TRIASSIC PERIOD. 531 



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 contain no lime, and are therefore not 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 about 1200 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, lime- 

 stone), from the abundance of fossil shells which it contains. 

 The Muschelkalk consists of compact grey or yellowish lime- 

 stones, sometimes dolomitic, and including occasional beds of 

 gypsum and rock-salt. 



III. The Upper Trias, or Keuper^ as it is generally called, 

 occurs in England ; but is not so well developed as it is in 

 Germany. In Britain the Keuper is about 1000 feet in thick- 

 ness, 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 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 Triassic, as they certainly agree 

 with the so-called St Cassian or Kossen 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 dark grey and 

 black shales, surmounted conformably by the lowest beds of 

 the Lias. The most characteristic fossils which they contain 

 are the three Bivalves Cardium Rhc&ticum, 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 



