76 GEOLOGY. 



delicate parts of various sorts of animals are preserved, shows that 

 the conditions of sedimentation were unusual. 



The uppermost horizons of the Jura are wanting in most of the 

 central provinces of Europe, but in England and northern Germany, 

 and at a few points elsewhere, brackish water deposits of the last stages 

 of the epoch are known. In England, these beds (Purbeck) are closely 

 associated with the oldest beds (Wealden) of the next period. 



The Upper Jurassic of southern France and of the Mediterranean 

 province, largely limestone, 1 differs from that of central Europe in 

 recording more uniform conditions. In Portugal, however, the higher 

 members of the Upper Jura are not altogether marine, and the system 

 grades up into the non-marine Lower Cretaceous. 2 Even where the 

 upper part of the Jurassic of southern Europe is marine, it is closely 

 connected with the Lower Cretaceous. In this respect, southern Europe 

 is in contrast with the central part of the continent, where the separa- 

 tion of the Jurassic from the Cretaceous is complete. 



In the eastern province of Europe, the Upper Jura (with late Middle 

 Jura) is widespread. The sea which covered this province is thought 

 to have come in from the north, and to have covered much of Russia. 

 The strata of this province are rather uniform in composition, and 

 mainly clastic, the sands being often glauconitic. In the eastern as 

 in the southern province, the Jura goes over into the Cretaceous with- 

 out stratigraphic break,, 



The Jurassic system attains a very considerable thickness both 

 in the central and southern provinces. 



The frequent alternations of muddy, sandy, and calcareous sedi- 

 ments, which are a marked feature of the system in England and north- 

 ern France, indicate frequent pauses and reversals of the changes 

 affecting either the depth of the water, or the height of the adjacent 

 land, or both. In the failure of petrographic characters to persist 

 through considerable thicknesses, the Jurassic system of the central 

 province is in contrast with most of the systems of the Paleozoic. 



In igneous rocks, the Jurassic system of the south and central 

 provinces of Europe is poor. Such rocks enter into the system in 

 western Scotland (Sky, Mull), and the date of their origin is about 

 the close of the Middle Jura. 



1 Geikie, op. cit., p. 1148. 



2 TJ~™ ~ 11C7 



2 Idem, p. 1157 



