1854.] PRESTWICH — LONDON CLAY. 409 



stoke, and. part of the sections at Pebble-hill near Hungerford, and 

 of Clarendon-hill near Salisbury *. 



At Alum Bay, in the Isle of Wight, where the London Clay is 

 not so thick, its paleeontological divisions are less marked, and similar 

 fossils are more generally prevalent throughout its entire mass. 



Confining ourselves to the London district, it vrould appear, that, 

 although a great proportion of the fossils range at intervals vertically 

 throughout the London Clay, yet their development is very dif- 

 ferent in different zones, being abundant in some and scarce in others, 

 whilst each zone is further marked by a few characteristic species, 

 thus forming distinct, although nearly related groups ; — that plant 

 remains occur sparingly throughout the mass of the London Clay, 

 but that their chief development is in the uppermost, or perhaps 

 superadded, beds in the eastern area, in which also are entombed the 

 great bulk of the reptilian and fish remains ; — that a profusion of 

 molluscs indicating waters of a more moderate depth characterise 

 its second descending stage ; — that deeper-sea forms prevail in the 

 third stage in the eastern area, but are replaced by a shallower water 

 fauna, more nearly allied to the fossils of the second stage, as we 

 proceed westward ; — and that the same change takes place also in the 

 fourth and lowest stage, many of the forms of the upper divisions 

 being common to it in the westward area, where the sea was evi- 

 dently shallow, whilst eastward we have indications of the maximum 

 depth of the London Clay sea, with a very scanty fauna of deep-sea 

 forms. 



It must not, however, be supposed that the fossils are dispersed 

 either vertically or horizontally with any uniformity as to numbers : 

 although, as before mentioned, certain zones are characterized by 

 certain groups of organic remains, yet the fossils tend to occur in 

 greater or less abundance in certain beds. This is especially the 

 case in the lower zone, which is frequently almost totally unfos- 

 siliferous ; the third zone, likevdse, is often marked by the absence 

 of fossils, and so again with the upper zone ; the fossils appear more 

 permanent in the second zone. These zones are necessarily artificial, 

 as no actual division exists, and the organic remains and mineral 

 characters are continuous ; but they serve to mark the distinct con- 

 ditions of the fauna and flora at particular periods, and show the 

 prevalence generally of like forms on the same levels ; they are not, 

 however, to be taken, by any means, as constant, but merely as local 

 centres for convenient reference and grouping. 



The following table shows this general order of succession, and 

 the position of the several fossiliferous zones. The divisions are not 

 given as very definite ; they may vary 50 feet either one way or 

 the other, according to the thickness of the London Clay itself in 

 different parts of its area ; but they vnll serve to show generally the 

 relative position of the beds at the different places named. Only 

 the principal localities are given. 



* Quart. Journ. Geol. Soc. vol. ii. p. 235, 236, and vol.iii. p. 367. 



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