Chapter 56

JURASSIC GROUP.—PURBECK BEDS AND OOLITE.

The Purbeck Beds a Member of the Jurassic Group. — Subdivisions of that Group. — Physical Geography of the Oolite in England and France. — Upper Oolite. — Purbeck Beds. — New Genera of fossil Mammalia in the Middle Purbeck of Dorsetshire. — Dirt-bed or ancient Soil. — Fossils of the Purbeck Beds. — Portland Stone and Fossils. — Kimmeridge Clay. — Lithographic Stone of Solenhofen. — Archæopteryx. — Middle Oolite. — Coral Rag. — Nerinæa Limestone. — Oxford Clay, Ammonites and Belemnites. — Kelloway Rock. — Lower, or Bath, Oolite. — 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. — Palæontological Relations of the several Subdivisions of the Oolitic group.

Classification 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 Jurassic. 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 p. 37). 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 principal subdivisions observed in the central and south-eastern parts of England.

OOLITE

Upper _a._ Purbeck beds. _b._ Portland stone and sand. _c._ Kimmeridge clay. Middle _d._ Coral rag. _e._ Oxford clay, and Kelloway rock. Lower _f._ Cornbrash and Forest marble. _g._ Great Oolite and Stonesfield slate. _h._ Fuller’s earth. _i._ Inferior Oolite.

The Upper Oolitic system of the above table 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 analogy 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 liassic 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.

Fig. 298: Configuration of surface.

Fig. 298 will give the reader an idea of the configuration of the surface now alluded to, such as may be seen in passing 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 analogous, 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 p. 105) 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 class 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 instituted, from the fossils of the overlying Hastings Sands and Weald Clay.

_Upper Purbeck._—The highest of the three divisions is purely fresh-water, the strata, about fifty feet in thickness, containing shells of the genera _Paludina, Physa, Limnæa, 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 Fig. 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.

Fig. 299: Cyprides from the Upper Purbecks.

_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.

Fig. 300: Ostrea distorta. Fig. 301: Hemicidaris Purbeckensis.

Immediately below is a great and conspicuous stratum, twelve feet thick, formed of a vast accumulation of shells of _Ostrea distorta_ (Fig. 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 (Fig. 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_ (Fig. 302, _a, b, c_), and with _Valvata, Paludina, Planorbis, Limnæa, Physa_ (Fig. 303), and _Cyclas,_ all different from any occurring higher in the series. It will be seen that _Cypris fasciculata_ (Fig. 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 shells and impressions of leaves like those of a large _Zostera,_ forming the base of the Middle Purbeck.

Fig. 302: Cyprides from the Middle Purbecks.

Fig. 303: Physa Bristovii

_Fossil Mammalia of the Middle Purbeck._—In 1852,[1] 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.” 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 class, more closely allied in its dentition 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 shells 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 dentition, 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.

The largest pre-molar (see Fig. 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,_ Fig. 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.

Fig. 304: Pre-molar of the recent Australian Hypsiprymnus Gaimardi, showing 7 grooves at right angles to the length of the jaw. Fig. 305: Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii, showing 7 diagonal grooves.

Fig. 306: Plagiaulax Becklessi. Right ramus of lower jaw. There are two fossil specimens of lower jaws of this genus evidently referable to two distinct species extremely unequal in size and otherwise distinguishable. The _Plagiaulax Becklesii_ (Fig. 306) was about as big as the English squirrel or the flying phalanger of Australia (_Petaurus Australis,_ Waterhouse). The smaller fossil, having only half the linear dimensions of the other, was probably only one-twelfth of its bulk. It is of peculiar geological interest, because, as shown by Dr. Falconer, its two back molars bear a decided resemblance to those of the Triassic _ Microlestes_ (Fig. 389), the most ancient of known mammalia, of which an account will be given in