Upper Oolite: Kimmeridge clay.1/4nat. size.Fig. 268.Gryphæa virgula.Fig. 269.Ostrea deltoidea.
Upper Oolite: Kimmeridge clay.1/4nat. size.
Fig. 268.Gryphæa virgula.
Fig. 269.Ostrea deltoidea.
Fig. 270.Trigonia gibbosa.1/2nat. size.a.the hinge.Portland Oolite, Tisbury.
Fig. 270.
Trigonia gibbosa.1/2nat. size.a.the hinge.
Portland Oolite, Tisbury.
The Kimmeridge clay consists, in great part, of a bituminous shale, sometimes forming an impure coal several hundred feet in thickness. In some places in Wiltshire it much resembles peat; and the bituminous matter may have been, in part at least, derived from the decomposition of vegetables. But as impressions of plants are rare in these shales, which contain ammonites, oysters, and other marine shells, the bitumen may perhaps be of animal origin.
The celebrated lithographic stone of Solenhofen, in Bavaria, belongs to one of the upper divisions of the oolite, and affords a remarkable example of the variety of fossils which may be preserved under favourable circumstances, and what delicate impressions of the tender parts of certain animals and plants may be retained where the sediment is of extreme fineness. Although the number of testacea in this slate is small, and the plants few, and those all marine, Count Munster had determined no less than 237 species of fossils when I saw his collection in 1833; and among them no less than sevenspeciesof flying lizards, or pterodactyls, six saurians, three tortoises, sixty species of fish, forty-six of crustacea, and twenty-six of insects. These insects, among which is a libellula, or dragon-fly, must have been blown out to sea, probably from the same land to which the flying lizards, and other contemporaneous reptiles, resorted.
Coral Rag.—One of the limestones of the Middle Oolite has been called the "Coral Rag," because it consists, in part, of continuous beds of petrified corals, for the most part retaining the position in which they grew at the bottom of the sea. They belong chiefly to the generaCaryophyllia(fig. 271.),Agaricia, andAstrea, and sometimes form masses of coral 15 feet thick. In the annexed figure of anAstrea, from this formation, it will be seen that the cup-shaped cavities are deepest on the right-hand side, and that they grow more and more shallow, till those on the left side are nearly filled up. The last-named stars are supposed to be Polyparia of advanced age.These coralline strata extend through the calcareous hills of the N.W. of Berkshire, and north of Wilts, and again recur in Yorkshire, near Scarborough.
Fig. 271.Caryophyllia annularis, Parkin. Coral rag, Steeple Ashton.
Fig. 271.
Caryophyllia annularis, Parkin. Coral rag, Steeple Ashton.
Fig 272.Astrea. Coralrag.
Fig 272.
Astrea. Coralrag.
One of the limestones of the Jura, referred to the age of the English coral rag, has been called "Nerinæan limestone" (Calcaire à Nérinées) by M. Thirria;Nerinæabeing an extinct genus of univalve shells, much resembling theCerithiumin external form. The annexed section (fig. 273.) shows the curious form of the hollow part of each whorl, and also the perforation which passes up the middle of the columella.N. Goodhallii(fig. 274.) is another English species of the same genus, from a formation which seems to form a passage from the Kimmeridge clay to the coral rag.[261-A]
Fig. 273.Nerinæa hieroglyphica.Coral rag.
Fig. 273.
Nerinæa hieroglyphica.Coral rag.
Fig. 274.Nerinæa Goodhallii, Fitton. Coral rag, Weymouth.1/4nat. size.
Fig. 274.
Nerinæa Goodhallii, Fitton. Coral rag, Weymouth.1/4nat. size.
A division of the oolite in the Alps, regarded by most geologists as coeval with the English coral rag, has been often named "Calcaire à Dicerates," or "Diceras limestone," from its containing abundantly a bivalve shell (seefig. 275.) of a genus allied to theChama.
Fig. 275.Cast ofDiceras arietina. Coral rag, France.
Fig. 275.
Cast ofDiceras arietina. Coral rag, France.
Fig. 276.Cidaris coronata.Coral rag.
Fig. 276.
Cidaris coronata.Coral rag.
Oxford Clay.—The coralline limestone, or "coral rag," above described, and the accompanying sandy beds, called "calcareous grits" of the Middle Oolite, rests on a thick bed of clay, called the Oxford clay, sometimes not less than 500 feet thick. In this there are no corals, but great abundance of cephalopoda of the genera Ammonite and Belemnite. (Seefig. 277.) In some of the clay of very fine texture ammonites are very perfect, although somewhat compressed, and are seen to be furnished on each side of the aperture with a single horn-like projection (seefig. 278.). These were discovered in the cuttings of the Great Western Railway, near Chippenham, in 1841, and have been described by Mr. Pratt.[262-A]
Fig. 277.Belemnites hastatus.Oxford Clay.
Fig. 277.
Belemnites hastatus.Oxford Clay.
Fig. 278.Ammonites Jason,Reinecke. Syn.A. Elizabethæ, Pratt. Oxford clay, Christian Malford, Wiltshire.
Fig. 278.
Ammonites Jason,Reinecke. Syn.A. Elizabethæ, Pratt. Oxford clay, Christian Malford, Wiltshire.
Fig. 279.Belemnites Puzosianus, D'Orb. Oxford Clay, Christian Malford.a, a.projecting processes of the shell or phragmocone.b, c.broken exterior of a conical shell called the phragmocone, which is chambered within, or composed of a series of shallow concave cells pierced by a siphuncle.c, d.The guard or osselet, which is commonly called the belemnite.
Fig. 279.
Belemnites Puzosianus, D'Orb. Oxford Clay, Christian Malford.
Similar elongated processes have been also observed to extend from the shells of some belemnites discovered by Dr. Mantell in the same clay (seefig. 279.), who, by the aid of this and other specimens, has been able to throw much light on the structure of this singular extinct form of cuttle-fish.[263-A]
The upper division of this series, which is more extensive than the preceding or Middle Oolite, is called in England the Cornbrash. It consists of clays and calcareous sandstones, which pass downwards into the Forest marble, an argillaceous limestone, abounding in marine fossils. In some places, as at Bradford, this limestone is replaced by a mass of clay. The sandstones of the Forest Marble of Wiltshire are often ripple-marked and filled with fragments of broken shells and pieces of drift-wood, having evidently been formed on a coast. Rippled slabs of fissile oolite are used for roofing, and have been traced over a broad band of country from Bradford, in Wilts, to Tetbury, in Gloucestershire. These calcareous tile-stones are separated from each other by thin seams of clay, which have been deposited upon them, and have taken their form, preserving the undulating ridges and furrows of the sand in such complete integrity, that the impressions of small footsteps, apparently of crabs, which walked over the soft wet sands, are still visible. In the same stone the claws of crabs, fragments of echini, and other signs of a neighbouring beach are observed.[263-B]
Great Oolite.—Although the name of coral-rag has been appropriated, as we have seen, to a member of the Upper Oolite before described, some portions of the Lower Oolite are equally intitled in many places to be called coralline limestones. Thus the Great Oolite near Bath contains various corals, among which theEunomia radiata(fig. 280.) is very conspicuous, single individuals forming masses several feet in diameter; and having probably required, like the large existing brain-coral (Meandrina) of the tropics, many centuries before their growth was completed.
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.
Fig. 280.
Eunomia radiata, Lamouroux.
Fig. 281.Apiocrinites rotundus, or Pear Encrinite;Miller. Fossilat Bradford, Wilts.a.Stem ofApiocrinites, 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 theApiocrinites rotundus.
Fig. 281.
Apiocrinites rotundus, or Pear Encrinite;Miller. Fossilat Bradford, Wilts.
Different species ofCrinoideans, 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 depositin which some now lie prostrate. These appearances are represented in the sectionb,fig. 281., where the darker strata represent the Bradford clay, which some geologists class 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 withserpulæ. Now theseserpulæcould 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 parasiticserpulæwere full grown, they had become incrusted over with a coral, calledBerenicea diluviana; and many generations of these polyps had succeeded each other in the pure water before they became fossil.
Fig. 282.a.Single plate or articulation of an Encrinite overgrown withserpulæandcorals. Naturalsize Bradford clay.b.Portion of the same magnified, showing the coralBerenicea diluvianacovering one of theserpulæ.
Fig. 282.
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 thestationsof 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 assemblages 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 minorformations has been due to the local influence ofstations, 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.
Fig. 283.Terebratula digona.Bradfordclay. Nat.size.
Fig. 283.
Terebratula digona.Bradfordclay. 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 shelly limestones, one of which, called the Bath Oolite, is much celebrated as a building stone. In parts of Gloucestershire, 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 shells are mingled with pebbles of rocks foreign to the neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood, and crabs' claws. The shelly 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 principally to the generaBuccinum,Pleurotoma,Rostellaria,Murex, andFusus, 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 slightlyoolitic shelly limestone, forming large spheroidal masses 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 shore at low water or during storms, and redeposited. The remains of belemnites, trigoniæ, and other marine shells, with fragments of wood, are common, and impressions of ferns, cycadeæ, and other plants. Several insects, also, and, among the rest, the wing-covers of beetles, are perfectly preserved (seefig. 284.), some of them approaching nearly to the genusBuprestis.[267-A]The remains, also, of many genera of reptiles, such asPlesiosaur,Crocodile, andPterodactyl, have been discovered in the same limestone.
Fig. 284.Elytron ofBuprestis? Stonesfield.
Fig. 284.
Elytron ofBuprestis? Stonesfield.
Fig. 285.Bone of a reptile, formerly supposed to be the ulna of a Cetacean; from the Great Oolite of Enstone, near Woodstock.
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 class. 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 Oxfordshire, was cited, on the authority of Cuvier, as referable to this class. 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 isthe 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 class the bone is probably referable.
Fig. 286.Amphitherium Prevostii. StonesfieldSlate.a. coronoid process.b. condyle.c. angle of jaw.d. double-fanged molars.
Fig. 286.
Amphitherium Prevostii. StonesfieldSlate.
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 ofAmphitheriumandPhascolotheriumhave been adopted. When Cuvier was first shown one of these fossils in 1818, he pronounced 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 (seefig. 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 arein situ, altogether amounting to sixteen teeth on each side of the lower jaw.
Fig. 287.Amphitherium Broderipii. Naturalsize. StonesfieldSlate.
Fig. 287.
Amphitherium Broderipii. Naturalsize. StonesfieldSlate.
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 classes. 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 classes of vertebrata. Lastly, the molar teeth in theAmphitheriumandPhascolotheriumhave complicated crowns, and two roots (seed,fig. 286.), instead of being simple and with single fangs.[269-A]
Fig. 288.Tupaia Tana.Right ramus of lower jaw, natural size. A recent insectivorous mammal from Sumatra.
Fig. 288.
Tupaia Tana.Right ramus of lower jaw, natural size. A recent insectivorous mammal from Sumatra.
Part of lower jaw ofTupaia Tana; twice natural size.Fig. 289. End view seen from behind, showing the very slight inflection of the angle atc.Fig. 290. Side view of same.
Part of lower jaw ofTupaia Tana; twice natural size.
Fig. 289. End view seen from behind, showing the very slight inflection of the angle atc.
Fig. 290. Side view of same.
Part of lower jaw ofDidelphis Azaræ; recent,Brazil. Naturalsize.Fig. 291. End view seen from behind, showing the inflection of the angle of the jaw,c. d.Fig. 292. Side view of same.
Part of lower jaw ofDidelphis Azaræ; recent,Brazil. Naturalsize.
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 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,figs. 291.and292.) of the lower jaw, as a character of the genusDidelphys; and Mr. Owen has since established its generality in the entire marsupial series. In all these pouched quadrupeds, this process is turned inwards, as atc d,fig. 291.in the Brazilian opossum, whereas in the placental series, as atc,figs. 290.and289.there is an almost entire absence of such inflection. TheTupaia Tanaof Sumatra has been selected by my friend Mr. Waterhouse, for this illustration, because that small insectivorous quadruped bears a great resemblance to those of the StonesfieldAmphitherium. By clearing away the matrix from the specimen ofAmphitherium Prevostiiabove 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, theAmphitheriumoffers some points of approximation in its osteology to the marsupials, especially to theMyrmecobius, 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 ofAmphitheriumhas been found at Stonesfield (fig. 287.p. 268.), which differs from the former (fig. 286.) principally in being larger.
Fig. 293.Phascolotherium Bucklandi, Owen.a.natural size.b.molar of same magnified.
Fig. 293.
Phascolotherium Bucklandi, Owen.
The second mammiferous genus discovered in the same slates was named originally by Mr. BroderipDidelphys Bucklandi(seefig. 293.), and has since been calledPhascolotheriumby 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 genusDidelphysin 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 theAmphitheriumandPhascolotheriumof Stonesfield represent both the placental and marsupial classes of mammalia; and if so, they warn us in a most emphatic manner, not to found rash generalizations respecting the non-existence of certain classes 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 assemblage of warm-blooded quadrupeds which flourished in the islands of the oolitic sea.
Mr. Owen has remarked that as the marsupial genera, to which thePhascolotheriumis 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 theCestracion, a cartilaginous fish which has a bony palate, allied to those calledAcrodusandPsammodus(seefigs. 307,308.p. 275.), so common in the oolite and lias. In the same Australian seas, also, near the shore, we find the livingTrigonia, 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 theThuja,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 structurein oolitic rocks, both above and below the Stonesfield slate, as, for example, thePodocaryaof Buckland, a fruit allied to thePandanus, found in the Inferior Oolite (seefig. 294.), and theCarpolithes conicaof 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.
Fig. 294.Portion of a fossil fruit ofPodocaryamagnified. (Buckland's Bridgew. Treat. Pl. 63.) Inferior Oolite, Charmouth, Dorset.
Fig. 294.
Portion of a fossil fruit ofPodocaryamagnified. (Buckland's Bridgew. Treat. Pl. 63.) Inferior Oolite, Charmouth, Dorset.
The Stonesfield slate, in its range from Oxfordshire to the north-east, is represented by flaggy and fissile sandstones, as at Collyweston in Northamptonshire, where, according to the researches of Messrs. Ibbetson and Morris, it contains many shells, such asTrigonia angulata, also found at Stonesfield. But the Northamptonshire strata of this age assume a more marine character, or appear at least to have been formed farther from land. They inclose, however, some fossil ferns, such asPecopteris polypodioides, of species common to the oolites of the Yorkshire 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.
Fig. 295.Pterophyllum comptum. (Syn.Cycadites comptus.) Uppersandstone and shale, Gristhorpe, near Scarborough.
Fig. 295.
Pterophyllum comptum. (Syn.Cycadites comptus.) Uppersandstone and shale, Gristhorpe, near Scarborough.
In the north-west of Yorkshire, 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 (seefigs. 295,296.). The lower shales are well exposed in the sea-cliffs at Whitby, and are chiefly characterizedby ferns and cycadeæ. They contain, also, a species of calamite, and a fossil calledEquisetum columnare, which maintains an upright position in sandstone strata over a wide area. Shells of the genusCyprisandUnio, collected by Mr. Bean from these Yorkshire coal-bearing beds, point to the estuary or fluviatile origin of the deposit.
Fig. 296.Hemitelites Brownii, Goepp. Syn.Phlebopteris contigua, Lind. & Hutt. Upper carbonaceous strata, Lower Oolite, Gristhorpe, Yorkshire.
Fig. 296.
Hemitelites Brownii, Goepp. Syn.Phlebopteris contigua, Lind. & Hutt. Upper carbonaceous strata, Lower Oolite, Gristhorpe, Yorkshire.
At Brora, in Sutherlandshire, 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 31/2feet thick, and there are several feet more of pyritous coal resting upon it.
Inferior Oolite.—Between the Great and Inferior Oolite, near Bath, an argillaceous deposit called "the fuller's earth," occurs, but is wanting in the north of England. The Inferior Oolite is a calcareous freestone, usually of small thickness, which sometimes rests upon, or is replaced by, yellow sands, called the sands of the Inferior Oolite. These last, in their turn, repose upon the lias in the south and west of England.
Among the characteristic shells of the Inferior Oolite, I may instanceTerebratula spinosa(fig. 297.), andPholadomya fidicula(fig. 298.). The extinct genusPleurotomariais also a form very common in this division as well as in the Oolitic system generally. It resembles theTrochusin form, but is marked by a singular cleft (a,fig. 299.) on the right side of the mouth.
Fig. 297.Terebratula spinosa.Inferior Oolite.
Fig. 297.
Terebratula spinosa.Inferior Oolite.
Fig. 298.a.Pholadomya fidicula,1/3nat. size. Inf. Ool.b.Heart-shaped anterior termination of the same.
Fig. 298.
Fig. 299.Pleurotomaria ornata.Ferruginous Oolite,Normandy. InferiorOolite, England.
Fig. 299.
Pleurotomaria ornata.Ferruginous Oolite,Normandy. InferiorOolite, England.
As illustrations of shells having a great vertical range, I mayallude toTrigonia clavellata, found in the Upper and Inferior Oolite, andT. costata, common to the Upper, Middle, and Lower Oolite; alsoOstrea Marshii(fig. 300.), common to the Cornbrash of Wilts and the Inferior Oolite of Yorkshire; andAmmonites striatulus(fig. 301.) common to the Inferior Oolite and Lias.
Fig. 300.Ostrea Marshii.1/2nat. size. Middle and Lower Oolite.
Fig. 300.
Ostrea Marshii.1/2nat. size. Middle and Lower Oolite.
Fig. 301.Ammonites striatulus, Sow.1/3nat. size. Inferior Oolite and Lias.
Fig. 301.
Ammonites striatulus, Sow.1/3nat. size. Inferior Oolite and Lias.
Such facts by no means invalidate the general rule, that certain fossils are good chronological tests of geological periods; but they serve to caution us against attaching too much importance to single species, some of which may have a wider, others a more confined vertical range. We have before seen that, in the successive tertiary formations, there are species common to older and newer groups, yet these groups are distinguishable from one another by a comparison of the whole assemblage of fossil shells proper to each.
Mineral character of Lias — Name of Gryphite limestone — Fossil shells and fish — Ichthyodorulites — Reptiles of the Lias — Ichthyosaur and Plesiosaur — Marine Reptile of the Galapagos Islands — Sudden destruction and burial of fossil animals in Lias — Fluvio-marine beds in Gloucestershire and insect limestone — Origin of the Oolite and Lias, and of alternating calcareous and argillaceous formations — Oolitic coal-field of Virginia, in the United States.
Mineral character of Lias — Name of Gryphite limestone — Fossil shells and fish — Ichthyodorulites — Reptiles of the Lias — Ichthyosaur and Plesiosaur — Marine Reptile of the Galapagos Islands — Sudden destruction and burial of fossil animals in Lias — Fluvio-marine beds in Gloucestershire and insect limestone — Origin of the Oolite and Lias, and of alternating calcareous and argillaceous formations — Oolitic coal-field of Virginia, in the United States.
Lias.—The English provincial name of Lias has been very generally adopted for a formation of argillaceous limestone, marl, and clay, which forms the base of the Oolite, and is classed by many geologists as part of that group. They pass, indeed, into each other in some places, as near Bath, a sandy marl called the marlstone of the Lias being interposed, and partaking of the mineral characters of the upper lias and inferior oolite. These last-mentioned divisions have also some fossils in common, such as theAvicula inæquivalvis(fig. 302.). Nevertheless the Lias may be traced throughout a great part of Europe as a separate and independent group, of considerablethickness, varying from 500 to 1000 feet, containing many peculiar fossils, and having a very uniform lithological aspect. Although usually conformable to the oolite, it is sometimes, as in the Jura, unconformable. In the environs of Lons-le-Saulnier, for instance, in the department of Jura, the strata of lias are inclined at an angle of about 45°, while the incumbent oolitic marls are horizontal.
Fig. 302.Avicula inæquivalvis, Sow.
Fig. 302.
Avicula inæquivalvis, Sow.
The peculiar aspect which is most characteristic of the Lias in England, France, and Germany, is an alternation of thin beds of blue or grey limestone with a surface becoming light-brown when weathered, these beds being separated by dark-coloured narrow argillaceous partings, so that the quarries of this rock, at a distance, assume a striped and riband-like appearance.[274-A]
Although the prevailing colour of the limestone of this formation is blue, yet some beds of the lower lias are of a yellowish white colour, and have been called white lias. In some parts of France, near the Vosges mountains, and in Luxembourg, M. E. de Beaumont has shown that the lias containingGryphæa arcuata,Plagiostoma giganteum(seefig. 303.), and other characteristic fossils, becomes arenaceous; and around the Hartz, in Westphalia and Bavaria, the inferior parts of the lias are sandy, and sometimes afford a building stone.
Fig. 303.Plagiostoma giganteum. Lias.
Fig. 303.
Plagiostoma giganteum. Lias.
Fig. 304.Gryphæa incurva, Sow. (G. arcuata, Lam.)
Fig. 304.
Gryphæa incurva, Sow. (G. arcuata, Lam.)
Fig. 305.Nautilus truncatus. Lias.
Fig. 305.
Nautilus truncatus. Lias.
The name of Gryphite limestone has sometimes been applied to the lias, in consequence of the great number of shells which it contains of a species of oyster, orGryphæa(fig. 304., see alsofig. 30.p. 29.). Many cephalopoda, also, such asAmmonite,Belemnite, andNautilus(fig. 305.), prove the marine origin of the formation.