228 NEOZOIC TIME. [CHAP. XI. 



lignitic series of the Paris basin, and the sudden appear- 

 ance in them and in their English equivalents of tropical 

 forms of mollusca, we may infer that a subsidence took place 

 which submerged part of the intervening land and allowed 

 the waters of the great Eocene Mediterranean to occupy a 

 portion of the low-lying tract on the northern side of the 

 barrier. The temperature was raised by this influx of 

 warm water, and a sub-tropical fauna and flora were 

 established on the shores of Britain. 



In Mr. Gardner's opinion, however, this southern sea 

 never reached far into Britain. He thinks that its shore 

 must have lain across the north of Hants and Sussex, 

 while the area of the London basin was at this time part 

 of a tract of land which stretched eastward and formed a 

 barrier that prevented any commingling of the northern 

 and southern seas. In other words, he holds that the 

 depression of France coincided with a ridging up of land 

 over England and Belgium, so that the waters of the two 

 seas were still kept apart. He bases this opinion on the 

 distinctness of the Lower Bracklesham fauna, and in a 

 letter to the author has expressed himself as follows : 

 " I have carefully separated out the Lower Bracklesham 

 forms, and find an utter absence of any approaching to 

 the London Clay types. I am sure that the waters they 

 lived in were perfectly isolated from any of our older 

 Eocene seas." He therefore concludes that some barrier 

 must have existed to keep out the northern forms, for 

 otherwise there would have been a greater or less mingling 

 of the faunas, and he supposes this barrier to have been 

 submerged in Barton Clay times, when London Clay types 

 again make their appearance in Hampshire, and are mingled 

 with the remnant of the southern fauna. Mr. Gardner 

 informs me that the tropical forms of the Lower Brackles- 

 ham fauna disappear, a certain number, e.g., the great 

 Bulla, Cyprcea, and corals, dropping out rather suddenly, 



