146 NEOZOIC TIME. [CHAP. IX. 



stones. Paludince are rare, and the principal fossils are 



freshwater bivalves and cyprids in the shales; marine 

 shells are still less frequent. 



2. Physical History and Geography. 



1. Lower Jurassic Time. The physical geography of 

 the Ehaetic and Liassic epochs was a simple and direct 

 modification of that which prevailed during the preceding 

 Triassic period. No local elevations and subsidences took 

 place in the British area, for the Ehsetic and Liassic beds 

 occupy the same basins of deposit as those which hold the 

 Keuper marls ; the great lakes or inland seas in which the 

 latter were accumulated became by submergence the seas 

 and bays in which the shales and limestones of the Lias 

 were laid down. 



This submergence set in doubtless toward the end of the 

 Triassic period, and affected the whole of the Triassic north- 

 European continent ; the epoch of the Avicula contorta 

 zone marks the time when the depression had proceeded so 

 far as to submerge the lowest tract of land which lay 

 between the great salt lakes and the wide-spreading southern 

 ocean. It is very probable that at this time the level of 

 the water in the salt lakes had been greatly reduced by 

 evaporation, and was perhaps several hundred feet below 

 that of the sea outside, and that when the dividing barrier 

 was submerged, the sea waters would rapidly invade the 

 lake basins and fill them up to a common level. 



Let us consider the nearly parallel case of the Caspian 

 Sea at the present day; the level of this sea is 85 feet 

 below that of the Black Sea, and it is surrounded by exten- 

 sive low-lying areas which were formerly covered by its 

 waters before the sea shrank to its present dimensions ; if 

 therefore the waters of the Black Sea were admitted to the 

 Caspian through the depression of the present barriers, 



