The Ancient Life History of the Earth Part 15

(19) 'Genera et Species Plantarum Fossilium.' Unger.

(20) "On the Red Rocks of England of older Date than the Trias"

--'Quart. Journ. Geol. Soc.,' vol. xxvii. Ramsay.

CHAPTER XV.

THE TRIa.s.sIC PERIOD.

We come now to the consideration of the great _Mesozoic_, or Secondary series of formations, consisting, in ascending order, of the Tria.s.sic, Jura.s.sic, and Cretaceous systems. The Tria.s.sic group forms the base of the Mesozoic series, and corresponds with the higher portion of the New Red Sandstone of the older geologists. Like the Permian rocks, and as implied by its name, the _Trias_ admits of a subdivision into three groups--a Lower, Middle, and Upper Trias. Of these sub-divisions the middle one is wanting in Britain; and all have received German names, being more largely and typically developed in Germany than in any other country. Thus, the Lower Trias is known as the _Bunter Sandstein_; the Middle Trias is called the _Muschelkalk_; and the Upper Trias is known as the _Keuper_.

I. The lowest division of the Trias is known as the _Bunter Sandstein_ (the _Gres bigarre_ of the French), from the generally variegated colours of the beds which compose it (German, _bunt_, variegated). The Bunter Sandstein of the continent of Europe consists of red and white sandstones, with red clays, and thin limestones, the whole attaining a thickness of about 1500 feet.

The term "marl" is very generally employed to designate the clays of the Lower and Upper Trias; but the term is inappropriate, as they may contain no lime, and are therefore not always genuine marls. In Britain the Bunter Sandstein consists of red and mottled sandstones, with unconsolidated conglomerates, or "pebble-beds,"

the whole having a thickness of 1000 to 2000 feet. The Bunter Sandstein, as a rule, is very barren of fossils.

II. The Middle Trias is not developed in Britain, but it is largely developed in Germany, where it const.i.tutes what is known as the _Muschelkalk_ (Germ. _Muschel_, mussel; _kalk_, limestone), from the abundance of fossil sh.e.l.ls which it contains. The Muschelkalk (the _Calcaire coquillier_ of the French) consists of compact grey or yellowish limestones, sometimes dolomitic, and including occasional beds of gypsum and rock-salt.

III. The Upper Trias, or _Keuper_ (the _Marnes irisees_ of the French), as it is generally called, occurs in England; but is not so well developed as it is in Germany. In Britain, the Keuper is 1000 feet or more in thickness, and consists of white and brown sandstones, with red marls, the whole topped by red clays with rock-salt and gypsum.

The Keuper in Britain is extremely unfossiliferous; but it pa.s.ses upwards with perfect conformity into a very remarkable group of beds, at one time cla.s.sed with the Lias, and now known under the names of the Penarth beds (from Penarth, in Glamorgans.h.i.+re), the Rhaetic beds (from the Rhaetic Alps), or the _Avicula contorta_ beds (from the occurrence in them of great numbers of this peculiar Bivalve). These singular beds have been variously regarded as the highest beds of the Trias, or the lowest beds of the Lias, or as an intermediate group. The phenomena observed on the Continent, however, render it best to consider them as Tria.s.sic, as they certainly agree with the so-called Upper St Ca.s.sian or Kossen beds which form the top of the Trias in the Austrian Alps.

The Penarth beds occur in Glamorgans.h.i.+re, Gloucesters.h.i.+re, Warwicks.h.i.+re, Staffords.h.i.+re, and the north of Ireland; and they generally consist of a small thickness of grey marls, white limestones, and black shales, surmounted conformably by the lowest beds of the Lias. The most characteristic fossils which they contain are the three Bivalves _Cardium Rhoetic.u.m, Avicula contorta_, and _Pecten Valoniensis_; but they have yielded many other fossils, amongst which the most important are the remains of Fishes and small Mammals (_Microlestes_).

In the Austrian Alps the Trias terminates upwards in an extraordinary series of fossiliferous beds, replete with marine fossils. Sir Charles Lyell gives the following table of these remarkable deposits:--

_Strata below the Lias in the Austrian Alps, in descending order._

/ Grey and black limestone, with calcareous marls having a thickness of about 50 feet. Among the fossils, Brachiopoda 1. Koessen beds. very numerous; some few species common (Synonyms, Upper to the genuine Lias; many peculiar.

St Ca.s.sian beds of < _avicula="" contorta,="" pecten="" valoniensis_,="" escher="" and="" merian.)="" _cardium="" rhoetic.u.m,="" avicula_="" _inoequivalvis,="" spirifer="" munsteri_,="" dav.="" strata="" containing="" the="" above="" fossils="" alternate="" with="" the="" dachstein="" beds,="" lying="" next="">

/ White or greyish limestone, often in beds three or four feet thick. Total thickness of the formation above 2000 feet. Upper part fossiliferous, with some strata 2. Dachstein beds. < composed="" of="" corals="" (_lithodendron_.)="" lower="" portion="" without="" fossils.="" among="" the="" characteristic="" sh.e.l.ls="" are="" _hemicardium_="" _wulfeni,="" megalodon="" triqueler_,="" and="" other="" large="">

/ Red, pink, or white marbles, from 800 to 1000 feet in thickness, containing more than 800 species of marine fossils, for 3. Hallstadt beds the most part mollusca. Many species of (or St Ca.s.sian). < _orthoceras_.="" true="" _ammonites_,="" besides="" _cerat.i.tes_="" and="" _goniat.i.tes,="" belemnites_="" (rare),="" _porcellia,="" pleurotomania,="" trochus_,="" _monotis="" salinaria_,="">

/ A. Black and grey Among the fossils 4. A. Guttenstein beds. limestone 150 feet are _Cerat.i.tes_ B. Werfen beds, base thick, alternating _ca.s.sia.n.u.s_, of Upper Trias? with the underlying _Myacites_ Lower Trias of < werfen="" beds.=""> _fa.s.saensis_, some geologists. B. Red and green _Naticella_ shale and sandstone, _costata_, &c.

with salt and gypsum./

In the United States, rocks of Tria.s.sic age occur in several areas between the Appalachians and the Atlantic seaboard; but they show no such triple division as in Germany, and their exact place in the system is uncertain. The rocks of these areas consist of red sandstones, sometimes shaly or conglomeratic, occasionally with beds of impure limestone. Other more extensive areas where Tria.s.sic rocks appear at the surface, are found west of the Mississippi, on the slopes of the Rocky Mountains, where the beds consist of sandstones and gypsiferous marls. The American Trias is chiefly remarkable for having yielded the remains of a small Marsupial (_Dromatherium_), and numerous footprints, which have generally been referred to Birds (_Brontozoum_), along with the tracks of undoubted Reptiles (_Otozoum, Anisopus_, &c.)

The subjoined section (fig. 139) expresses, in a diagrammatic manner, the general sequence of the Tria.s.sic rocks when fully developed, as, for example, in the Bavarian Alps:--

[Ill.u.s.tration: Fig. 139. GENERALIZED SECTION OF THE TRIa.s.sIC ROCKS OF CENTRAL EUROPE.]

With regard to the _life_ of the Tria.s.sic period, we have to notice a difference as concerns the different members of the group similar to that which has been already mentioned in connection with the Permian formation. The arenaceous deposits of the series, namely, resemble those of the Permian, not only in being commonly red or variegated in their colour, but also in their conspicuous paucity of organic remains. They for the most part are either wholly unfossiliferous, or they contain the remains of plants or the bones of reptiles, such as may easily have been drifted from some neighbouring sh.o.r.e. The few fossils which may be considered as properly belonging to these deposits are chiefly Crustaceans (_Estheria_) or Fishes, which may well have lived in the waters of estuaries or vast inland seas. We may therefore conclude, with considerable probability, that the barren sandy and marly acc.u.mulations of the Bunter Sandstein and Lower Keuper were not laid down in an open sea, but are probably brackish-water deposits, formed in estuaries or land-locked bodies of salt water. This at any rate would appear to be the case as regards these members of the series as developed in Britain and in their typical areas on the continent of Europe; and the origin of most of the North American Trias would appear to be much the same. Whether this view be correct or not, it is certain that the beds in question were laid down in _shallow_ water, and in the immediate vicinity of _land_, as shown by the numerous drifted plants which they contain and the common occurrence in them of the footprints of air-breathing animals (Birds, Reptiles, and Amphibians). On the other hand, the middle and highest members of the Trias are largely calcareous, and are replete with the remains of undoubted marine animals. There cannot, therefore, be the smallest doubt but that the Muschelkalk and the Rhaetic or Kossen beds were slowly acc.u.mulated in an open sea, of at least a moderate depth; and they have preserved for us a very considerable selection from the marine fauna of the Tria.s.sic period.

[Ill.u.s.tration: Fig. 140.--_Zamia spiralis_, a living Cycad.

Australia.]

The _plants_ of the Trias are, on the whole, as distinctively Mesozoic in their aspect as those of the Permian are Palaeozoic.

In spite, therefore, of the great difficulty which is experienced in effecting a satisfactory stratigraphical separation between the Permian and the Trias, we have in this fact a proof that the two formations were divided by an interval of time sufficient to allow of enormous changes in the terrestrial vegetation of the world. The _Lepidodendroids, Asterophyllites_, and _Annularioe_, of the Coal and Permian formations, have now apparently wholly disappeared: and the Tria.s.sic flora consists mainly of Ferns, Cycads, and Conifers, of which only the two last need special notice. The _Cycads_ (fig. 140) are true exogenous plants, which in general form and habit of growth present considerable resemblance to young Palms, but which in reality are most nearly related to the Pines and Firs (_Coniferoe_). The trunk is unbranched, often much shortened, and bears a crown of feathery pinnate fronds.

The leaves are usually "circinate"--they unroll in expanding, like the fronds of ferns. The seeds are not protected by a seed-vessel, but are borne upon the edge of altered leaves, or are carried on the scales of a cone. All the living species of Cycads are natives of warm countries, such as South America, the West Indies, j.a.pan, Australia, Southern Asia, and South Africa.

The remains of Cycads, as we have seen, are not known to occur in the Coal formation, or only to a very limited extent towards its close; nor are they known with certainty as occurring in Permian deposits. In the Tria.s.sic period, however, the remains of Cycads belonging to such genera as _Pterophyllum_ (fig. 141, b), _Zamites_, and _Podozamites_ (fig. 141, c), are sufficiently abundant to const.i.tute quite a marked feature in the vegetation; and they continue to be abundantly represented throughout the whole Mesozoic series. The name "Age of Cycads," as applied to the Secondary epoch, is therefore, from a botanical point of view, an extremely appropriate one. The _Conifers_ of the Trias are not uncommon, the princ.i.p.al form being _Veltzia_ (fig. 141, a), which possesses some peculiar characters, but would appear to be most nearly related to the recent Cypresses.

[Ill.u.s.tration: Fig. 141.--Tria.s.sic Conifers and Cycads. a, _Voltzia_ (_Schizoneura_) _heterophylla_, portion of a branch, Europe and America; b, Part of the frond of _Pterophyllum Joegeri_, Europe; c, Part of the frond of _Podozamites lanceolatus_, America.]

As regards the _Invertebrate animals_ of the Trias, our knowledge is still princ.i.p.ally derived from the calcareous beds which const.i.tute the centre of the system (the Muschelkalk) on the continent of Europe, and from the St Ca.s.sain and Rhaetic beds still higher in the series; whilst some of the Tria.s.sic strata of California and Nevada have likewise yielded numerous remains of marine Invertebrates. The _Protozoans_ are represented by _Foraminifera_ and _Sponges_, and the _Coelenterates_ by a small number of _Corals_; but these require no special notice. It may be mentioned, however, that the great Palaeozoic group of the _Rugose_ corals has no known representative here, its place being taken by corals of Secondary type (such as _Montlivaltia, Synastoea_, &c.)

The _Echinoderms_ are represented princ.i.p.ally by _Crinoids_, the remains of which are extremely abundant in some of the limestones. The best-known species is the famous "Lily-Encrinite"

(_Encrinus liliiformis_, fig. 142), which is characteristic of the Muschelkalk. In this beautiful species, the flower-like head is supported upon a rounded stem, the joints of which are elaborately articulated with one another; and the fringed arms are composed each of a double series of alternating calcareous pieces. The Palaeozoic Urchins, with their supernumerary rows of plates, the Cystideans, and the Pentremites have finally disappeared; but both Star-fishes and Brittle-stars continue to be represented.

One of the latter--namely, the _Aspidura loricata_ of Goldfuss (fig. 143)--is highly characteristic of the Muschelkalk.

[Ill.u.s.tration: Fig. 142.--Head and upper part of the column of _Encrinus liliiformis_. The lower figure shows the articulating surface of one of the joints of the column. Muschelkalk, Germany.]

[Ill.u.s.tration: Fig. 143.--_Aspidura loricata_, a Tria.s.sic Ophiuroid.

Muschelkalk, Germany.]

The remains of _Articulate Animals_ are not very abundant in the Trias, if we except the bivalved cases of the little Water-fleas (_Ostracoda_), which are occasionally very plentiful. There are also many species of the h.o.r.n.y, concentrically-striated valves of the _Estherioe_ (see fig. 122, b), which might easily be taken for small Bivalve Molluscs. The "Long-tailed" Decapods of the type of the Lobster, are not without examples but they become much more numerous in the succeeding Jura.s.sic period.

Remains of insects have also been discovered.

Amongst the _Mollusca_ we have to note the disappearance, amongst the lower groups, of many characteristic Palaeozoic types. Amongst the _Polyzoans_, the characteristic "Lace-corals," _Fenestella, Retepora_,[22] _Synocladia, Polypora_, &c., have become apparently extinct. The same is true of many of the ancient types of _Brachiopods_, and conspicuously so of the great family of the _Productidoe_, which played such an important part in the seas of the Carboniferous and Permian periods.

[Footnote 22: The genus _Retefora_ is really a recent one, represented by living forms; and the so-called _Reteporoe_ of the Palaeozoic rocks should properly receive another name (_Phyllopora_), as being of a different nature. The name _Retepora_ has been here retained for these old forms simply in accordance with general usage.]

[Ill.u.s.traton: Fig. 144. Tria.s.sic Lamellibranchs. a, _Daonella_ (_Halobia_) _Lommelli_; b, _Pecten Valoniensis_; c, _Myophoria lineata_; d. _Cardium Rhoetic.u.m_; e. _Avicula contorta_; f. _Avicula socialis_.]

_Bivalves_ (_Lamellibranchiata_) and _Univalves_ (_Gasteropoda_) are well represented in the marine beds of the Trias, and some of the former are particularly characteristic either of the formation as a whole or of minor subdivisions of it. A few of these characteristic species are figured in the accompanying ill.u.s.tration (fig. 144).

Bivalve sh.e.l.ls of the genera _Daonella_ (fig. 144, a) and _Halobia_ (_Monotis_) are very abundant, and are found in the Tria.s.sic strata of almost all regions. These groups belong to the family of the Pearl-oysters (_Aviculidoe_), and are singular from the striking resemblance borne by some of their included forms to the _Strophomenoe amongst the Lamp-sh.e.l.ls, though, of course, no real relation exists between the two. The little Pearl-oyster, _Avicula socialis_ (fig. 144, f), is found throughout the greater part of the Tria.s.sic series, and is especially abundant in the Muschelkalk. The genus _Myophoria_ (fig. 144, c), belonging to the _Trigoniadoe_, and related therefore to the Permian _Schizodus_, is characteristically Tria.s.sic, many species of the genus being known in deposits of this age. Lastly, the so-called "Rhaetic" or "Kossen" beds are characterised by the occurrence in them of the Scallop, _Pecten Valoniensis_ (fig. 144, b); the small c.o.c.kle, _Cardium Rhoetic.u.m_ (fig. 144, d); and the curiously-twisted Pearl-oyster, _Avicula contorta_ (fig. 144, e)--this last Bivalve being so abundant that the strata in question are often spoken of as the "Avicula contorta beds."

[Ill.u.s.tration: Fig. 145.--_Cerat.i.tes nodosus_, viewed from the side and from behind. Muschelkalk.]

Pa.s.sing over the groups of the _Heteropods_ and _Pteropods_, we have to notice the _Cephalopoda_, which are represented in the Trias not only by the chambered sh.e.l.ls of _Tetrabranchiates_, but also, for the first time, by the internal skeletons of _Dibranchiate_ forms. The Trias, therefore, marks the first recognised appearance of true Cuttle-fishes. All the known examples of these belong to the great Mesozoic group of the _Belemnitidoe_; and as this family is much more largely developed in the succeeding Jura.s.sic period, the consideration of its characters will be deferred till that formation is treated of. Amongst the chambered _Cephalopods_ we find quite a number of the Palaeozoic _Orthocerat.i.tes_, some of them of considerable size, along with the ancient _Cyrtoceras_ and _Goniat.i.tes_; and these old types, singularly enough, occur in the higher portion of the Trias (St Ca.s.sian beds), but have, for some unexplained reason, not yet been recognised in the lower and equally fossiliferous formation of the Muschelkalk. Along with these we meet for the first time with true _Ammonites_, which fill such an extensive place in the Jura.s.sic seas, and which will be spoken of hereafter. The form, however, which is most characteristic of the Trias is _Cerat.i.tes_ (fig. 145). In this genus the sh.e.l.l is curved into a flat spiral, the volutions of which are in contact; and it further agrees with both _Goniat.i.tes_ and _Ammonites_ in the fact that the septa or part.i.tions between the air-chambers are not simple and plain (as in the _Nautilus_ and its allies), but are folded and bent as they approach the outer wall of the sh.e.l.l. In the _Goniat.i.te_ these foldings of the septa are of a simply lobed or angulated nature, and in the _Ammonite_ they are extremely complex; whilst in the _Cerat.i.te_ there is an intermediate state of things, the special feature of which is, that those foldings which are turned towards the mouth of the sh.e.l.l are merely rounded, whereas those which are turned away from the mouth are characteristically toothed. The genus _Cerat.i.tes_, though princ.i.p.ally Tria.s.sic, has recently been recognised in strata of Carboniferous age in India.

From the foregoing it will be gathered that one of the most important points in connection with the Tria.s.sic _Mollusca_ is the remarkable intermixture of Palaeozoic and Mesozoic types which they exhibit.

It is to be remembered, also, that this intermixture has. .h.i.therto been recognised, not in the Middle Tria.s.sic limestones of the Muschelkalk, in which--as the oldest Tria.s.sic beds with marine fossils--we should naturally expect to find it, but in the St Ca.s.sian beds, the age of which is considerably later than that of the Muschelkalk. The intermingling of old and new types of Sh.e.l.l-fish in the Upper Trias is well brought out in the annexed table, given by Sir Charles Lyell in his 'Student's Elements of Geology' (some of the less important forms in the table being omitted here):--

GENERA OF FOSSIL MOLLUSCA IN THE ST Ca.s.sIAN AND HALLSTADT BEDS.

Common to Characteristic of Common to Older Rocks. Tria.s.sic Rocks Newer Rocks.

Orthoceras. Cerat.i.tes. Ammonites.

Bactrites. Cochloceras. Chemnitzia.

Macrocheilus. Rhabdoceras. Cerithium.

Loxonema. Aulacoceras. Monodonta.

Holopella. Naticella. Sph.o.e.ra.

Murchisonia. Platystoma. Cardita.

Porcellia. Halobia. Myoconcha.

Athyris. Hornesia. Hinnites.

Retzia. Koninckia. Monotis.

Cyrtina. Scoliostoma. Plicatula.