The Ancient Life History of the Earth Part 17

(23) "On Evidences of Theriodonts in Permian Deposits," &c.--'Quart.

Journ. Geol. Soc.,' vol. x.x.xii., 1876. Owen.

(24) "On the Stagonolepis Robertsoni," &c.--'Quart. Journ. Geol.

Soc.,' vol. xv., 1859. Huxley.

(25) "On a New Specimen of Telerpeton Elginense"--'Quart. Journ.

Geol. Soc.,' vol. xxiii., 1866. Huxley.

(26) "On Hyperodapedon"--'Quart. Journ. Geol. Soc.,' vol. xxv., 1869. Huxley.

(27) "On the Affinities between the Deinosaurian Reptiles and Birds"--'Quart. Journ. Geol. Soc.,' vol. xxvi., 1870. Huxley.

(28) "On the Cla.s.sification of the Deinosauria," &c.--'Quart. Journ.

Geol. Soc.,' vol. xxvi., 1870. Huxley.

(29) "Palaeontologica Indica"--'Memoirs of the Geol. Survey of India.'

(30) "On the Geological Position and Geographical Distribution of the Dolomitic Conglomerate of the Bristol Area"--'Quart. Journ.

Geol. Soc.,' vol. xxvi., 1870. R. Etheridge, sen.

(31) "Remains of Labyrinthodonta from the Keuper Sandstone of Warwick"--'Quart. Journ. Geol. Soc.,' vol. x.x.x., 1874 Miall.

(32) 'Manual of Geology.' Dana.

(33) 'Synopsis of Extinct Batrachia and Reptilia of North America.'

Cope.

(34) 'Fossil Footmarks.' Hitchc.o.c.k.

(35) 'Ichnology of New England.' Hitchc.o.c.k.

(36) 'Traite de Paleontologie Vegetale.' Schimper.

(37) 'Histoire des Vegetaux Fossiles.' Brongniart.

(38) 'Monographie der Fossilen Coniferen.' Goeppert.

CHAPTER XVI.

THE JURa.s.sIC PERIOD.

Resting upon the Trias, with perfect conformity, and with an almost undeterminable junction, we have the great series of deposits which are known as the _Oolitic Rocks_, from the common occurrence in them of oolitic limestones, or as the _Jura.s.sic Rocks_, from their being largely developed in the mountain-range of the Jura, on the western borders of Switzerland. Sediments of this series occupy extensive areas in Great Britain, on the continent of Europe, and in India. In North America, limestones and marls of this age have been detected in "the Black Hills, the Laramie range, and other eastern ridges of the Rocky Mountains; also over the Pacific slope, in the Uintah, Wahsatch, and Humboldt Mountains, and in the Sierra Nevada" (Dana); but in these regions their extent is still unknown, and their precise subdivisions have not been determined. Strata belonging to the Jura.s.sic period are also known to occur in South America, in Australia, and in the Arctic zone. When fully developed, the Jura.s.sic series is capable of subdivision into a number of minor groups, of which some are clearly distinguished by their mineral characters, whilst others are separated with equal certainty by the differences of the fossils that they contain. It will be sufficient for our present purpose, without entering into the more minute subdivisions of the series, to give here a very brief and general account of the main sub-groups of the Jura.s.sic rocks, as developed in Britain--the arrangement of the Jura-formation of the continent of Europe agreeing in the main with that of England.

I. THE LIAS.--The base of the Jura.s.sic series of Britain is formed by the great calcareo-argillaceous deposit of the "Lias," which usually rests conformably and almost inseparably upon the Rhaetic beds (the so-called "White Lias"), and pa.s.ses up, generally conformably, into the calcareous sandstones of the Inferior Oolite.

The Lias is divisible into the three princ.i.p.al groups of the Lower, Middle, and Upper Lias, as under, and these in turn contain many well-marked "zones;" so that the Lias has some claims to be considered as an independent formation, equivalent to all the remaining Oolitic rocks. The _Lower Lias_ (_Terrain Sinemurien_ of D'Orbigny) sometimes attains a thickness of as much as 600 feet, and consists of a great series of bluish or greyish laminated clays, alternating with thin bands of blue or grey limestone--the whole, when seen in quarries or cliffs from a little distance, a.s.suming a characteristically striped and banded appearance. By means of particular species of _Ammonites_, taken along with other fossils which are confined to particular zones, the Lower Lias may be subdivided into several well-marked horizons. The _Middle Lias_, or _Marlstone Series_ (_Terrain Liasien_ of D'Orbigny), may reach a thickness of 200 feet, and consists of sands, arenaceous marls, and argillaceous limestones, sometimes with ferruginous beds. The _Upper Lias_ (_Terrain Toarcien_ of D'Orbigny) attains a thickness of 300 feet, and consists princ.i.p.ally of shales below, pa.s.sing upwards into arenaceous strata.

II. THE LOWER OOLITES.--Above the Lias comes a complex series of partly arenaceous and argillaceous, but princ.i.p.ally calcareous strata, of which the following are the more important groups: a, The _Inferior Oolite_ (_Terrain Bajocien_ of D'Orbigny), consisting of more than 200 feet of oolitic limestones, sometimes more or less sandy; b, The _Fuller's Earth_, a series of shales, clays, and marls, about 120 feet in thickness; c, The _Great Oolite_ or _Bath Oolite_ (_Terrain Bathonien_ of D'Orbigny), consisting princ.i.p.ally of oolitic limestones, and attaining a thickness of about 130 feet. The well-known "Stonesfield Slates"

belong to this horizon; and the locally developed "Bradford Clay,"

"Corn brash," and "Forest-marble" may be regarded as const.i.tuting the summit of this group.

III. THE MIDDLE OOLITES.--The central portion of the Jura.s.sic series of Britain is formed by a great argillaceous deposit, capped by calcareous strata, as follows: a, The _Oxford Clay_ (_Terrain Callovien_ and _Terrain Oxfordien_ of D'Orbigny), consisting of dark-coloured laminated clays, sometimes reaching a thickness of 700 feet, and in places having its lower portion developed into a hard calcareous sandstone ("Kelloway Rock"); b, The Coral-Rag (_Terrain Corallien_ of D'Orbigny, "Nerinean Limestone" of the Jura, "Diceras Limestone" of the Alps), consisting, when typically developed, of a central ma.s.s of oolitic limestone, underlaid and surmounted by calcareous grits.

IV. THE UPPER OOLITES.--a, The base of the Upper Oolites of Britain is const.i.tuted by a great thickness (600 feet or more) of laminated, sometimes carbonaceous or bituminous clays, which are known as the _Kimmeridge Clay_ (_Terrain Kimmeridgien_ of D'Orbigny); b, The _Portland Beds_ (_Terrain Portlandien_ of D'Orbigny) succeed the Kimmeridge clay, and consist inferiorly of sandy beds surmounted by oolitic limestones ("Portland Stone"), the whole series attaining a thickness of 150 feet or more, and containing marine fossils; c, The _Purbeck_ Beds are apparently peculiar to Great Britain, where they form the summit of the entire Oolitic series, attaining a total thickness of from 150 to 200 feet. The Purbeck beds consist of arenaceous, argillaceous, and calcareous strata, which can be shown by their fossils to consist of a most remarkable alternation of fresh-water, brackish-water, and purely marine sediments, together with old land-surfaces, or vegetable soils, which contain the upright stems of trees, and are locally known as "Dirt-beds."

One of the most important of the Jura.s.sic deposits of the continent of Europe, which is believed to be on the horizon of the Coral-rag or of the lower part of the Upper Oolites, is the "_Solenhofen Slate_" of Bavaria, an exceedingly fine-grained limestone, which is largely used in lithography, and is celebrated for the number and beauty of its organic remains, and especially for those of Vertebrate animals.

The subjoined sketch-section (fig. 159) exhibits in a diagrammatic form the general succession of the Jura.s.sic rocks of Britain.

Regarded as a whole, the Jura.s.sic formation is essentially marine; and though remains of drifted plants, and of insects and other air-breathing animals, are not uncommon, the fossils of the formation are in the main marine. In the Purbeck series of Britain, antic.i.p.atory of the great river-deposit of the Wealden, there are fresh-water, brackish-water, and even terrestrial strata, indicating that the floor of the Oolitic ocean was undergoing upheaval, and that the marine conditions which had formerly prevailed were nearly at an end. In places also, as in Yorkshire and Sutherlandshire, are found actual beds of coal: but the great bulk of the formation is an indubitable sea-deposit; and its limestones, oolitic as they commonly are, nevertheless are composed largely of the comminuted skeletons of marine animals. Owing to the enormous number and variety of the organic remains which have been yielded by the richly fossiliferous strata of the Oolitic series, it will not be possible here to do more than to give an outline-sketch of the princ.i.p.al forms of life which characterise the Jura.s.sic period as a whole. It is to be remembered, however, that every minor group of the Jura.s.sic formation has its own peculiar fossils, and that by the labours of such eminent observers as Quenstedt, Oppel, D'Orbigny, Wright, De la Beche, Tate, and others, the entire series of Jura.s.sic sediments admits of a more complete and more elaborate subdivision into zones characterised by special life-forms than has as yet been found practicable in the case of any other rock-series.

[Ill.u.s.tration: Fig. 159. GENERALIZED SECTION OF THE JURa.s.sIC ROCKS OF ENGLAND.]

[Ill.u.s.tration: Fig. 160.--_Mantellia_ (_Cycadeoidea_) _megalophylla_, a Cycad from the Purbeck "dirt-bed." Upper Oolites, England.]

The _plants_ of the Jura.s.sic period consist princ.i.p.ally of Ferns, Cycads, and Conifers--agreeing in this respect, therefore, with those of the preceding Tria.s.sic formation. The _Ferns_ are very abundant, and belong partly to old and partly to new genera. The _Cycads_ are also very abundant, and, on the whole, const.i.tute the most marked feature of the Jura.s.sic vegetation, many genera of this group being known (_Pterophyllum, Otozamites, Zamites, Crossozamia, Williamsonia, Bucklandia,_ &c.) The so-called "dirt-bed" of the Purbeck series consists of an ancient soil, in which stand erect the trunks of Conifers and the silicified stools of Cycads of the genus _Mantellia_ (fig.160). The _Coniferoe_ of the Jura.s.sic are represented by various forms more or less nearly allied to the existing _Araucarioe_; and these are known not only by their stems or branches, but also in some cases by their cones. We meet, also, with the remains of undoubted Endogenous plants, the most important of which are the fruits of forms allied to the existing Screw-pines (_Pandaneoe_), such as _Podocarya_ and _Kaidacarpum_. So far, however, no remains of Palms have been found; nor are we acquainted with any Jura.s.sic plants which could be certainly referred to the great "Angiospermous" group of the Exogens, including the majority of our ordinary plants and trees.

Amongst animals, the _Protozoans_ are well represented in the Jura.s.sic deposits by numerous _Foraminifers_ and _Sponges_; as are the _Coelenterates_ by numerous _Corals_. Remains of these last-mentioned organisms are extremely abundant in some of the limestones of the formation, such as the "Coral-rag" and the Great Oolite; and the former of these may fairly be considered as an ancient "reef." The _Rugose Corals_ have not hitherto been detected in the Jura.s.sic rocks; and the "_Tabulate Corals_,"

so-called, are represented only by examples of the modern genus _Millepora_. With this exception, all the Jura.s.sic Corals belong to the great group which predominates in recent seas (_Zoantharia sclerodermata_); and the majority belong to the important reef-building family of the "Star-corals" (_Astroeidoe_). The form here figured (_Thecosmilia annularis_, fig. 161) is one of the characteristic species of the Coral-rag.

[Ill.u.s.tration: Fig. 161.--_Thecosmilia annularis_, Coral-rag, England.]

[Ill.u.s.tration: Fig. 162.--_Pentacrinus fasciculos_, Lias. The left-hand figure shows a few or the joints of the column; the middle figure shows the arms, and the summit of the column with its side-arms; and the right-hand figure shows the articulating surface of one of the column-joints.]

The _Echinoderms_ are very numerous and abundant fossils in the Jura.s.sic series, and are represented by Sea-lilies, Sea-urchins, Star-fishes, and Brittle-stars. The _Crinoids_ are still common, and some of the limestones of the series are largely composed of the _debris_ of these organisms. Most of the Jura.s.sic forms resemble those with which we are already familiar, in having the body permanently attached to some foreign object by means of a longer or shorter jointed stalk or "column." One of the most characteristic Jura.s.sic genera of these "stalked" Crinoids (though not exclusively confined to this period) is _Pentacrinus_ (fig. 162). In this genus, the column is five-sided, with whorls of "side-arms;" and the arms are long, slender, and branched.

The genus is represented at the present day by the beautiful "Medusa-head Pentacrinite" (_Pentacrinus caput-medusoe_). Another characteristic Oolitic genus is _Apiocrinus_, comprising the so-called "Pear Encrinites." In this group the column is long and rounded, with a dilated base, and having its uppermost joints expanded so as to form, with the cup itself, a pear-shaped ma.s.s, from the summit of which spring the comparatively short arms.

Besides the "stalked" Crinoids, the Jura.s.sic rocks have yielded the remains of the higher group of the "free" Crinoids, such as _Saccosoma_. These forms resemble the existing "Feather-stars"

(_Comatula_) in being attached when young to some foreign body by means of a jointed stem, from which they detach themselves when fully grown to lead an independent existence. In this later stage of their life, therefore, they closely resemble the Brittle-stars in appearance. True Star-fishes (_Asteroids_) and Brittle-stars (_Ophiuroids_) are abundant in the Jura.s.sic rocks, and the Sea-urchins (_Echinoids_) are so numerous and so well preserved as to const.i.tute quite a marked feature of some beds of the series.

All the Oolitic urchins agree with the modern _Echinoids_ in having the sh.e.l.l composed of no more than twenty rows of plates.

Many different genera are known, and a characteristic species of the Middle Oolites (_Hemicidaris crenularis_, fig. 163) is here figured.

[Ill.u.s.tration: Fig. 163.--_Hemicidaris crenularis_, showing the great tubercles on which the spines were supported. Middle Oolites.]

Pa.s.sing over the _Annelides_, which, though not uncommon, are of little special interest, we come to the _Articulates_, which also require little notice. Amongst the _Crustaceans_, whilst the little Water-fleas (_Ostracoda_) are still abundant, the most marked feature is the predominance which is now a.s.sumed by the _Decapods_--the highest of the known groups of the cla.s.s.

True Crabs (_Brachyura_) are by no means unknown; but the princ.i.p.al Oolitic Decapods belonged to the "Long-tailed" group (_Macrura_), of which the existing Lobsters, Prawns, and Shrimps are members.

The fine-grained lithographic slates of Solenhofen are especially famous as a depot for the remains of these Crustaceans, and a characteristic species from this locality (_Eryon arctiformis_, fig. 164) is here represented. Amongst the air-breathing _Articulates_, we meet in the Oolitic rocks with the remains of Spiders (_Arachnida_), Centipedes (_Myriapoda_), and numerous true Insects (_Insecta_). In connection with the last-mentioned of these groups, it is of interest to note the occurrence of the oldest known fossil b.u.t.terfly--the _Paloeontina Oolitica_ of the Stonesfield slate--the relationships of which appear to be with some of the living b.u.t.terflies of Tropical America.

[Ill.u.s.tration: Fig. 164.--_Eryon arctiformis_, a "Long-tailed Decapod," from the Middle Oolites (Solenhofen Slate).]

Coming to the _Mollusca_, the _Polyzoans_, numerous and beautiful as they are, must be at once dismissed; but the _Brachiopods_ deserve a moment's attention. The Jura.s.sic Lamp-sh.e.l.ls (fig.

165) do not fill by any means such a predominant place in the marine fauna of the period, as in many Palaeozoic deposits, but they are still individually numerous. The two ancient genera _Leptoena_ (fig. 165, a) and _Spirifera_ (fig. 165, b), dating the one from the Lower and the other from the Upper Silurian, appear here for the last time upon the scene, but they have not hitherto been recognised in deposits later than the Lias. The great majority of the Jura.s.sic _Brachiopods_, however, belong to the genera _Terebratula_ (fig. 165, c, e, f) and _Rhynchonella_ (fig. 165. d), both of which are represented by living forms at the present day. The _Terebratuloe_, in particular, are very abundant, and the species are often confined to special horizons in the series.

[Ill.u.s.tration: Fig. 165.--Jura.s.sic Brachiopod. a. _Leptoena Lia.s.sica_, enlarged, the small cross below the figure indicating the true size of the sh.e.l.l--Lias; b, _Spirifera rostrata_, Lias; c, _Terebratula quadrifida_, Lias; d, d', _Rhynchonella varians_, Fulter's Earth and Kelloway Rock; e, _Terebratula sph.o.e.roidalis_, Inferior Oolite; f, _Terebratula digona_, Bradford Clay, Forest-marble, and Great Oolite. (After Davidson).]

[Ill.u.s.tration: Fig. 166.--_Ostrea Marshii_. Middle and Lower Oolites.]

[Ill.u.s.tration: Fig. 167.--_Gryphoea incurva_. Lias.]

Remains of _Bivalves_ (_Lamellibranchiata_) are very numerous in the Jura.s.sic deposits, and in many cases highly characteristic.

In the marine beds of the Oolites, which const.i.tute by far the greater portion of the whole formation, the Bivalyes are of course marine, and belong to such genera as _Trigonia, Lima, Pholadomya, Cardinia, Avicula, Hippopodium_, &c.; but in the Purbeck beds, at the summit of the series, we find bands of Oysters alternating with strata containing fresh-water or brackish-water Bivalves, such as _Cyrenoe_ and _Corbuloe_. The predominant Bivalves of the Jura.s.sic, however, are the _Oysters_, which occur under many forms, and often in vast numbers, particular species being commonly restricted to particular horizons. Thus of the true Oysters, _Ostrea distorta_ is characteristic of the Purbeck series, where it forms a bed twelve feet in thickness, known locally as the "Cinder-bed;" _Ostrea expansa_ abounds in the Portland beds; _Ostrea deltoidea_ is characteristic of the Kimmeridge clay; _Ostrea gregaria_ predominates in the Coral-rag; _Ostrea ac.u.minata_ characterises the small group of the Fuller's Earth; whilst the plaited _Ostrea Marshii_ (fig. 166) is a common sh.e.l.l in the Lower and Middle Oolites. Besides the more typical Oysters, the Oolitic rocks abound in examples of the singularly unsymmetrical forms belonging to the genera _Exogyra_ and _Gryphoea_ (fig.

167). In the former of these are included Oysters with the beaks "reversed"--that is to say, turned towards the hinder part of the sh.e.l.l; whilst in the latter are Oysters in which the lower valve of the sh.e.l.l is much the largest, and has a large incurved beak, whilst the upper valve is small and concave. One of the most characteristic _Exogyroe_ is the _E. Virgula_ of the Oxford Clay, and of the same horizon on the Continent; and the _Gryphoea incurva_ (fig. 167) is equally abundant in, and characteristic of, the formation of the Lias. Lastly, we may notice the extraordinary sh.e.l.ls belonging to the genus _Diceras_ (fig. 168), which are exclusively confined to the Middle Oolites. In this formation in the Alps they occur in such abundance as to give rise to the name of "Calcaire a Dicerates," applied to beds of the same age as the Coral-rag of Britain. The genus _Diceras_ belongs to the same family as the "Th.o.r.n.y Clams" (Chama) of the present day--the sh.e.l.l being composed of nearly equally-sized valves, the beaks of which are extremely prominent and twisted into a spiral. The sh.e.l.l was attached to some foreign body by the beak of one of its valves.

[Ill.u.s.tration: Fig. 168.--_Diceras arietina_. Middle Oolite.]

[Ill.u.s.tration: Fig. 169.--_Nerinoea Goodhallii_, one-fourth of the natural size. The left-hand figure shows the appearance presented by the sh.e.l.l when vertically divided. Coral-rag, England.]

Amongst the Jura.s.sic Univalves (_Gasteropoda_) there are many examples of the ancient and long-lived _Pleurotomaria_; but on the whole the Univalves begin to have a modern aspect. The round-mouthed ("holostomatous"), vegetable-eating Sea-snails, such as the Limpets (_Patellidoe_), the Nerites (_Nerita_), the _Turritelloe, Chemnitzioe_, &c., still hold a predominant place.

The two most noticeable genera of this group are _Cerithium_ and _Nerinoea_--the former of these attaining great importance in the Tertiary and Recent seas, whilst the latter (fig. 169) is highly characteristic of the Jura.s.sic series, though not exclusively confined to it. One of the limestones of the Jura, believed to be of the age of the Coral-rag (Middle Oolite) of Britain, abounds to such an extent in the turreted sh.e.l.ls of _Nerinoea_ as to have gained the name of "Calcaire a Nerinees."

In addition to forms such as the preceding, we now for the first time meet, in any force, with the Carnivorous Univalves, in which the mouth of the sh.e.l.l is notched or produced into a ca.n.a.l, giving rise to the technical name of "siphonostomatous" applied to the sh.e.l.l. Some of the carnivorous forms belong to extinct types, such as the _Purpuroidea_ of the Great Oolite; but others are referable to well-known existing genera. Thus we meet here with species of the familiar groups of the Whelks (_Buccinum_), the Spindle-sh.e.l.ls (_Fusus_), the Spider-sh.e.l.ls (_Pteroceras_), _Murex, Rostellaria_, and others which are not at present known to occur in any earlier formation.

Amongst the Wing-sh.e.l.ls (_Pteropoda_), it is sufficient to mark the final appearance in the Lias of the ancient genus _Conularia_.