-4. Strahan — Explosive Slickensides. 405 



liis Elements of Geology in 1865, remarked, "these phenomena and 

 their causes (probably connected with electrical action) seem scarcely 

 to have attracted the attention they deserve." In subsequent editions 

 this suggestion of a connection with electrical action was omitted. 



Mr. Leonard Maltby, of the Mill Dam Mine, Great Hucklow, 

 informs me ^ that he has had experience of the explosion of slicken- 

 sides. The explosions take place at the present time in the vein at 

 the Cockersfield Shaft ; pieces of minei'al burst from the face with 

 a loud noise and with great force, so as to necessitate great care on 

 the part of the men when working. There are also several other 

 places in the Mill Dam Mine where slickensides of an explosive 

 tendency have occurred, as well as in the Silence Mine on the same 

 vein, and in a vein near Eyam, called the Brookhead Vein. At the 

 Lady Wash Mine also, on the eastern range of the vein worked at Mill 

 Dam, the miners noted the same phenomenon. Its prevalence in 

 this neighbourhood leads Mr. Maltby to infer that slickensides will 

 explode more or less, while being cut, wherever they occur. He 

 remarks further that where slickensides occur, the vein is always as 

 hard and fast as it is possible to be, and seems to be under great 

 pressure. " When we work with a pick, cutting one side of the 

 vein, as soon as we have made a little opening, it seems then that 

 the air gets in, and the mineral swells and bursts off with loud noises, 

 and where the vein is hardest and most nipped, the explosions are 

 strongest. It always bursts off just as far as the opening is made." 

 He considers both the slickeuside and the explosions to be the result 

 of pressure. 



Though some of the veins in which explosions have occurred con- 

 tain much fluor-spar, yet the phenomenon has been more frequently 

 ol)served in the hard and tight veins which contain cale-spar, heavy 

 spar (sulphate of baryta) and galena. Fluor-spar, as Mr. Maltby 

 informs me,^ OGCurs more commonly in soft veins, such as that at 

 the Dasty Pits, near Eyam, where it was very abundant. In this 

 vein no slickenside was seen and explosions were unknown. 



The late Mr. J. A. Phillips, F.R.S., F.G.S., etc., informed me that 

 he had known of several instances of the flying off of fragments of 

 mineral from the slickensided face of a vein, with a sharp report. 

 In one case a fragment was thrown off with sufficient force to break 

 the leg of a man who was passing. The explosions occurred after a 

 portion of the vein had been undercut. Mr. Phillips suggested that 

 the removal of one side of a vein would leave the remaining side in 

 a condition of strain, resembling that of a strung bow, with a 

 tendency to bulge outwards into the workings. The undercutting 

 would free, so to speak, one end of the bow. 



Mr. W. Bowman, of Alport, writes ^ that he has seen pieces of 

 limestone in the Ecton Mine fly off with a sharp small crack, some 

 short time after it has been broken by blasting. In one instance, in 

 1SS5, two miners were drilling a hole by hand in the Clayton Adit- 

 level, when a piece of rock burst from the face with a loud report, 



1 In a letter dated 22nd October, 1886. 

 - In a letter dated 23rd November, 1886. 

 3 In a letter dated 15tli October, 1886. 



