18^2.] Geology of the Isle of Wight, S^c, 349 



and upper marine formations of Mr. Webster. No part of the 

 coast appears to offer any trace of the upper freshwater de- 

 posit. 



2. Upper Marine Formation. By this we understand all the 

 beds of argillaceous marl, sand, Sec. which in Headen Hill are 

 interposed between the two freshwater formations. (Geol. Trans, 

 vol. ii.pl. 11.) They occupy a very wide superficial extent in the 

 north of the island ; yet there are few places in which their 

 natural history can be studied with much advantage, if we except 

 the cliff between Headen Hill and Colwell Bay, the argillaceous 

 beds of Hampstead Hill, and the upper part of the escarpment 

 in WhiteclifF Bay. Mr. Webster derived from the first of these 

 localities all the facts which were connected with the zoological 

 history of the formation. In our examination of this deposit, we 

 were convinced that its true limits could not be perfectly ascer- 

 tained without a more rigid determination of the genera of its 

 fossil inhabitants than had yet been attempted ; we did not, 

 however, by any means, arrive at the conclusion stated by Mr. 

 G. Sowerby {Annals of Philosophy/ , p. 219), viz. " that if we 

 depend upon fossil as the principal means of identifying strata, 

 we shall see great reason to beUeve that there does not exist any 

 marine formation between the two freshwater ones." As the 

 determination of this question is of considerable importance, we 

 think it proper briefly to state the grounds of our opinion. 



(1.) In general, the lower calcareous beds appear to have been 

 tranquilly deposited in freshwater. But if we ascend to the 

 argillaceous marl which rests immediately upon them, we not 

 only find a complete change in the physical circumstances of the 

 deposit, but a new suite of organic remains, some of which are 

 of marine origin, others of a doubtful character, and a few are 

 identical with those in the lower beds. iVU this seems to indi- 

 cate a marine inundation. Without some such interruption, it is 

 not possible to conceive how a single marine shell should have 

 found its way into a freshwater deposit. On the contrary, it is 

 perfectly conceivable that any quantity of freshwater shells 

 should have been drifted down into a marine deposit, and become 

 so much mixed with it as to have altered its whole character. 



(2.) There is much direct evidence to prove that the marine 

 inundation lasted for a considerable time. The oyster bed 

 above the freshwater rock in Whitecliff Bay has been already 

 mentioned. Two oyster beds may be traced in some parts of 

 Headen Hill ; and single oysters are dispersed in many other 

 portions of the formation. 



Mr. Webster pointed out a great bank of oysters in Colwell 

 Bay, many of which have their valves united, and are locked 

 together in the way in which they usually live. This bank is 

 several feet thick, and the species could hardly have existed in 

 their present state had they not been propagated on the v^ry 



