OF PYRITES AND GYPSUM. 323 



-origin — and thus it conies about that we may sometimes search 

 for fossils, and find only crystals of gypsum. The appearance of 

 the gypsum is in truth correlative with the disappearance of the 

 fossils. Less commonly the acid from the disintegration of 

 tlie pyrites attacks the silicate of alumina in clay, and so 

 produces an aluminous sulphate, like the mineral known as 

 Wehsterite or aliunitiite. 



In an old journal entitled The Mining Review there appeared 

 a series of articles on "The Mineral Topography of Great 

 Britain," by Mr. A. W. Tooke, M.A., F.G.S., and in the 

 number for February, 1837, the minerals oi Essex are enumerated. 

 Excluding rocks, which are not here separated, the Essex 

 minerals include iron-pyrites, copper-pyrites (which can have 

 been only occasionally, if ever, found) and selenite, or crystallized 

 gypsum. With regard to the selenite the following local infor- 

 mation is given : 



" Upon the sea-shore, between Little Holland and Harwich, copper-pyrites and 

 ■selenite, there called Frinton glass, from being most plentiful opposite that 

 •village." 



Probably the good folk of Frintoi never used this " glass " 

 for glazing their windows, but, ages before Mr. Tooke wrote, the 

 mineral was undoubtedly employed elsewhere for a similar pur- 

 pose. It is said tliat the Lapis Speculavis, which most authorities 

 believe to have been gypsum, was introduced at Rome in the 

 time of Seneca, and that Tiberius got cucumbers at his table 

 almost every month in the year by using it for the protection of 

 ithe plants. Pliny, describing this glass-stone, cites the opinion 

 •of certain authorities to the effect that it was (to borrow from 

 the old translator, Holland) "a liquid humour of the earth, 

 •congealed to an ice after the manner of crystal." And to this 

 •day workmen are in the habit of calling crystals of gypsum 

 '• congealed water." Samuel Dale, in referring to the selenite of 

 Harwich, calls it " Mock Crystal," composed of the same 

 material as Talc. '• Certeine it is," said parson Harrison, "that 

 antiquitie used it before glasse was know en, under the name of 

 Selenites." 



Transparent gypsum is still called Selenite — a word supposed 

 to express the character of the lustre exhibited by the mineral on 

 its cleavage-planes. This lustre has been compared to the 

 soft brightness of moonliglit, and hence the word selenite or 



