1850.] PRESTWICH ON THE LOWER TERTIARY STRATA- 275 



to reduce into harmony with the pheenomena of the surrounding 

 district that, which in this case would become an exceptional phse- 

 nomenon. We should rather seek, by an inquiry into the conditions 

 regulating the distribution of organic remains in the London clay, 

 whether a sufficient reason can be assigned for the absence of organic 

 remains in these portions of the London clay, and to discover how 

 far the phaenomenon is a local or a general one. 



In Hampshire and the western part of the London tertiary district, 

 the organic remains of the London clay are dispersed with tolerable 

 regularity throughout the whole of its mass, whereas eastward of 

 London the lower beds of the London clay contain, as a general rule, 

 few or no fossils. The fact therefore of the scarcity of organic remains 

 in the lowest clay beds of the London clay in a large portion of its range 

 is a prevailing and not an exceptional feature. It is not alone apparent 

 in those sections where we find only a small extent of the London 

 clay exposed, and on which consequently doubts have been throvra, 

 but also in those sections where we have the successive beds of the 

 London clay exposed from its base up to its well-characterized central 

 beds. Thus in the extensive section in the cliffs adjoining Heme 

 Bay, the base of the London clay is almost, or entirely destitute of 

 fossils, whereas as we reach the beds higher in the series, which are 

 seen gradually setting in, the well-known fossils of this formation 

 become far from scarce. So also at Guildford the lower beds are 

 unfossiliferous, but in proceeding along the dip of the beds towards 

 Woking, organic remains become tolerably abundant. 



I have in a previous paper * argued the probability of the London 

 clay of Hampshire having been deposited during a period of a nearly 

 constant, regular, and tranquil subsidence of the bed of the sea, whereby 

 a nearly uniform condition of the sea-bottom, favourable to the pro- 

 longed existence of the same group of testacea, was maintained. I 

 also concluded that the subsidence had been greater to the north-east 

 than to the south-west of the Tertiary district, and it therefore follows 

 that it must have been more rapid in the former direction either 

 throughout the whole, or else during particular intervals, of the 

 London clay period ; and consequently if the rate of that subsidence 

 was at any time more rapid than the silting up of the sea-bottom, it 

 would result that at such times the sea would become deeper in the 

 north-east than the south-west. Under these conditions we might 

 expect a distribution of the fauna in the north-east of the tertiary 

 district different to that prevailing in the south-west. Now, if the 

 accumulation of the lower part of the great mass of clay forming the 

 London clay commenced during a sudden or even a tolerably rapid 

 subsidence of the sea-bottom in the north-eastern portions of the 

 tertiary area, then the shallow sea fauna of the basement bed could 

 not under these altered conditions of increased depth have been tran- 

 quilly transmitted upwards as in the Hampshire area, but must for 

 a time have ceased to live in districts so aifected, and, before a deeper 

 sea fauna were introduced, strata might have been deposited with 

 few or no organic remains. 



* Quart. Journ. Geol. Soc. vol. ill, p. 354. 



