218 Mr. Sowerby on the Geology [Sept. 



nuous ; and he speaks of the nodules contained in it as " lime- 

 stone," as if he did not think it identical with the London clay. 

 But here I must object, first, that there is just as much apparent 

 continuity in that bed of clay as in the one marked B ; secondly, 

 that the nodules in it are exactly of the same nature as those he 

 calls " septaria " in the stratum B ; thirdly, that the fossils found 

 in these septaria in stratum d are identical with those constantly 

 found in the London clay almost wherever it occurs. 



I beg leave to state one more fact connected with this 

 *' lowest * marine formation above the chalk," the singularly 

 disintegrated siliceous pebbles found in the white sand marked 

 q \ : also occur in the white sand D, which, according to Mr. 

 W. forms the bottom of Totland and Colwell Bays — a fact fur- 

 ther confirmative of the identity of the sand and plastic clay 

 with the London clay. I may also add that similar pebbles have 

 been taken up, attached to septaria at Highgate. 



The next stratum upon which I shall trouble you with some 

 observations is that announced by Mr. W. as an " upper marine 

 formation," which, he says, " contains a vast number of fossil 

 shells wholly marine." Though the evidence which 1 must here 

 produce is not sufficient to prove this formation to be not marine, 

 yet it will go very far towards producing the conviction that it is 

 not wholly marine, and evince the probability of its being a 

 Jreshwater formation, or, at least, the produce of an estuary, in 

 which, owing to some peculiar vicissitudes, some marine produc- 

 tions have become mixed with those of freshwater. 



It appears proper for me to begin by observing, that notwith- 

 standing Mr. W. has said of this stratum, that it contains wholly 

 marine shells ; yet in the list of those shells, he mentions five 

 species, though two of them with doubt, of three genera, which 

 are known to exist in a recent state only in freshwater ; viz. 

 Cyclas, 1 ; Amputtaria, 2 ; and Melania, 2. He mentions, but 

 also with doubt, a genus of which all the recent species known, 

 are land shells, viz. Helicina. Of the remaining species which 

 he mentions as being found in that stratum, I will not presume 

 to say absolutely that the following do not occur, but, I think, 

 some of them are wrongly named, and others I do not believe 

 are to be found in it. Cerithium lapidum orlapidorum ; ancilla 

 buccinoides ; murex reticulatus ; natica caurena. There exist 

 in it, however, in great profusion, a species very much resembling 

 cerithium lapidum ; but this, with the remaining six species, 

 which he gives under the generic name cerithium, may very 

 probably be freshwater shells. Their recent analogues are found 

 in the freshwaters of the Islands of Bourbon, Guadaloupe, 



• I think the following observations will prove this to be, strictly speaking, the only 

 marine formation above the chalk at Headen Hill and Alum Bay. 



+ Whether this sand have changed its place since Mr. W. described it, or whether by 

 some accident or mistake he placed it wrongly, I know not ; but it is now seen to the N. 

 of the most beautifully coloured bed of sand and clay. 



