48 GEOLOGICAL HISTORY. [CH. 



formation; the more restricted German Trias, on the other 

 hand, is a shallow shore, bay or inland sea formation^ 



In the Keuper beds of southern Sweden there are found 

 workable seams of coal, and the beds of this district have 

 yielded numerous well-preserved examples of the Triassic flora. 

 A more impure coal occurs in the lower Keuper of Thuringia 

 and S.-W. Germany, and to this group of rocks the term 

 Lettenkohle is occasionally applied. 



In the Rhaetic Alps of Lombardy, in the Tyrol, and in 

 England, from Yorkshire to Lyme Regis, Devonshire, Somer- 

 setshire, and other districts there are certain strata at the top 

 of the Triassic system known as the Rhaetic or Penarth beds. 

 The uppermost Rhaetic beds, often described as the White 

 Lias, afford evidence of a change from the salt lakes of the 

 Trias to the open sea of the succeeding Jurassic period. 

 Passing beyond this period of salt lakes and wind-swept barren 

 tracts of land, we enter on another phase of the earth's history. 



IX. Jurassic. 



The Jura mountains of western Switzerland consist in 

 great part of folded and contorted rocks which were originally 

 deposited on the floor of a Jurassic sea. In England the 

 Jurassic rocks are of special interest, both for geological and 

 historical reasons, as it is in them that we find a rich fauna and 

 flora of Mesozoic age, and it was the classification of these beds 

 by means of their fossil contents that gained for William Smith 

 the title of the Father of English Geology. A glance at a 

 geological map of England shows a band of Jurassic rocks 

 stretching across from the Yorkshire coast to Dorset. These are 

 in a large measure calcareous, argillaceous, and arenaceous 

 sediments of an open sea ; but towards the upper limit of the 

 series, both freshwater and terrestrial beds are met with. Nu- 

 merous fragments of old coral reefs, sea-urchins, crinoids, and 

 other marine fossils are especially abundant ; in the freshwater 

 beds and old surface-soils, as well as in the marine sandstones 

 1 Kayser and Lake (96), p. 196. 



