258 PLASTIC CLAY FORMATION. 



pebbles; the latter evidently formed of broken chalk flints 

 rounded by attrition, from - - - 10 to 15 feet. 



The ochraceous clay (No. 2) contains the substance that has ren- 

 dered Castle HiU so interesting to the mineralogist — the subsulphate of 

 alumine. As this mineral is peculiar to Sussex, or at least has not been 

 discovered elsewhere in England, a particular description of it must not 

 be omitted. 



Hydrate and submlphate of alumine. Brit. Min. Tab. 499. Annals 

 of Philosophy, Vol. ii. p. 238. 



This substance is imbedded in a layer of ochraceous clay that lies im- 

 mediately upon the chalk. The bed is situate nearly midway between 

 the summit of the cliffs and the sea-shore, and therefore cannot be ex- 

 amined without much difficulty, and exposure to considerable danger*. 



The first specimen of the subsiJphate was discovered by the author 

 among some gravel that had been brought from Newhaven, and was 

 lying in a wharf near Lewes -f. A few months afterwards, Mr. Webster, 

 in a geological excursion along the Sussex coast, collected a specimen at 

 Newhaven. This was analyzed by Dr. WoUaston, and found to consist 

 of alumine, in combination with sulphuric acid, and a small proportion 

 of silex, lime, and oxide of iron. 



This mineral occurs massive, in veins, and in tabular and tuberose 

 masses; the former frequently attaining several feet in length, and the 

 latter exceeding three or four pounds in weight. It appears to have 

 been of stalactitical origin, and is supposed to result from the decomposition 

 of iron pyrites, and the reaction of other substances. As the superin- 

 cumbent strata contain aU the elements necessary for its production, it 

 probably has been introduced into its present situation by infiltration:}:. 

 When pure, it is perfectly white, but is generally more or less dis- 



* 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 Newhaven harbour. 



■\ Vide Mr. Sowerby's description of this substance in British Mineralogy. 



X Professor Buckland on the Plastic Clay. Geological Transact io?i^s, Vol. iv. p. 294. 



