CHAP. XIV.] SUMMARY OF GEOGRAPHICAL EVOLUTION. 311 



faratively small one ; it was simply that the subsidence of 

 the land, which probably had been in progress for some 

 time, led at last to the submergence of the French Triassic 

 plains beneath the waters of the Liassic sea, which thus 

 obtained access to the British area and converted the 

 Triissic lake into a gulf of that sea. 



So far as we can tell, the geography of Liassic Britain 

 was precisely the same as in Triassic time, except that the 

 site of the great lake was occupied by a sea which opened 

 southward, and probably that the area covered by water 

 was somewhat larger. The position of the land- tracts was 

 not, so far as we know, otherwise altered, but they were 

 probably exposed to more rapid processes of surface dis- 

 integration, and the removal of material from higher to 

 lower levels was correspondingly accelerated. 



Neither was this geographical arrangement materially 

 altered during the formation of the sands and limestones 

 of the Middle Jurassic series. We find proofs, however, in 

 certain portions of this series that large rivers were in 

 existence and poured large quantities of material into the 

 sea, so as to fill up portions of it and convert them into 

 shallow bays surrounded by tracts of swampy alluvial land. 

 Further, it would appear that subsidence ceased for a time, 

 and that some portions of the sea-bottom were raised 

 while other parts remained stationary, the result of these 

 movements being the formation of several distinct basins 

 separated from one another by submarine ridges like those 

 of the Mediterranean Sea ; one of these ridges was a pro- 

 longation of the Mendip uplift, and another crossed the 

 central part of England. 



The period of the Upper Jurassic clays was one of 

 general and extensive subsidence ; there can be little doubt 

 that during their deposition the coast-lines were carried 

 backward and the area of the sea was greatly widened, but 

 how far this recession continued, and how much of the 



