56 
CASTLE HILL. 
9. Ochraceous day, containing hydrate and mh- 
mlphate of alamine, with gypsum, &c., — foot. 
10. Chalk witli flints, — about 100 feet. 
Of the strata above enumerated. Nos. 3, 4. 6. 
are considered by Professor Buckland as analogous 
to the plastic clay beds of Woolwich (Nos. 7 j 8* 
PI. xiii. Geolog. Trans, vol. iv.), which contain 
also the same species of Potamides and cyclas. 
No. 7- of Castle Hill, is the ash-coloured sand of 
Woolwich, in diminished thickness. The hreccia, 
No. 8., corresponds with the Reading oyster bed, 
which “ though inconsiderable in thickness, seems 
constantly to occur immediately above the chalk ; 
although organic remains have been noticed in it 
only at Reading.” 
The ochraceous clay (No. 9.) contains the sub- 
stance that has rendered Castle Hill so interesting 
to tlie mineralogist — the suhsulphate of alumine. 
As this mineral is peculiar to Sussex, a particular 
description of it is subjoined. 
(^Hydrate and suhsulphate of alumine. Brit. Min. 
Tab. 499. Annals of Philosophy, vol. ii. p. 238.) 
This substance is imbedded in a layer of ochra- 
ceous clay that lies immediately upon the chalk. 
The bed is situated nearly midway between the 
summit of the cliffs and the sea-shore, and there- 
fore cannot be examined without much difficulty, 
and exposure to considerable danger.* 
The first specimen of the subsulphate was dis- 
covered by the author among some gravel that 
* Specimens may generally be found among the ruins of the cliffs 
that lie scattered on the shore, from half a mile to a mile west of New- 
haven harbour. 
