408 A. Strahan — Explosive SUckemides. 



by an explosion of violent expansive power. It will be noticed that 

 the cause of the great explosion of 1790 is distinctly stated by Mr. 

 W. Watson, though writing nearly forty years after the event, to have 

 been a slide joint of slickensides going across a vein. 



By the kindness of Mr. Maltby I have been supplied with some 

 specimens of the explosive ore from the Cockersfield Shaft of the 

 Milldam Mine at Great Hucklow, near Eyam. The specimens 

 consist of sulphate of baryta and galena arranged in more or less 

 vertical but irregular ribs. The planes of slickenside are beautifully 

 polished, so much so in places as to possess the reflective power of 

 a looking-glass, but they show also the usual ribbing or striation 

 which so strongly conveys the idea of slickenside having been pro- 

 duced by the rubbing of two surfaces together. The planes of 

 slickenside were clearlv formed after the fillino; in of the vein by 

 the minerals mentioned above. They cut through the galena and 

 baryta impartially, nor in any of the specimens in my possession 

 is there any appearance of a rearrangement of the minerals having 

 resulted from the existence of the plane of slickenside, except that 

 the slickenside surfaces are slightly stained by iron oxide, or coated 

 by a microscopically thin film of galena. 



The spar in these specimens has the granular appearance of white 

 lump sugar, and readily crumbles into a gritty powder. Whether 

 this granular structure is the result of the explosions by which the 

 specimens were detached, there is no evidence to show. The speci- 

 mens themselves average from half an inch to three inches across, 

 and are of all shapes. Attempts have been made to prepare 

 microscopic sections across the planes of slickenside, but up to the 

 present without success. 



The first explanation that suggested itself to me was that the 

 mass of rock, separating two planes of slickenside, was comparable 

 to a huge sheet of very brittle glass, placed on its edge ; a slight 

 blow on the lower part of which might bring down the whole mass 

 in fragments. This, however, provides no explanation of the 

 explosive power, which is so clearly brought out in the above 

 quotations. 



Secondly, it will be familiar to all who have been in mines, that 

 newly bared shale swells and crumbles on exposure to air and 

 moisture. This is probably due in many cases to chemical processes 

 set up in the mass of the rock, principally no doubt in connection 

 with the salts of iron. It seemed conceivable that such processes, 

 taking place within a brittle rock, might place it in a condition 

 of strain, under which it would fly to pieces with a mere touch. But 

 there is no sign of any chemical alteration having taken place in 

 these explosive spars. 



The explanation, which perhaps best satisfies the requirements of 

 the problem, appears to be that the spars are in a state of molecular 

 strain, resembling that of the Eupert's Drop, or of toughened glass, 

 and that this condition of strain is the result of the earth-movements, 

 which produced the slickensides. 



