DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 501 



in diameter. This was placed in a copper tube, i inch (25.4 mm.) 

 high and otherwise of the same dimensions as those usually employed, . 

 namely 1.0625 inches (26.98 mm.) in internal diameter, and having 

 a wall o. 125 inch (3.175 mm.) thick. Melted alum was used as an 

 embedding material and a brass plate was used to form the top and 

 bottom of the tube. The whole was then placed in the press and 

 squeezed down until the tube was reduced to o. 75 inch (19.05 mm.) 

 in height. When, this, having assumed a symmetrical bulge, com- 

 menced to develop minute fissures in its most distended portion, the 

 experiment was brought to a close. The time occupied in the 

 deformation was 50 minutes, the pressure being gradually raised 

 until it reached a load of 42,500 pounds. On dissolving away the 

 alum, the group of fluorite crystals was obtained as a firm coherent 

 mass, but the deformation had been so great that while portions of 

 two cubes could be recognized, the rest of these two cubes and the 

 third cube had been so welded together into a lump that it was impos- 

 sible to distinguish them or to ascertain which part of the mass they 

 represented. The green color of the original mineral had disappeared 

 except in one or two spots, and its place had been taken by a pale 

 violet tint, and the mineral, which was originally transparent to 

 translucent, had become practically opaque. 



A thin section of the deformed mass was then prepared, which, 

 when examined under the microscope, showed that the fluorite was 

 still clear and transparent, except along a few lines which traversed 

 the slide in sinuous curves. Here the mineral presented a turbid 

 appearance. The three individuals composing the mass were seen 

 to be traversed by their respective cleavage lines, evidently developed 

 in grinding the section, which made it possible to determine their 

 boundaries in a general way. Each cleavage line was seen to follow 

 a straight course, until it approached the turbid lines above mentioned, 

 when it bent with a sharp curve or sudden twist; the crystal along 

 these lines where the movement was greatest being broken into a mass 

 of minute grains, still, however, firmly coherent. When examined 

 between crossed Nicols the fluorite was seen to remain perfectly 

 isotropic, except along the lines of most intense movement and 

 granulation, where it can frequently be seen to be distinctly doubly 

 refracting;. 



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