WEALD, OR OAK TREE CLAY, 67 



and is separated by thin layers of marl into slabs, from six to twelve inches 

 thick, A spring of excellent water issues from beneath the limestone 

 beds, in a well thirty feet deep. 



In concluding this account of the Sussex marble, it may be proper to 

 remark, that it appeared unnecessary to specify every variation in its tex_ 

 ture or colour, since these differences are merely accidental, and do not 

 affect the geological history of the substance under examination. 



Organic Remains, 



The fossils of the Weald clay, consist only of the shells contained in 

 the beds of limestone, and these do not present much variety. They are 

 referable but to one genus, and consist of the following species : 



1, Vivipara fluviorum? Tab. xvij. fig. 5, 6. 



2, extensa. Min. Conch, tab. xxxi. fig. 2. 



From the apparent resemblance of these shells to the recent helices 

 (Helta' vivipara, et H. tentaculata, of Linne) the limestone in which they 

 are imbedded, has been supposed to be of fresh water origin. The correct- 

 ness of such an opinion is, however, very questionable, since the characters 

 of the fossils are not sufficiently defined, to admit of accurate comparison 

 with their assumed prototypes *, 



The analogy between tliese strata, and the upper beds of the Purbeck, 

 is so striking, that some eminent geologists have been induced to con- 

 sider the Sussex marble as belonging to that series of deposits. But the 

 testace^E of the Purbeck hmestone, although corresponding in many 



* Mr. G. Sowerby has favoured me with the following remarks on the specimens repre- 

 sented in tab. xvij. figs. 5, 6. 



" I will briefly state the reasons which induce me to think the specimens in question are 

 not of fresh water origin, and consequently not pahidincB. After a careful examination, I can- 

 not perceive the least appearance of that kind of erosion, so characteristic of fresh water tes- 

 taceae, and which, though not constant, is in most cases observable in a greater or less degree. 

 The substance of these shells is also considerably thicker than in any species of paludina ; in- 

 deed for size, their thickness is very considerable. 



" These fossils must not be confounded with the viviparce or paludina;, so common in the 

 marl that lies over the fresh water beds at Headen-hill, in the Isle of Wight, and which 

 are distinctly eroded, very thin, and of undoubted fresh water origin. 



" The only recent shells that resemble the fossils you have submitted to my examination, 

 are a small species of turbo, (of Linne) approaching in its characters to tm-bo liitoreus." 



k2 



