J. S. GARDNEE ON THE LOWER LONDON TERTIAR1ES. 199 



sea, recognizable in the Brackleshams, occupied the Hampshire 

 basin and covered over the London Clay to within 20 or 30 miles 

 of London. That these formations were deposited from opposite 

 directions is apparent from their relative easterly and westerly 

 developments. The thinning of the London Clay in Hampshire is 

 not due to denudation by the Bracklesham sea, as it was everywhere 

 protected by thick intervening freshwater beds, and its original 

 thickness is preserved entire. The Barton beds show for the first 

 time an admixture of the two faunas, though only the less tropical 

 of the Bracklesham species remain. The Brockenhurst fauna shows 

 an increased preponderance of southern forms, while the Hempstead 

 fauna, if it can be said to show any thing, recalls the northern 

 types. 



jNow marine faunas could only be kept distinct in adjacent areas, 

 whose conditions of depth and sea-bottom were so similar, either by 

 very sharply dehned cold and warm currents, or by intervening 

 land. These were clearly not separated by currents, since each 

 formation is limited by a shore-line, and freshwater strata intervene 

 in every case, showing the area to have become land after the 

 deposition of each ; and there is no supposition open that will explain 

 the facts, except the continued existence of an isthmus, connecting 

 England with the mainland, throughout the Eocene, until at least 

 the Barton period. This isthmus was not stationary, however, but 

 undulated from north to south and south to north without being 

 broken through. Thus during the Thanet-Sand time it must have 

 stretched from Dieppe along the "Weald ; but in the London-Clay 

 period it could only have joined Erance to the south of the Isle of 

 "Wight and Purbeck; whilst in Bracklesham times it probably 

 stretched from Belgium across the Weald to Hertfordshire. These 

 various positions of this isthmus are not purely conjectural, except 

 where is now sea; for its northern shore is distinctly traceable in 

 the London Clay, and its southern shore in the Bracklesham beds. 

 Further, the vast Eocene river, whose presence is felt in every deposit, 

 had its estuary in the direction of the Thames valley throughout all 

 the Lower Eocenes, but had its course diverted to the south by the 

 change in the position of the isthmus which caused the London-Clay 

 sea to recede ; and its estuary remained in Hampshire as long as 

 any further record of it is preserved. The detritus of the Eocene 

 river is mainly quartzose and felspathic, and is such as to show that 

 it probably drained a palaeozoic area, while its bulk and the enormous 

 variety of the forest vegetation imbedded in its silts leave no alter- 

 native but the belief, supported in many other ways which cannot 

 be entered into here, that it drained a vast continent with an in- 

 definite westerly extension, even connected in some mysterious way 

 with America. The breadth of this river in its purely freshwater 

 reaches is actually seen, in Hampshire and Dorsetshire, to have been 

 at least 17 or 18 miles* ; and the extent of homogeneous or similar 

 fluviatile and estuarine deposits, stretching as they do from England 

 over France on this area and over no other in Eocene time, shows 

 * Dollfus compares its bulk in France to that of the Amazons. 



