370 Dr. Filton on the Strata [Nov. 



The Firestone holds its place, and is visible every where along 

 the coast of Dorsetshire, as far as Whitenore Point, where 

 the chalk retires from the sea; and it exhibits the same cha- 

 racters throughout. It can also be traced through the greater 

 part of Sussex and Surry ; but in Kent seems gradually to 

 become less conspicuous, and to lose some of its more pro- 

 minent characters as it approaches the sea, on the east of 

 Godstone. There can, I believe, be no doubt of its geological 

 identity with the lower part of the Cambridgeshire Clioich:* 

 The fossils, so far as they are known, are nearly the same in 

 both ; — and they are in both cases different from those of the 

 green-sand. 



Gault. — Clay of the Undercliff. — Either of these denomina- 

 tions for the marly clay, which immediately succeeds the fire- 

 stone, appears preferable to that oiblue marl, which is taken from 

 a character neither constant nor peculiar to this stratum; and 

 all the evidence that I have had an opportunity of examining, is 

 in favour of its identification with the Cambridgeshire gault, and 

 the blue marl of Folkestone ; the principal difference, which 

 certainly is remarkable, consisting in the great abundance and 

 variety of the fossils at the place last mentioned, and their com- 

 parative rarity in the clay beneath the Firestone of the Isle of 

 Wight. Mr. Webster has mentioned this peculiarity, and though 

 I searched carefully in various parts of the island, I could 

 not find any of the more characteristic Folkestone shells ; the 

 place of the bed, however, is not only remarkably well defined 

 along the whole southern coast, but is plainly discernible also 

 to the west, as far as Durdle Cove. Its characters in all 

 these places are very uniform : the clay being of a dull bluish 

 grey colour, harsh to the touch, adhering not very strongly 

 to the tongue, and containing numerous minute glittering par- 

 ticles, which have been taken for mica, but which I believe are 

 more frequently crystalline plates of gypsum, — distinct crystals 

 of that substance beingfound in it in great abundance, originating 

 probably in the decomposition of the pyrites,^ which it every 

 where contains. It effervesces strongly with acids, and besides 

 the shells, which (in the Isle of Wight) are dispersed through 

 it in small numbers, frequently exhibits traces of slender cylin- 

 drical ramifications, probably derived from organized bodies. 

 The fossils which I succeeded in preserving (for in general they 

 are very fragile), are the following : 



* See Hailstone and Warburton, Geol. Trans, vol. iii. pp.248 — 250. 



+ The specimens, however, have been generally taken in places where the clay has 

 been long exposed to the action of air and moisture : its characters may be different 

 when freshly opened, at considerable depths from the surface. Mr. Aikin has informed 

 me, that at the Highgate Tunnel, the London clay when fresh dug out wasuniform, soft, 

 and saponaceous to the touch, not containing any crystalline particles ; but after ex- 

 posure for a few weeks to the air, the surface was found to be covered more or less 

 with small rhomboidal crystals of gypsum. 



