Chap. 31.] Curious Mineral. 183 



is compofed in general of laminae which have nearly 

 the colour and afpect of lead, but are more brilliant, 

 and very brittle. A great variety of thefe ores have 

 been difcovered, which it will not be neceffary to 

 enumerate. One fpecies however is too curious in 

 its nature and effects to be omitted. It is called by 

 the miners Jlickenfides , and coniifts of galena united 

 with phofphorated hydrogen. The mineral has the 

 appearance of black marble, and breaks with a polifhed 

 furface, not truly plane, but lying in waves. It is 

 found in fiffures of lime-ftone in Haycliff and Lady- 

 wafh mines at Eyam, and in Oden at Caftleton, in 

 Derbymire. It is divided into two equal parts or 

 (labs, by a line parallel to the fides of the fiffure, and 

 thefe flabs are joined by two polifhed faces, which 

 feem to be in perfect contact without any cohefion. 

 The furfaces are of the colour of lead, but the covering- 

 is as thin as that from a black lead pencil. If a fharp- 

 pointed tool, which the workmen call a pkk, is drawn 

 over the vein with fome force, the mineral begins to 

 crackle like fulphur excited by electricity ; in a few 

 minutes after which the laminse explode with violence, 

 and fly out as if they had been blafted with gun-pow- 

 der, infomuch that the weight of forty tons has been 

 blown out together. Thefe dangerous effects deterred 

 the workmen from proceeding for fevera) years ; but 

 at length it occurred to them that this power might be 

 ufed for the carrying on of their works with better 

 advantage than by the common method of blafting 

 with gun-powder. Accordingly a workman makes 

 afcratch with his tool upon the joint of the Aickenfides, 

 and runs away as faft as he can, to efcape the explo- 

 fion, which, it is faid, loofens as much of the rock 

 as ten men would have brought away in three months, 

 by ctye ordinary methods. 



N 4 From 



