320 ON THE NATURAL HISTORY 



vitriolization in pure pyrite. Moreover he finds the fibrous 

 structure, contrary to usual beHef, much more common in pyrite 

 than in marcasite.^^ 



During the oxidation of pyrites lieat is evolved, and cases 

 have occurred in which bituminous shales rich in pyrites have 

 caught fire spontaneously from this cause. A famous case 

 occurred in the cliffs of Kimeridge clay at Ringstead Bay^ 

 opposite Weymouth, in the autumn of 1826. The alum-shale of 

 Whitby is a bituminous pyritic rock in the Upper Lias, which 

 owed its economic value to the ready decompositioxi of its 

 pyrites. 



Iron-pyrites is a common mineral in beds of coal, and the 

 oxidation of the pyrites when the coal is stored in the bunks of 

 ships has often given rise to so elevated a temperature as to con- 

 tribute to the spontaneous combustion of the coal, especially in 

 hot climates, as when vessels are passing through the Red Sea» 

 The actual cause of ignition, however, is probably to be sought 

 elsewhere. 



The pyrites is often seen in brassy films on the joint-faces 

 of pieces of coal in the domestic scuttle. Darwin, in his " Botanic 

 Garden," has a poetic reference to this pyrites: 

 " Hence sable coal his massv couch extends 

 And stars of gold the sparkling Pyrite blends." 



The mention of " stars of gold " reminds us that the 

 inexperienced prospector has occasionally mistaken pyrites for 

 the precious metal ; and hence in mining camps, where speech 

 is apt to be more direct than courteous, the mineral has been 

 called " Fool's Gold." 



In coal mines the nodules of pyrites are often known as 

 " brasses," though a so-called " brassy coal " in some cases 

 contains very little pyrites. In certain collieries the pyritic 

 lumps are picked out, and sold for sulphuric acid manufacture. 

 The coal brass is usually marcasite and not true pyrite, though 

 cubic crystals are occasionally found. 



It is worth noting that pyrites occurs in coal not only in 

 brass-like lumps and bands, but also in a finely-divided form, so 

 intimately associated with the coal that it fails to appeal directly 

 to the eye ; in this form it has been called " black pyrites."^* 



13 Bulletin of U.S. Geol. Survey, No. i86, 1901. 



14 Royal Commission on Arsenical jHoisoning, Blue-book of Evidence, vol. ii., 1903. 



