A Manual of Elementary Geology Part 40

[Ill.u.s.tration: Fig. 280. _Eunomia radiata_, Lamouroux.

_a._ section transverse to the tubes.

_b._ vertical section, showing the radiation of the tubes.

_c._ portion of interior of tubes magnified, showing striated surface.]

[Ill.u.s.tration: Fig. 281. _Apiocrinites rotundus_, or Pear Encrinite; Miller. Fossil at Bradford, Wilts.

_a._ Stem of _Apiocrinites_, and one of the articulations, natural size.

_b._ Section at Bradford of great oolite and overlying clay, containing the fossil encrinites. See text.

_c._ Three perfect individuals of Apiocrinites, represented as they grew on the surface of the Great Oolite.

_d._ Body of the _Apiocrinites rotundus_.]

Different species of _Crinoideans_, or stone-lilies, are also common in the same rocks with corals; and, like them, must have enjoyed a firm bottom, where their root, or base of attachment, remained undisturbed for years (_c_, fig. 281.). Such fossils, therefore, are almost confined to the limestones; but an exception occurs at Bradford, near Bath, where they are enveloped in clay. In this case, however, it appears that the solid upper surface of the "Great Oolite" had supported, for a time, a thick submarine forest of these beautiful zoophytes, until the clear and still water was invaded by a current charged with mud, which threw down the stone-lilies, and broke most of their stems short off near the point of attachment. The stumps still remain in their original position; but the numerous articulations once composing the stem, arms, and body of the zoophyte, were scattered at random through the argillaceous deposit in which some now lie prostrate. These appearances are represented in the section _b_, fig. 281., where the darker strata represent the Bradford clay, which some geologists cla.s.s with the Forest marble, others with the Great Oolite. The upper surface of the calcareous stone below is completely incrusted over with a continuous pavement, formed by the stony roots or attachments of the Crinoidea; and besides this evidence of the length of time they had lived on the spot, we find great numbers of single joints, or circular plates of the stem and body of the encrinite, covered over with _serpulae_. Now these _serpulae_ could only have begun to grow after the death of some of the stone-lilies, parts of whose skeletons had been strewed over the floor of the ocean before the irruption of argillaceous mud. In some instances we find that, after the parasitic _serpulae_ were full grown, they had become incrusted over with a coral, called _Berenicea diluviana_; and many generations of these polyps had succeeded each other in the pure water before they became fossil.

[Ill.u.s.tration: Fig. 282.

_a._ Single plate or articulation of an Encrinite overgrown with _serpulae_ and corals. Natural size Bradford clay.

_b._ Portion of the same magnified, showing the coral _Berenicea_ _diluviana_ covering one of the _serpulae_.]

We may, therefore, perceive distinctly that, as the pines and cycadeous plants of the ancient "dirt bed," or fossil forest, of the Lower Purbeck were killed by submergence under fresh water, and soon buried beneath muddy sediment, so an invasion of argillaceous matter put a sudden stop to the growth of the Bradford Encrinites, and led to their preservation in marine strata.[265-A]

Such differences in the fossils as distinguish the calcareous and argillaceous deposits from each other, would be described by naturalists as arising out of a difference in the _stations_ of species; but besides these, there are variations in the fossils of the higher, middle, and lower part of the oolitic series, which must be ascribed to that great law of change in organic life by which distinct a.s.semblages of species have been adapted, at successive geological periods, to the varying conditions of the habitable surface. In a single district it is difficult to decide how far the limitation of species to certain minor formations has been due to the local influence of _stations_, or how far it has been caused by time or the creative and destroying law above alluded to. But we recognize the reality of the last-mentioned influence, when we contrast the whole oolitic series of England with that of parts of the Jura, Alps, and other distant regions, where there is scarcely any lithological resemblance; and yet some of the same fossils remain peculiar in each country to the Upper, Middle, and Lower Oolite formations respectively. Mr. Thurmann has shown how remarkably this fact holds true in the Bernese Jura, although the argillaceous divisions, so conspicuous in England, are feebly represented there, and some entirely wanting.

[Ill.u.s.tration: Fig. 283. _Terebratula digona._ Bradford clay. Nat. size.]

The Bradford clay above alluded to is sometimes 60 feet thick, but, in many places, it is wanting; and, in others, where there are no limestones, it cannot easily be separated from the clays of the overlying "forest marble"

and underlying "fuller's earth."

The calcareous portion of the Great Oolite consists of several sh.e.l.ly limestones, one of which, called the Bath Oolite, is much celebrated as a building stone. In parts of Gloucesters.h.i.+re, especially near Minchinhampton, the Great Oolite, says Mr. Lycett, "must have been deposited in a shallow sea, where strong currents prevailed, for there are frequent changes in the mineral character of the deposit, and some beds exhibit false stratification. In others, heaps of broken sh.e.l.ls are mingled with pebbles of rocks foreign to the neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood, and crabs' claws.

The sh.e.l.ly strata, also, have occasionally suffered denudation, and the removed portions have been replaced by clay."[266-A] In such shallow-water beds cephalopoda are rare, and, instead of ammonites and belemnites, numerous genera of carnivorous trachelipods appear. Out of one hundred and forty-two species of univalves obtained from the Minchinhampton beds, Mr. Lycett found no less than forty-one to be carnivorous. They belong princ.i.p.ally to the genera _Buccinum_, _Pleurotoma_, _Rostellaria_, _Murex_, and _Fusus_, and exhibit a proportion of zoophagous species not very different from that which obtains in warm seas of the recent period. These conchological results are curious and unexpected, since it was imagined that we might look in vain for the carnivorous trachelipods in rocks of such high antiquity as the Great Oolite, and it was a received doctrine that they did not begin to appear in considerable numbers till the Eocene period when those two great families of cephalopoda, the ammonites and belemnites, had become extinct.

_Stonesfield slate._--The slate of Stonesfield has been shown by Mr.

Lonsdale to lie at the base of the Great Oolite.[266-B] It is a slightly oolitic sh.e.l.ly limestone, forming large spheroidal ma.s.ses imbedded in sand, only 6 feet thick, but very rich in organic remains. It contains some pebbles of a rock very similar to itself, and which may be portions of the deposit, broken up on a sh.o.r.e at low water or during storms, and redeposited. The remains of belemnites, trigoniae, and other marine sh.e.l.ls, with fragments of wood, are common, and impressions of ferns, cycadeae, and other plants. Several insects, also, and, among the rest, the wing-covers of beetles, are perfectly preserved (see fig. 284.), some of them approaching nearly to the genus _Buprestis_.[267-A] The remains, also, of many genera of reptiles, such as _Plesiosaur_, _Crocodile_, and _Pterodactyl_, have been discovered in the same limestone.

[Ill.u.s.tration: Fig. 284. Elytron of _Buprestis_? Stonesfield.]

[Ill.u.s.tration: Fig. 285. Bone of a reptile, formerly supposed to be the ulna of a Cetacean; from the Great Oolite of Enstone, near Woodstock.]

But the remarkable fossils for which the Stonesfield slate is most celebrated, are those referred to the mammiferous cla.s.s. The student should be reminded that in all the rocks described in the preceding chapters as older than the Eocene, no bones of any land quadruped, or of any cetacean, have been discovered. Yet we have seen that terrestrial plants were not rare in the lower cretaceous formation, and that in the Wealden there was evidence of freshwater sediment on a large scale, containing various plants, and even ancient vegetable soils with the roots and erect stumps of trees. We had also in the same Wealden many land-reptiles and winged-insects, which renders the absence of terrestrial quadrupeds the more striking. The want, however, of any bones of whales, seals, dolphins, and other aquatic mammalia, whether in the chalk or in the upper or middle oolite, is certainly still more remarkable. Formerly, indeed, a bone from the great oolite of Enstone, near Woodstock, in Oxfords.h.i.+re, was cited, on the authority of Cuvier, as referable to this cla.s.s. Dr. Buckland, who stated this in his Bridgewater Treatise[267-B], had the kindness to send me the supposed ulna of a whale, that Mr. Owen might examine into its claims to be considered as cetaceous. It is the opinion of that eminent comparative anatomist that it cannot have belonged to the cetacea, because the fore-arm in these marine mammalia is invariably much flatter, and devoid of all muscular depressions and ridges, one of which is so prominent in the middle of this bone, represented in the above cut (fig. 285.). In saurians, on the contrary, such ridges exist for the attachment of muscles; and to some animal of that cla.s.s the bone is probably referable.

[Ill.u.s.tration: Fig. 286. _Amphitherium Prevostii_. Stonesfield Slate. Natural size.

_a_. coronoid process.

_b_. condyle.

_c_. angle of jaw.

_d_. double-fanged molars.]

These observations are made to prepare the reader to appreciate more justly the interest felt by every geologist in the discovery in the Stonesfield slate of no less than seven specimens of lower jaws of mammiferous quadrupeds, belonging to three different species and to two distinct genera, for which the names of _Amphitherium_ and _Phascolotherium_ have been adopted. When Cuvier was first shown one of these fossils in 1818, he p.r.o.nounced it to belong to a small ferine mammal, with a jaw much resembling that of an opossum, but differing from all known ferine genera, in the great number of the molar teeth, of which it had at least ten in a row. Since that period, a much more perfect specimen of the same fossil, obtained by Dr. Buckland (see fig. 286.), has been examined by Mr. Owen, who finds that the jaw contained on the whole twelve molar teeth, with the socket of a small canine, and three small incisors, which are _in situ_, altogether amounting to sixteen teeth on each side of the lower jaw.

[Ill.u.s.tration: Fig. 287. _Amphitherium Broderipii_. Natural size.

Stonesfield Slate.]

The only question which could be raised respecting the nature of these fossils was, whether they belonged to a mammifer, a reptile, or a fish. Now on this head the osteologist observes that each of the seven half jaws is composed of but one single piece, and not of two or more separate bones, as in fishes and most reptiles, or of two bones, united by a suture, as in some few species belonging to those cla.s.ses. The condyle, moreover (_b_, fig. 286.), or articular surface, by which the lower jaw unites with the upper, is convex in the Stonesfield specimens, and not concave as in fishes and reptiles. The coronoid process (_a_, fig. 286.) is well developed, whereas it is wanting or very small, in the inferior cla.s.ses of vertebrata.

Lastly, the molar teeth in the _Amphitherium_ and _Phascolotherium_ have complicated crowns, and two roots (see _d_, fig. 286.), instead of being simple and with single fangs.[269-A]

[Ill.u.s.tration: Fig. 288. _Tupaia Tana._ Right ramus of lower jaw, natural size. A recent insectivorous mammal from Sumatra.]

[2 Ill.u.s.trations: Part of lower jaw of _Tupaia Tana_; twice natural size.

Fig. 289. End view seen from behind, showing the very slight inflection of the angle at _c_.

Fig. 290. Side view of same.]

[2 Ill.u.s.trations: Part of lower jaw of _Didelphis Azarae_; recent, Brazil. Natural size.

Fig. 291. End view seen from behind, showing the inflection of the angle of the jaw, _c. d._

Fig. 292. Side view of same.]

The only question, therefore, which could fairly admit of controversy was limited to this point, whether the fossil mammalia found in the lower oolite of Oxfords.h.i.+re 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_, figs.

291. and 292.) of the lower jaw, as a character of the genus _Didelphys_; and Mr. Owen has since established its generality in the entire marsupial series. In all these pouched quadrupeds, this process is turned inwards, as at _c d_, fig. 291. in the Brazilian opossum, whereas in the placental series, as at _c_, figs. 290. and 289. there is an almost entire absence of such inflection. The _Tupaia Tana_ of Sumatra has been selected by my friend Mr. Waterhouse, for this ill.u.s.tration, because that small insectivorous quadruped bears a great resemblance to those of the Stonesfield _Amphitherium_. By clearing away the matrix from the specimen of _Amphitherium Prevostii_ above represented (fig. 286.), Mr. Owen ascertained that the angular process (_c_) bent inwards 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 turns the scale in favour of its affinities to the placental insectivora. Nevertheless, the _Amphitherium_ offers 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.[269-B]

Another species of _Amphitherium_ has been found at Stonesfield (fig.

287. p. 268.), which differs from the former (fig. 286.) princ.i.p.ally in being larger.

[Ill.u.s.tration: Fig. 293. _Phascolotherium Bucklandi_, Owen.

_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 fig. 293.), 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 premolar and molar teeth, is complete.[270-A]

On reviewing, therefore, the whole of the osteological evidence, it will be seen that we have every reason to presume that the _Amphitherium_ and _Phascolotherium_ of Stonesfield represent both the placental and marsupial cla.s.ses of mammalia; and if so, they warn us in a most emphatic manner, not to found rash generalizations respecting the non-existence of certain cla.s.ses of animals at particular periods of the past, on mere negative evidence. The singular accident of our having as yet found nothing but the lower jaws of seven individuals, and no other bones of their skeletons, is alone sufficient to demonstrate the fragmentary manner in which the memorials of an ancient terrestrial fauna are handed down to us. We can scarcely avoid suspecting that the two genera above described, may have borne a like insignificant proportion to the entire a.s.semblage of warm-blooded quadrupeds which flourished in the islands of the oolitic sea.

Mr. Owen has remarked that as the marsupial genera, to which the _Phascolotherium_ is most nearly allied, are now confined to New South Wales and Van Diemen's Land, so also is it in the Australian seas, that we find the _Cestracion_, a cartilaginous fish which has a bony palate, allied to those called _Acrodus_ and _Psammodus_ (see figs. 307, 308. p. 275.), so common in the oolite and lias. In the same Australian seas, also, near the sh.o.r.e, we find the living _Trigonia_, a genus of mollusca so frequently met with in the Stonesfield slate. So, also, the Araucarian pines are now abundant, together with ferns, in Australia and its islands, as they were in Europe in the oolitic period. Many botanists incline to the opinion, that the _Thuja_, _Pine_, _Cycas_, _Zamia_, in short, all the gymnogens, belong to a less highly developed type of flowering plants than do the exogens; but even if this be admitted, no naturalist can ascribe a low standard of organization to the oolitic flora, since we meet with endogens of the most perfect structure in oolitic rocks, both above and below the Stonesfield slate, as, for example, the _Podocarya_ of Buckland, a fruit allied to the _Panda.n.u.s_, found in the Inferior Oolite (see fig. 294.), and the _Carpolithes conica_ of the Coral rag. The doctrine, therefore, of a regular series of progressive development at successive eras in the animal and vegetable kingdoms, from beings of a more simple to those of a more complex organization, receives a check, if not a refutation, from the facts revealed to us by the study of the Lower Oolites.

[Ill.u.s.tration: Fig. 294. Portion of a fossil fruit of _Podocarya_ magnified. (Buckland's Bridgew. Treat. Pl. 63.) Inferior Oolite, Charmouth, Dorset.]

The Stonesfield slate, in its range from Oxfords.h.i.+re to the north-east, is represented by flaggy and fissile sandstones, as at Collyweston in Northamptons.h.i.+re, where, according to the researches of Messrs. Ibbetson and Morris, it contains many sh.e.l.ls, such as _Trigonia angulata_, also found at Stonesfield. But the Northamptons.h.i.+re strata of this age a.s.sume a more marine character, or appear at least to have been formed farther from land. They inclose, however, some fossil ferns, such as _Pecopteris polypodioides_, of species common to the oolites of the Yorks.h.i.+re coast[271-A], where rocks of this age put on all the aspect of a true coal-field; thin seams of coal having actually been worked in them for more than a century.

[Ill.u.s.tration: Fig. 295. _Pterophyllum comptum._ (Syn. _Cycadites comptus_.) Upper sandstone and shale, Gristhorpe, near Scarborough.]

In the north-west of Yorks.h.i.+re, the formation alluded to consists of an upper and a lower carbonaceous shale, abounding in impressions of plants, divided by a limestone considered by many geologists as the representative of the Great Oolite; but the scarcity of marine fossils makes all comparisons with the subdivisions adopted in the south extremely difficult. A rich harvest of fossil ferns has been obtained from the upper carbonaceous shales and sandstones at Gristhorpe, near Scarborough (see figs. 295, 296.). The lower shales are well exposed in the sea-cliffs at Whitby, and are chiefly characterized by ferns and cycadeae. They contain, also, a species of calamite, and a fossil called _Equisetum columnare_, which maintains an upright position in sandstone strata over a wide area. Sh.e.l.ls of the genus _Cypris_ and _Unio_, collected by Mr. Bean from these Yorks.h.i.+re coal-bearing beds, point to the estuary or fluviatile origin of the deposit.

[Ill.u.s.tration: Fig. 296. _Hemitelites Brownii_, Goepp. Syn.

_Phlebopteris contigua_, Lind. & Hutt. Upper carbonaceous strata, Lower Oolite, Gristhorpe, Yorks.h.i.+re.]

At Brora, in Sutherlands.h.i.+re, a coal formation, probably coeval with the above, or 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 3-1/2 feet thick, and there are several feet more of pyritous coal resting upon it.