CHAPTER XXI.

THE TRIASSIC SYSTEM.

_Classification._ The term Triassic has been applied to these rocks on account of the threefold division into which those of Germany naturally fall. These three divisions are:--

Keuper, Muschelkalk, Bunter;

but above the Keuper beds we find a group of deposits of some importance, which shew affinities with both Triassic and Jurassic rocks, which may be looked upon as true passage beds, though they are generally placed in the Triassic System. They are known as Rhætic or locally in Britain as Penarth Beds. The Muschelkalk is usually considered to be unrepresented in Britain, and accordingly the British deposits may be, and are usually grouped as under:--

Rhætic or Penarth beds Keuper { Keuper Marls { Keuper Sandstones [Muschelkalk] absent { Upper Red and Mottled Sandstones Bunter { Bunter Pebble Beds { Lower Red and Mottled Sandstones.

The threefold grouping has been applied more or less universally, but when used outside the north-west European area, it loses its significance, as the conditions which enable one to differentiate the rocks of the three divisions were naturally only prevalent over a limited area.

_Description of the strata._ The British Triassic rocks possess a certain sameness as regards their general characters, consisting mainly of mechanical sediments coloured red by peroxide of iron, with occasional chemical precipitates of rock-salt and gypsum. They have a wider distribution over Britain than have the Permian rocks, and the lithological characters of the different subdivisions do not as a rule vary to a remarkable degree when traced laterally. The differences in detail in the characters of the various deposits are noteworthy, and an explanation of the exact origin of some of these abnormal deposits which will satisfy everyone is not yet forthcoming. Leaving the details out of consideration for the moment, and looking at the general aspect of the deposits, the prevalence of conditions generally similar to those which existed over the British Isles in the preceding Permian period is decidedly indicated by the nature of the strata, though the continental conditions appear to have been more widely established over our area, as shewn by the general absence of any calcareous deposits resembling the Magnesian Limestone. We find chemical precipitates, millet-seed sandstones, and scree-like breccias in the British Triassic rocks as well as in those of Permian age, and the paucity of a marine invertebrate fauna in the Triassic rocks of Britain is even more apparent than in the Permian strata. It is only at the extreme close of the Triassic period, during the deposition of the rocks which are admitted on all hands to be of Rhætic age, that we note the incoming of those marine conditions over our area, which prevailed so extensively, with few local exceptions, during the remainder of the Mesozoic and the early part of Tertiary times; the Rhætic beds, in fact, mark the commencement of the third marine period. Referring to the strata in further detail, we may proceed to consider the character of the different subdivisions in the order of their formation, commencing as usual with the oldest. The Bunter deposits rest in places upon those of Permian age with an unconformity at the junction, but as these unconformities occur frequently among the British Triassic rocks, it is doubtful whether this unconformity marks more than very local change of physical conditions. The lower and upper divisions of the Bunter sandstone consist of false-bedded red and variegated sandstones, and there is no great difficulty in explaining their formation in desert areas with tracts of water, but the great change which marks the appearance and disappearance of the middle division, the Bunter pebble beds, requires some explanation, for the contrast between the lithological characters of the rocks of this division and those of the rocks appertaining to the preceding and succeeding division is very marked. The matrix differs, but the main difference is the abundance of pebbles, mostly of fairly uniform size, well rounded, and largely consisting of liver-coloured quartzite. Much difference of opinion exists as to the exact origin of these pebble beds, and the source of the pebbles, but without entering into this vexed question, it may be remarked that the agency of rivers has been somewhat generally invoked to account for their transport, and the conditions during their accumulation need not have been very different from those which are now found in northern India where the torrential rivers of the south side of the Himalayan chains debouch upon the plain, and spread an abundant deposit of well-worn pebbles over the finer silts which were previously laid down thereon.

The junction of the Bunter and Keuper beds requires a short notice. It is usually if not always an unconformable one in Britain, and it is generally assumed that the absence of the Muschelkalk of the Continent is due to the presence of land undergoing denudation in Britain during the time when the Muschelkalk was elsewhere deposited, though it is quite possible that the Muschelkalk epoch is represented in Britain not only by the time which elapsed when the unconformity was being impressed on the rocks, but also during the true deposition of the upper part of the Bunter beds, or the lower part of the Keuper, or both.

The Keuper sandstones and marls contain a great development of chemical deposits, of millet-seed sands, and of many other features pointing to desert conditions, such as sun-cracks, tracks of animals impressed upon a rapidly drying surface, and pseudomorphs of mud after rock salt in the form of cubes and hopper-crystals; furthermore we find the scree-like breccias at different horizons of the Keuper beds where they abut against the old Mendip ridge composed largely of mountain-limestone which furnished the fragments, as was the case with the brockrams abutting against the Pennine ridge. It must be noted that the chemical precipitates of Triassic age consist of the less soluble substances dissolved in ocean water, namely, gypsum and rock salt, whilst the more deliquescent potash and magnesia salts are not represented in Britain.

Turning to these continental beds, we get evidence of a general approach to open sea conditions as we pass away from Britain in a south-easterly direction as roughly shewn in the following diagram (fig. 22), where _B_ represents the Bunter beds, _M_ the Muschelkalk, and _K_ the Keuper.

It will be seen that the mechanical sediments gradually die out and become replaced by calcareous material as one passes from Britain towards Switzerland; the Muschelkalk is very thin in the east of France and thickens out in Germany, while in Switzerland Keuper, Muschelkalk and Bunter are alike largely represented by calcareous deposits, and the mechanical deposits are chiefly argillaceous, the only important sandstone being situated at the extreme base of the Bunter series.

The marine development of the Triassic system is naturally the one which is most widely spread, though full appreciation of its importance has only taken place as the result of researches in distant climes of recent years. It is found in southern Europe, in Spitsbergen, in considerable tracts of Asia, including India, and along the Pacific coast region of North America, and everywhere possesses much the same characters.

It will be seen from the above remarks that the physical conditions which prevailed in the continental area of Triassic times which is now partly occupied by the British Isles are most closely represented by those of the desert regions of central Asia, hemmed in by the mountain ranges which intercept the vapour-laden winds of the oceans, and cause them to precipitate the great bulk of their vapour on the seaward slopes of the mountains, so that they blow over the deserts as dry winds, causing the fall of any large amount of rain to be a rare though by no means unknown event in the desert regions.

_Flora and Fauna of the Period._ The Triassic flora is essentially similar to that of the higher Permian strata, though many of the genera are different.

The invertebrate fauna of the British deposits is, as might be expected, very poor until the beds of the Rhætic series are reached. In the beds below the Rhætics, the principal invertebrate remains are the tests of the crustacean genus _Estheria_, though a few obscure lamellibranch shells have been recorded. The vertebrate fauna is of great interest. A number of fishes have been found, the most remarkable of which is the genus _Ceratodus_, occurring in the Rhætic beds of Britain and lower Triassic strata of foreign countries. It is closely related to the Barramunda of the Queensland rivers belonging to the order Dipnoi. As in the Permian strata, abundance of Labyrinthodont amphibians have been discovered, and the reptiles belong to the orders Anomodontia and Rhynchocephalia. In the Rhætic beds of Britain and in still lower Triassic beds abroad the orders Ichthyopterygia and Sauropterygia (represented by _Ichthyosaurus_ and _Plesiosaurus_) are found.

The Triassic rocks also yield the earliest known mammals, the best known, _Microlestes_, occurring in the Triassic rocks of Britain and the Continent. These mammals are now placed in a subclass Metatheria of the order Monotremata.

The marine invertebrate fauna of the normal Triassic rocks presents some points of considerable interest. As already remarked, the fauna may be looked upon as a passage fauna between that of Palæozoic and that of Mesozoic times, the number of Palæozoic forms which pass into the Trias being approximately comparable with those which appear here and range upwards into higher Mesozoic strata. This may be well seen by examining the table given in Chapter XXI. of the Second Edition of Sir Charles Lyell's _Student's Elements of Geology_, in which three columns shew the genera of Mollusca common to older rocks, those characteristic of the Trias, and those common to newer rocks. Amongst the first are _Orthoceras_, _Bactrites_, _Loxonema_, _Murchisonia_, and _Euomphalus_, in the second column are _Ceratites_, _Halobia_ (_Daonella_), _Koninckina_, and _Myophoria_, and in the third, Ammonites, _Cerithium_, _Opis_, _Plicatula_ and _Thecidium_[98].

[Footnote 98: It has been seen that some of the Ammonites appear earlier, namely, in Permian strata. _Myophoria_ is extremely abundant in the Trias, but ranges into newer strata.]

The Ammonites are largely utilised in the case of the Mesozoic strata for separation of these strata into zones, each zone being characterised by some species of Ammonite, and the researches of Mojsisovics have proved that this zonal subdivision, long adopted for Jurassic rocks, is also applicable to those of Triassic age[99]. He gives the following table of the classification of the Triassic rocks of the Mediterranean Province, which is reproduced, as it is founded upon Palæontological evidence, and will probably be widely adopted.

[Footnote 99: von Mojsisovics, Dr E., "Faunistische Ergebnisse aus der Untersuchung der Ammoneen-faunen der Mediterranen Trias." _Abhandl. der k. k. Geologisch. Reichsanstalt_, VI. Band 2 Abtheilung. Vienna, 1893.]

Series Zonal Divisions --------------+-------------------+-------------------------------------- Rhætic | | 1. Zone of _Avicula Contorta_ --------------+-------------------+-------------------------------------- | | 2. Zone of _Sirenites Argonautae_ | Upper Juvavic | 3. Zone of _Pinnacoceras | | Metternichi_ Juvavic | Middle Juvavic | 4. Zone of _Cyrtopleurites | | bicrenatus_ | | 5. Zone of _Cladiscites ruber_ | Lower Juvavic | 6. Zone of _Sagenites Giebeli_ --------------+-------------------+-------------------------------------- | Upper Carnic | 7. Zone of _Tropites subbullatus_ Carnic | Middle Carnic | 8. Zone of _Trachyceras Aonoides_ | Lower Carnic | 9. Zone of _Trachyceras Aon_ --------------+-------------------+-------------------------------------- | Upper Noric | 10. Zone of _Protrachyceras Noric | | Archelaus_ | Lower Noric | 11. Zone of _Protrachyceras Curionii_ --------------+-------------------+-------------------------------------- | Upper Muschelkalk | 12. Zone of _Ceratiles trinodosus_ Muschelkalk | | | Lower Muschelkalk | 13. Zone of _Ceratiles binodosus_ --------------+-------------------+-------------------------------------- Buntsandstein | Werfener Schichten| 14. Zone of _Tirolites Cassianus_ --------------+-------------------+--------------------------------------