DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 503 



in the last experiment, and inclosing it in alum in a piece of thicker 

 copper tube. This tube was o. 75 inch (19.05 nim.) high, and made 

 of metal 0.187 i^^h (4.65 mm.) thick. The octahedron was set in 

 the alum so that it rested on an edge. For the ends of the tube, 

 instead of brass plates, plates of steel were used. That at the bottom 

 of the tube was made of machine steel one inch (25 .4 mm.) thick, and 

 that at the top was a plate of nickel armor steel o . 063 inch (i . 6 mm.) 

 thick. The tube was slowly squeezed down to a height of 0.384 

 inch (9.75 mm.) . This occupied 30 minutes, the load finally reaching 

 112,000 pounds (50,804 kilos). The fiuorite octahedron was found 

 to have been squeezed into a nearly square tabular mass (Plate II, 

 Fig. h) measuring about three-quarters of an inch across, the 

 movements being of the same nature as those described in the last 

 experiment. Crossing the top and bottom of this mass diagonally 

 were two faint ridges representing a survival of the edges of the 

 octahedron. These by the pressure had been brought into contact 

 with the steel plates at either end of the tube and had actually 

 embedded themselves in the latter, the upper and sharper edge 

 sinking into the nickel steel, leaving a deep, well-marked depression 

 in the steel along its whole length (Plate II, Fig. c), and at the same 

 time distinctly bending the plate. The lower edge, which was 

 blunter, left a similar depression in the machine steel below. These 

 edges of the fiuorite crystal, although having in this way forced 

 themselves into the steel, showed no signs of breaking or granulation, 

 but were intact. It would without doubt be possible, by changing 

 the conditions of the experiment somewhat, to force a crystal of 

 fiuorite completely through a piece of steel armor plate. 



As in the other experiments, the color as well as the form of the 

 mineral was found to have been altered by the pressure. Two of the 

 opposite solid angles of the octahedron still retained a green color 

 though much paler than before, but the rest of the flattened crystal, 

 including the edges which had embedded themselves in the steel, 

 that is to say, that portion of the mineral which had been submitted 

 to the most intense pressure, was found to have assumed a distinct 

 violet or purple color. 



Another experiment, in which parafUne wax was used instead of 

 alum as an embedding material, showed that with this medium a 



