEDS.

These strata, which we cla.s.s as the uppermost member of the Oolite, are of limited geographical extent in Europe, as already stated, but they acquire importance when we consider the succession of three distinct sets of fossil remains which they contain. Such repeated changes in organic life must have reference to the history of a vast lapse of ages. The Purbeck beds are finely exposed to view in Durdlestone Bay, near Swanage, Dorsetshire, and at Lulworth Cove and the neighbouring bays between Weymouth and Swanage. At Meup's Bay, in particular, Professor E. Forbes examined minutely, in 1850, the organic remains of this group, displayed in a continuous sea-cliff section, and it appears from his researches that the Upper, Middle, and Lower Purbecks are each marked by peculiar species of organic remains, these again being different, so far as a comparison has yet been inst.i.tuted, from the fossils of the overlying Hastings Sands and Weald Clay.

UPPER PURBECK.

(FIGURE 299. Cyprides from the Upper Purbeck.

a. Cypris gibbosa, E. Forbes.

b. Cypris tuberculata, E. Forbes.

c. Cypris leguminella, E. Forbes.)

The highest of the three divisions is purely fresh-water, the strata, about fifty feet in thickness, containing sh.e.l.ls of the genera Paludina, Physa, Limnaea, Planorbis, Valvata, Cyclas, and Unio, with Cyprides and fish. All the species seem peculiar, and among these the Cyprides are very abundant and characteristic (see Figure 299, a, b, c.)

The stone called "Purbeck Marble," formerly much used in ornamental architecture in the old English cathedrals of the southern counties, is exclusively procured from this division.

MIDDLE PURBECK.

Next in succession is the Middle Purbeck, about thirty feet thick, the uppermost part of which consists of fresh-water limestone, with cyprides, turtles, and fish, of different species from those in the preceding strata. Below the limestone are brackish-water beds full of Cyrena, and traversed by bands abounding in Corbula and Melania. These are based on a purely marine deposit, with Pecten, Modiola, Avicula, and Thracia. Below this, again, come limestones and shales, partly of brackish and partly of fresh-water origin, in which many fish, especially species of Lepidotus and Microdon radiatus, are found, and a crocodilian reptile named Macrorhynchus. Among the mollusks, a remarkable ribbed Melania, of the section Chilina, occurs.

(FIGURE 300. Ostrea distorta, Sowerby. Cinder-bed. Middle Purbeck.)

(FIGURE 301. Hemicidaris Purbeckensis, E. Forbes. Middle Purbeck.)

(FIGURE 302. Cyprides from the Middle Purbecks.

a. Cypris striato-punctata, E. Forbes.

b. Cypris fasciculata, E. Forbes.

c. Cypris granulata, Sowerby.)

(FIGURE 303. Physa Bristovii, E. Forbes. Middle Purbeck.)

Immediately below is a great and conspicuous stratum, twelve feet thick, formed of a vast acc.u.mulation of sh.e.l.ls of Ostrea distorta (Figure 300), long familiar to geologists under the local name of "Cinder-bed." In the uppermost part of this bed Professor Forbes discovered the first echinoderm (Figure 301) as yet known in the Purbeck series, a species of Hemicidaris, a genus characteristic of the Oolitic period, and scarcely, if at all, distinguishable from a previously known Oolitic fossil. It was accompanied by a species of Perna. Below the Cinder-bed fresh-water strata are again seen, filled in many places with species of Cypris (Figure 302, a, b, c), and with Valvata, Paludina, Planorbis, Limnaea, Physa (Figure 303), and Cyclas, all different from any occurring higher in the series. It will be seen that Cypris fasciculata (Figure 302, b) has tubercles at the end only of each valve, a character by which it can be immediately recognised. In fact, these minute crustaceans, almost as frequent in some of the shales as plates of mica in a micaceous sandstone, enable geologists at once to identify the Middle Purbeck in places far from the Dorsetshire cliffs, as, for example, in the Vale of Wardour in Wiltshire. Thick beds of chert occur in the Middle Purbeck filled with mollusca and cyprides of the genera already enumerated, in a beautiful state of preservation, often converted into chalcedony. Among these Professor Forbes met with gyrogonites (the spore-vessels of Chara), plants never until 1851 discovered in rocks older than the Eocene.

About twenty feet below the "Cinder-bed" is a stratum two or three inches thick, in which fossil mammalia presently to be mentioned occur, and beneath this a thin band of greenish shales, with marine sh.e.l.ls and impressions of leaves like those of a large Zostera, forming the base of the Middle Purbeck.

FOSSIL MAMMALIA OF THE MIDDLE PURBECK.

In 1852, after alluding to the discovery of numerous insects and air-breathing mollusca in the Purbeck strata, I remarked that, although no mammalia had then been found, "it was too soon to infer their non-existence on mere negative evidence." (Elements of Geology 4th edition.) Only two years after this remark was in print, Mr. W.R. Brodie found in the Middle Purbeck, about twenty feet below the "Cinder-bed" above alluded to, in Durdlestone Bay, portions of several small jaws with teeth, which Professor Owen recognised as belonging to a small mammifer of the insectivorous cla.s.s, more closely allied in its dent.i.tion to the Amphitherium (or Thylacotherium) than to any existing type.

Four years later (in 1856) the remains of several other species of warm-blooded quadrupeds were exhumed by Mr. S.H. Beckles, F.R.S., from the same thin bed of marl near the base of the Middle Purbeck. In this marly stratum many reptiles, several insects, and some fresh-water sh.e.l.ls of the genera Paludina, Planorbis, and Cyclas, were found.

Mr. Beckles had determined thoroughly to explore the thin layer of calcareous mud from which in the suburbs of Swanage the bones of the Spalacotherium had already been obtained, and in three weeks he brought to light from an area forty feet long and ten wide, and from a layer the average thickness of which was only five inches, portions of the skeletons of six new species of mammalia, as interpreted by Dr. Falconer, who first examined them. Before these interesting inquiries were brought to a close, the joint labours of Professor Owen and Dr.

Falconer had made it clear that twelve or more species of mammalia characterised this portion of the Middle Purbeck, most of them insectivorous or predaceous, varying in size from that of a mole to that of the common polecat, Mustela putorius. While the majority had the character of insectivorous marsupials, Dr.

Falconer selected one as differing widely from the rest, and pointed out that in certain characters it was allied to the living Kangaroo-rat, or Hypsiprymnus, ten species of which now inhabit the prairies and scrub-jungle of Australia, feeding on plants, and gnawing scratched-up roots. A striking peculiarity of their dent.i.tion, one in which they differ from all other quadrupeds, consists in their having a single large pre-molar, the enamel of which is furrowed with vertical grooves, usually seven in number.

(FIGURE 304. Pre-molar of the recent Australian Hypsiprymnus Gaimardi, showing 7 grooves, at right angles to the length of the jaw, magnified 3 1/2 diameters.)

(FIGURE 305. Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii, magnified 5 1/2 diameters, showing 7 diagonal grooves.)

(FIGURE 306. Plagiaulex Becklesii, Falconer. Middle Purbeck. Right ramus of lower jaw, magnified two diameters.

a. Incisor.

b, c. Line of vertical fracture behind the pre-molars.

pm. Three pre-molars, the third and last (much larger than the other two taken together) being divided by a crack.

m. Sockets of two missing molars.)

The largest pre-molar (see Figure 305) in the fossil genus exhibits in like manner seven parallel grooves, producing by their termination a similar serrated edge in the crown; but their direction is diagonal-- a distinction, says Dr.

Falconer, which is "trivial, not typical." As these oblique furrows form so marked a character of the majority of the teeth, Dr. Falconer gave to the fossil the generic name of Plagiaulax. The shape and relative size of the incisor, a, Figure 306, exhibit a no less striking similarity to Hypsiprymnus. Nevertheless, the more sudden upward curve of this incisor, as well as other characters of the jaw, indicate a great deviation in the form of Plagiaulax from that of the living kangaroo-rats.