500 FRANK D. ADAMS 



paraffine wedges driven into it. Even in this case, the crystal would 

 develop a filigree pattern; but if the conditions of the experiment be 

 slightly altered so as to prevent the formation of the wedges of paraf- 

 fine, it is believed that a salt crystal, on account of its great plasticity, 

 might be flattened out to almost any required extent and might be 

 molded into any desired form. 



Iceland spar. — Since in considering the deformation of marble 

 the effect of differential pressure on the constituent calcite grains is 

 described, it is unnecessary here to repeat these descriptions. It 

 may, however, be of interest to refer to a single experiment on the 

 deformation of a large cleavage fragment of Iceland spar. 



In this a cleavage rhombohedron of Iceland spar, measuring 

 o . 73 inch (i8 . 54 mm.) between the acute angles of the rhombohedron, 

 was embedded in alum in a copper tube of the usual type, having a 

 height of 1.25 inches (31.75 mm.) , and a wall thickness of o. 1 2 5 inch 

 (3.175 mm.), the tube being closed by a thick plate of cast iron placed 

 against one end and a plate of machinery steel placed against the 

 other, the rhombohedron being so set that its acute edges would come 

 against the metal plates at either end as the deformation progressed. 

 The tube was then squeezed down to a height of 0.473 inch (12.01 

 mm.), under a load of 83,000 pounds. On dissolving away the alum 

 it was found that the calcite rhombohedron had been pressed into the 

 metal plates at either end, leaving a faint but clearly perceptible 

 impression on the machinery steel at one end and a somewhat more 

 distinct one in the cast iron at the other. Neither of these, however, 

 was so distinct as those produced by the fiuorite (see below). The 

 edges of the calcite which produced the indentations remained quite 

 sharp and showed no granulation, but the crystal under the pressure 

 has been converted into a perfect twin crystal, the plane of twinning 

 being at right angles to the direction of maximum pressure (see 

 Plate II, Fig. a). 



Fiuorite. — Passing to the next higher member of Mohs's Scale of 

 Hardness, the action of differential pressure on fiuorite was investi- 

 gated. Five experiments were made with this mineral. For the 

 first, a group of twinned fiuorite crystals, green in color and consisting 

 of three interpenetrating cubes from Weardale, Durham (England), 

 was selected, the largest of these crystals being 0.82 inch (20.8 mm.) 



