CHAP. IX.] JURASSIC PERIOD. 159 



Once more, therefore, we find limestone in the south of 

 England and shale in the north, and in this case it is clear 

 that the formation of limestone was possible because of the 

 intervening barrier which prevented the influx of the 

 northern mud-bearing currents. 



We have now brought the history of the British Jurassic 

 sea to a close, and have arrived at the epoch of the great 

 Purbeck-Wealden continent ; but before we can with any 

 confidence restore the physical geography of the Purbeck 

 and Wealden times, we must pause to ask what systems of 

 river-drainage would be likely to come into existence under 

 the circumstances of the Portlandian upheaval. In the 

 first place, the uprise of the central barrier which caused 

 the restriction of the north-eastern part of the Jurassic sea, 

 must have similarly affected the north-western branch of 

 the sea, and if we consider that this had probably been a 

 land-locked gulf throughout the Jurassic period, we are in- 

 evitably led to conclude that the Portlandian upheaval 

 must have converted it into a large inland lake. 



In the absence of any Jurassic deposits of later date than 

 the Oxford Clay over this area there is, of course, no posi- 

 tive evidence for the existence of such a lake, but the pro- 

 bability of its having existed may be increased by another 

 process of inductive reasoning. If there were no such 

 catchment basin on the site of the gulf, this must have 

 formed a wide valley traversed by a river which would 

 receive many large tributaries from the mountains on 

 either side, and, flowing southward across what is now the 

 Irish Sea, would necessarily be a very large and powerful 

 stream, carrying down a great quantity of detritus. If it 

 continued its course over the newly-emerged bed of the 

 Jurassic sea in spite of the local upheaval across England, 1 



1 There is no reason to suppose that the Irish Channel was then in 

 existence any more than the English Channel was, nor is it probable 

 that there was any opening westward into the Atlantic. 





