OF PYRITES AND GYPSUM. 325 



This selenite of Friedrichroda is in Permian rocks, and it 

 is notable that the greatest deposits of gypsum in our own country 

 also occur in the New Red Sandstone formation. At Newark in 

 Nottinghamshire, at Fauld in vStaffordshire, and at Chellaston 

 in Derbyshire, gypsum is, or has been, very extensively worked 

 in the Keuper marl, whilst in the north of England the mineral 

 occurs, as Mr. J. G. Goodchild has pointed out, at a lower 

 horizon.^'' The gypsum of our New, Red rocks is mostly in the 

 granular form called alahastev, and, with its veins and cloudings 

 of iron oxide, is valued as an ornamental stone, especially for 

 internal ecclesiastical decoration. As it is extremely soft it is 

 carved into elaborate forms much more readily than true 

 marble. 



These great masses of alabaster, occur as balls, or 

 lenticular cakes, or irregular beds, in themarV^ and often enclose 

 ■cavites, the walls of which are studded with crystals of selenite, 

 not unlke those disseminated through many clays. In the 

 Triassic and Permian rocks, however, the gypsum seems to 

 liave been precipitated from the salt water of lakes, which 

 received the inland drainage of a region probably of desert-like 

 features; or it may perhaps have been deposited in aland-locked 

 sheet of water, representing an arm cut off from the sea. 



Large deposits of gypsum occur not only in the Midlands 

 and the North of England, but also in the south-east, notably in 

 the Purbeck beds of Sussex. Here it was discovered by the 

 famous sub-Wealden boring at Netherfield, near Battle, 

 commenced in 1872 to commemorate the visit of the British 

 Association to Brighton. At the present day this gypsum is 

 largely worked, chiefly for conversion into "Plaster of Paris" 

 — a use to which much of our gypsum elsewhere is applied. 

 By careful calcination a large proportion of the water of this 

 hydrous sulphate is expelled, and the partially dehydrated 

 gypsum becomes a valuable material, of wide application in the 

 industrial arts. If all the water be expelled the mineral has the 

 composition of anliydvite, and becomes spoilt as a cement, being 

 as the workmen say, " over-burnt." 



The gypsum of the London clay, althcugh occurring 



16 An admirable account of gypsum will be found in IMi. Goodchild's paper entitled 

 Some Ooservations upon the Natural History of Gypsum." Fvocccdiiigs Geologists' 



Assoc, \o\. X., p. 425. For the selenite of Utah see Rep. Museums Assoc, 1S97, p. 47. 



17 " The Gyppum Deposits of Nottinghamshire and Derbyshire." By A. T. Metcalfe, 

 Tnins. Fed. Inst. Miit., Vol. XII. (1896), p. 107. 



