II IK JURASSIC SYSTEM 383 



Purbeck Beds 1 ., 



Portland Beds } Portlands. 



f Pu 

 Po 



I'l'I'iT Jurassic -\ Kimeridge clay Kiiiierid^ui 



I Corallian Beds Sequaniun. 



VOxford clay Oxfordian. 



MIIII f Great Oolite Bathonian. 



Middle Jurassic-! T r\ v* 



(Inferior Oolite Bajocian. 



Lower Jurassic 



flipper Lias Toarciau. 



I Middle Lias Charniouthian. 



I Lower Lias and Rhajtic ( Sinemurian. 



V (Hettangian. 



It will be noticed that I follow French geologists in regarding 

 the Rhaetic or zone of Pteria contorta as the basal part of the 

 Liassic Series, while the Germans have generally included it in the 

 Trias. These beds were briefly referred to on p. 375, and reason 

 was there given for restricting the name to this zone, which succeeds 

 that of Gervillia exilis in the Eastern Alps but spreads far and 

 wide beyond the latter over large parts of Europe. There is no 

 doubt that it marks an epoch when the sea once more occupied the 

 Triassic lowlands of Northern and Western Europe, and that over 

 these lowlands it is a mere zone, from 30 to 40 feet thick, which is 

 more intimately connected with the overlying Lias than with the 

 underlying Keuper. 



In dealing with the stratigraphy of this system it will be 

 convenient to take each of the three great divisions or series 

 separately, and to give a condensed account of the component 

 strata of each series, both in Britain and on the Continent, before 

 proceeding to the next one. We can afterwards review the general 

 history of the whole period, and indicate the geographical changes 

 which took place at its close. 



B. LIFE OF THE PERIOD 



The reader may be reminded that throughout Triassic times our 

 islands formed part of a continent which occupied the northern 

 part of Europe, and some part at least of the North Atlantic region, 

 and that it was not until the Rhaetic epoch that the inland lakes 

 of this continent were finally submerged beneath the waters of an 

 open sea. Further, the fauna of the British Rhaetic beds or zone 

 of Pteria [ = Avicula] contorta in Northern Europe contains but a 

 small number of species, and it was not till the time of the Lias 

 that conditions were favourable for the establishment of a new and 

 abundant marine fauna over the whole region. We are thus 

 suddenly presented with an assemblage of fossils which is very 

 different from those of the Carboniferous and Permian periods, and 



