502 FRANK D. ADAMS 



It is thus evident that the mineral fluorite is plastic to a marked 

 ■degree. It may be bent and twisted without any signs of disruption 

 and it is only along certain lines of very intense movement that the 

 mineral breaking develops a cataclastic structure, just as marble 

 does in places when deformed under low differential pressures. The 

 little broken grains of fluorite thus produced, however, as in the case 

 of the marble, remain firmly coherent, and it is highly probable that, 

 as in the case of marble, if the deformation were carried on under 

 much higher pressures or at a higher temperature, fluorite could be 

 deformed without any fracture or the development of any cataclastic 

 structure whatsoever. 



While, however, the plasticity of the mineral is remarkable, its 

 resistance to movement and the force required to bring about its 

 deformation appear to be considerably greater than in the case of 

 calcite, and it was observed that an angle of one of the fluorite crystals 

 which cleared itself from the alum and came into contact with the 

 brass plate at one end of the tube made a distinct triangular depression 

 in it. 



Several other experiments with a perfect cleavage octahedron of 

 fluorite from Westmoreland, Cheshire Co., New Hampshire, carried 

 out under identical conditions, resulted in the flattening of the rhom- 

 bohedron, the movement being of the nature of a plastic flow, except 

 possibly where here and there a few little opaque white lines indicated 

 the development of a minute cataclastic structure. In these experi- 

 ments also the fluorite showed the same stiffness or resistance to 

 deformation, which was seen not only in the very high pressure 

 required to deform it, but also in the fact that as the copper tube was 

 squeezed down and the alum flowed away from above and below it, 

 leaving the mineral in contact with the brass plates at either end, the 

 octahedral faces of the mineral, where they came upon the brass plate 

 below, sank into it, leaving a well-marked depression, while the two 

 octahedral edges bounding the face in contact with the upper brass 

 plate, which was 0.075 i^^^h (i .9 mm.) thick,. cut completely through 

 it, leaving a wide rent, and having passed through this plate, forced 

 their way into a second brass plate behind it. 



Another experiment employing much higher pressures was then 

 made by taking an octahedron of fluorite, similar to that employed 



