The Student's Elements of Geology Part 41

(FIGURE 341. Side view of same.))

(Figures 342 and 343. Part of lower jaw of Didelphys Azarae; recent, Brazil.

Natural size.

(FIGURE 342. End view seen from behind, showing the inflection of the angle of the jaw, c, d.)

(FIGURE 343. Side view of same.))

(FIGURE 344. Amphitherium Prevostii, Cuvier sp. Stonesfield Slate. Syn.

Thylacotherium Prevostii, Valenc.

a. Coronoid process.

b. Condyle.

c. Angle of jaw.

d. Double-fanged molars.)

(FIGURE 345. Amphitheriumm Broderipii, Owen. Natural size. Stonesfield Slate.)

It is now generally admitted that these or really the remains of mammalia (although it was at first suggested that they might be reptiles), and the only question open to controversy is limited to this point, whether the fossil mammalia found in the Lower Oolite of Oxfordshire ought to be referred to the marsupial quadrupeds, or to the ordinary placental series. Cuvier had long ago pointed out a peculiarity in the form of the angular process (c, Figures 342 and 343) of the lower jaw, as a character of the genus Didelphys; and Professor Owen has since confirmed the doctrine of its generality in the entire marsupial series. In all these pouched quadrupeds this process is turned inward, as at c, d, Figure 342, in the Brazilian opossum, whereas in the placental series, as at c, Figures 340 and 341, there is an almost entire absence of such inflection.

The Tupaia Tana of Sumatra has been selected by Mr. Waterhouse for this ill.u.s.tration, because the jaws of that small insectivorous quadruped bear a great resemblance to those of the Stonesfield Amphitherium. By clearing away the matrix from the specimen of Amphitherium Prevostii here represented (Figure 344), Professor Owen ascertained that the angular process (c) bent inward in a slighter degree than in any of the known marsupialia; in short, the inflection does not exceed that of the mole or hedgehog. This fact made him doubt whether the Amphitherium might not be an insectivorous placental, although it offered some points of approximation in its osteology to the marsupials, especially to the Myrmecobius, a small insectivorous quadruped of Australia, which has nine molars on each side of the lower jaw, besides a canine and three incisors. (A figure of this recent Myrmecobius will be found in my Principles of Geology chapter 9.) Another species of Amphitherium has been found at Stonesfield (Figure 345), which differs from the former (Figure 344) princ.i.p.ally in being larger.

(FIGURE 346. Phascolotherium Bucklandi, Broderip, sp.

a. Natural size.

b. Molar of same, magnified.)

The second mammiferous genus discovered in the same slates was named originally by Mr. Broderip Didelphys Bucklandi (see Figure 346), and has since been called Phascolotherium by Owen. It manifests a much stronger likeness to the marsupials in the general form of the jaw, and in the extent and position of its inflected angle, while the agreement with the living genus Didelphys in the number of the pre-molar and molar teeth is complete. (Owen's British Fossil Mammals page 62.)

In 1854 the remains of another mammifer, small in size, but larger than any of those previously known, was brought to light. The generic name of Stereognathus was given to it, and, as is usually the case in these old rocks (see above), it consisted of part of a lower jaw, in which were implanted three double-fanged teeth, differing in structure from those of all other known recent or extinct mammals.

PLANTS OF THE OOLITE.

(FIGURE 347. Portion of a fossil fruit of Podocarya Bucklandi, Ung., magnified.

(Buckland's Bridgewater Treatise Plate 63.) Inferior Oolite, Charmouth, Dorset.)

(FIGURE 348. Cone of fossil Araucaria sphaerocarpa, Carruthers. Inferior Oolite.

Bruton, Somersetshire. One-third diameter of original. In the collection of the British Museum.)

The Araucarian pines, which are now abundant in Australia and its islands, together with marsupial quadrupeds, are found in like manner to have accompanied the marsupials in Europe during the Oolitic period (see Figure 348). In the same rock endogens of the most perfect structure are met with, as, for example, fruits allied to the Panda.n.u.s, such as the Kaidacarpum oolitic.u.m of Carruthers in the Great Oolite, and the Podocarya of Buckland (see Figure 347) in the Inferior Oolite.

FULLER'S EARTH.

(FIGURE 349. Ostrea ac.u.minata. Fuller's Earth.)

Between the Great and Inferior Oolite near Bath, an argillaceous deposit, called "the fuller's earth," occurs; but it is wanting in the north of England. It abounds in the small oyster represented in Figure 349. The number of mollusca known in this deposit is about seventy; namely, fifty Lamellibranchiate Bivalves, ten Brachiopods, three Gasteropods, and seven or eight Cephalopods.

INFERIOR OOLITE.

This formation consists of a calcareous freestone, usually of small thickness, but attaining in some places, as in the typical area of Cheltenham and the Western Cotswolds, a thickness of 250 feet. It sometimes rests upon yellow sands, formerly cla.s.sed as the sands of the Inferior Oolite, but now regarded as a member of the Upper Lias. These sands repose upon the Upper Lias clays in the south and west of England. The Collyweston slate, formerly cla.s.sed with the Great Oolite, and supposed to represent in Northamptonshire the Stonesfield slate, is now found to belong to the Inferior Oolite, both by community of species and position in the series. The Collyweston beds, on the whole, a.s.sume a much more marine character than the Stonesfield slate. Nevertheless, one of the fossil plants Aroides Stutterdi, Carruthers, remarkable, like the Pandanaceous species before mentioned (Figure 347) as a representative of the monocotyledonous cla.s.s, is common to the Stonesfield beds in Oxfordshire.

(FIGURE 350. Hemitelites Brownii, Goepp. Syn. Phlebopteris contigua, Lind. and Hutt. Lower carbonaceous strata, Inferior Oolite shales. Gristhorpe, Yorkshire.)

The Inferior Oolite of Yorkshire consists largely of shales and sandstones, which a.s.sume much the aspect of a true coal-field, thin seams of coal having actually been worked in them for more than a century. A rich harvest of fossil ferns has been obtained from them, as at Gristhorpe, near Scarborough (Figure 350). They contain also Cycadeae, of which family a magnificent specimen has been described by Mr. Williamson under the name Zamia gigas, and a fossil called Equisetum Columnare (see Figure 397), which maintains an upright position in sandstone strata over a wide area. Sh.e.l.ls of Estheria and Unio, collected by Mr.

Bean from these Yorkshire coal-bearing beds, point to the estuary or fluviatile origin of the deposit.

At Brora, in Sutherlandshire, a coal formation, probably coeval with the above, or at least belonging to some of the lower divisions of the Oolitic period, has been mined extensively for a century or more. It affords the thickest stratum of pure vegetable matter hitherto detected in any secondary rock in England. One seam of coal of good quality has been worked three and a half feet thick, and there are several feet more of pyritous coal resting upon it.

(FIGURE 351. Terebratula fimbria, Sowerby. Inferior Oolite marl. Cotswold Hills.)

(FIGURE 352. Rhynchonella spinosa, Schloth. Inferior Oolite.)

(FIGURE 353. Pholadomya fidicula, Sowerby. One-third natural size. Inferior Oolite.)

(FIGURE 354. Pleurotomaria granulata, Sowerby. Ferruginous Oolite, Normandy.

Inferior Oolite, England.)

(FIGURE 355. Pleurotomaria ornata, Sowerby Sp. Inferior Oolite.)

(FIGURE 356. Collyrites (Dysaster) ringens, Aga.s.siz. Inferior Oolite, Somersetshire.)

(FIGURE 357. Ammonites Humphresia.n.u.s, Sowerby. Inferior Oolite.)

(FIGURE 358. Ammonites Braikenridgii, Sowerby. Oolite, Scarborough. Inferior Oolite, Dundry; Calvados; etc.)

(FIGURE 359. Ostrea Marshii. One-half natural size. Middle and Lower Oolite.)

Among the characteristic sh.e.l.ls of the Inferior Oolite, I may instance Terebratula fimbria (Figure 351), Rhynchonella spinosa (Figure 352), and Pholadomya fidicula (Figure 353). The extinct genus Pleurotomaria is also a form very common in this division as well as in the Oolitic system generally. It resembles the Trochus in form, but is marked by a deep cleft (a, Figures 354, 355) on one side of the mouth. The Collyrites (Dysaster) ringens (Figure 356) is an Echinoderm common to the Inferior Oolite of England and France, as are the two Ammonites (Figures 357, 358).

PALAEONTOLOGICAL RELATIONS OF THE OOLITIC STRATA.

Observations have already been made on the distinctness of the organic remains of the Oolitic and Cretaceous strata, and the proportion of species common to the different members of the Oolite. Between the Lower Oolite and the Lias there is a somewhat greater break, for out of 256 mollusca of the Upper Lias, thirty- seven species only pa.s.s up into the Inferior Oolite.

In ill.u.s.tration of sh.e.l.ls having a great vertical range, it may be stated that in England some few species pa.s.s up from the Lower to the Upper Oolite, as, for example, Rhynchonella obsoleta, Lithodomus inclusus, Pholadomya ovalis, and Trigonia costata.

(FIGURE 360. Ammonites macrocephalus, Schloth. One-third natural size. Great Oolite and Oxford Clay.)

Of all the Jura.s.sic Ammonites of Great Britain, A. macrocephalus (Figure 360), which is common to the Great Oolite and Oxford Clay, has the widest range.

We have every reason to conclude that the gaps which occur, both between the larger and smaller sections of the English Oolites, imply intervals of time, elsewhere represented by fossiliferous strata, although no deposit may have taken place in the British area. This conclusion is warranted by the partial extent of many of the minor and some of the larger divisions even in England.

CHAPTER XX.

JURa.s.sIC GROUP-- CONTINUED.-- LIAS.

Mineral Character of Lias.

Numerous successive Zones in the Lias, marked by distinct Fossils, without Unconformity in the Stratification, or Change in the Mineral Character of the Deposits.

Gryphite Limestone.

Sh.e.l.ls of the Lias.

Fish of the Lias.