On the Silica Strata of the Lower Chalk, 235 



The following- samples are from the land of Sir A. K. Mac- 

 donald, Bart., at Selborne, in Hampshire : — 



No. 



Silica. 



Carbonate 

 of Lime. 



Remarks. 



1 



45-41 





A light yellow rock, very similar to No. 73. 



2 



55-68 





White rock, analogous in position to No. 39. 



3 



26-80 



13-30 



Kesembling No. 31. 



A piece of firestone rock, from Merstham in Sun-ey, was 

 found to contain 40 per cent, of soluble silica. 



We have recently visited the UnderclifF of the Isle of Wight 

 for the purpose of examining these strata, and of obtaining 

 specimens for analysis. As a whole, they are much more 

 heavy and indurated than in the neighbourhood of Farn- 

 ham, and generally exhibit too a more sandy-looking structure. 

 We judge that the whole section we took was about 170 feet 

 thick — -that is, from the gault to the grey marl above the 

 green phosphate bed, which always shows itself very con- 

 spicuously. The gault is trifling as compared with that near 

 Farnham. Immediately above the gault, the " malm," as it is here 

 termed, has a hardened rocky appearance (8), and is somewhat 

 discoloured by contact with the black gault below ; about 10 

 feet above it assumes a light brown or cream-colour (7), which 

 gradually becomes more compact, forming huge masses of rubbly 

 stone, altogether 30 feet thick ; above this there is a mass of 

 rock 25 to 30 feet thick, very similar in appearance to, though 

 heavier than, the high percentage silica layer at Farnham (6). 

 This bed is continued upwards perhaps 40 feet more, but is in- 

 terrupted by several thin seams of " chert " or compact flint. We 

 now arrive at the freestone of the island, one bed of which is 

 well wwthy of notice, as forming the principal building-stone 

 at Ventnor. It is called " the Freestone" (4). It is easily sawn 

 when dug, and hardens on exposure to the weather. It occupies 

 one continuous layer between 4 and 5 feet thick, lying between 

 two other beds of "false freestone " (5), of about the same 

 thickness, being separated from each by a seam of blue lime- 

 stone. These thin beds, therefore, are about 15 feet thick. 

 Above this there is a rubbly bed of rock used chiefly for 

 building rough walls, &c., about 20 feet (3). Thence upwards 

 to the chloritic bed of phosphatic marl, from 30 to 40 feet, 

 there are thin beds of the silica rock interlocated with seams 

 of blue limestone (2) and cherty flint (1). These thin layers 

 of compact limestone and chert give rise to the weathered 

 protuberances of the upper part of the Undercliff, for they resist 

 to a great extent the decomposing influence of the atmosphere, 



