CHAPTER XV.
THE TRIASSIC PERIOD.
We come now to the consideration of the great Mesozoic, or Secondary series of formations, consisting, in ascending order, of the Triassic, Jurassic, and Cretaceous systems. The Triassic 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 Grès bigarré 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 constitutes what is known as the Muschelkalk (Germ. Muschel, mussel; kalk, limestone), from the abundance of fossil shells 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 irisées 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 passes upwards with perfect conformity into a very remarkable group of beds, at one time classed with the Lias, and now known under the names of the Penarth beds (from Penarth, in Glamorganshire), the Rhætic beds (from the Rhætic 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 Triassic, as they certainly agree with the so-called Upper St Cassian or Kössen beds which form the top of the Trias in the Austrian Alps.
The Penarth beds occur in Glamorganshire, Gloucestershire, Warwickshire, Staffordshire, 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 Rhœticum, 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.