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learnt to discover, by the sounding of their tools against the 

 spot, that the next stroke may detach the slickensides, they 

 stand beneath, if the situation allows, and, advising their 

 friends to arrange themselves in safety, spring the mine as it 

 were in triumph. I presume that the compressed air, allowed 

 relief from a space so broad and suddenly coming into con- 

 tact with the common air, causes the noise, and the expan- 

 sion impels the stones to a proportionate distance, and frag- 

 ments fly afar over the operator's head, who has a reasonable 

 cause to triumph, as his knowledge has taught him to make 

 sport with safety where he might have suffered by ignorance. 



The specimen figured in the front of the plate is from 

 Ecton Mine, where so many brilliant coloured and variegated 

 specimens of Pyrites occur; and this is not a common va- 

 riety, because it partakes of the splendid prismatic colours 

 added to the highly polished surface. The piece figured be- 

 hind is more usual, and at first sight very much resembles 

 the Free-stone of a chimney when shining with common 

 Black Lead or Plumbago, which sometimes by means of 

 smoke has the iridescent or prismatic colours, but in this 

 it does not resemble it. This specimen has Mineral Pitch 

 and Pyrites about it in veins. 



Other substances are liable to be polished in the same 

 manner, and apparently from the same causes. 



