302 Ancient Geological Changes in England. 



a river or lake ; and the habits of the recent species more nearly 

 related to the latter, warrant a similar conclusion, since they are 

 well known to frequent the rivers and marshy tracts of tropical 

 regions, in the sands and banks of which they deposit their 

 eggs;* 



*' 5. The group thus accumulated is distinguished by many 

 peculiar circumstances. Among these are, the marked differ- 

 ence in the character of the fossils from those of the marine 

 strata, both below it and above; — the novelty of the fossils 

 themselves, many of them not having hitherto been found in 

 any other situation; — the proofs which they afford of a great 

 subsequent change in the climate of this part of the globe; — 

 the limited space which the formation appears \o have occupied^ 

 — and its gradual diminution in thickness towards its borders, 

 so far at least as it has yet been possible to trace the subterra- 

 nean boundaries of a group, of which, unfortunately, such small 

 portions are disclosed. All these facts, it will be observed, ac-^ 

 cord with the hypothesis of its origin in fresh water communi* 

 eating with the sea. 



'* 6. After the depression of the surface last mentioned, — to 

 a depth not yet beyond the access of deposits from fresh water, 

 — ^next comes a fartlier depression of the surface, still covered 

 with water, and along with it, most probably, of the land from 

 which the fresh water was supplied, — to such a depth, that it 

 became accessible to sea- water ^one; for above the Wealden 

 group, we find a numerous succession of strata, — the green- 

 sands, the gauk, and the chalk,— abounding in fossils, not one 

 of them belonging to any genus of' which the existing species 

 inhabit fresh water ; and it may be added (and the observation, 

 indeed, applies to all the strata we have mentioned), not a single 

 one of which belongs to any species at present Jcnoxe^n to exist in 

 any recent sea 1 



" 7. The duration of this last epoch of submersion, that during 

 which the greensands, gault, and chalk, were deposited, we are 

 not enabled to measure, except by the mass of the strata accu- 

 mulated during its progress^ — a thickness, at the lowest estimate, 

 of not less than 1200 feet. But, though the contrast of the fos- 

 sils di the Wealden, and of these incumbent bcds^ is sudden and 

 • M antell, in Sussex, p. 57. 



