DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 505 



this remnant of the crystal approximately parallel to a pyramidal face-, 

 and thus in a direction approximately at right angles to the direction of 

 pressure, some half-dozen planes could be seen along which movement 

 of the nature of a minute faulting had taken place. The mineral, 

 however, was firmly coherent where traversed by these planes, indi- 

 cating that apatite under the experimental conditions, although break- 

 ing along certain lines, was firmly welded together again by the 

 pressure and was thus slightly plastic. A further evidence of plasticity 

 is afforded by the fact that one of the prismatic faces shows a slight but 

 distinct bending or curving. 



The evidence afforded by the experiment, therefore, shows that 

 while apatite is very much more brittle than the softer minerals of 

 the series, it nevertheless possesses the property of plasticity in a 

 slight degree at least, a conclusion which is confirmed by the occasional 

 discovery of apatite crystals which are distinctly curved or bent in 

 the highly contorted crystalline limestones and associated rocks of 

 Laurentian age in the Ottawa district. That the mineral is, however, 

 but slightly plastic even under the conditions of very great pressure 

 which obtain during the contortion of the limestones above mentioned, 

 is shown by the fact that the curved crystals to which reference has 

 been made are always found to be broken when the bending becomes 

 very pronounced. 



Diopside. — A number of clear crystals of pale green diopside from 

 De Kalb, New York, were secured, and from these two were selected. 

 These, together with an octahedron of magnetite from Mineville, 

 New York, were embedded in alum, inclosed in a copper tube, and 

 submitted to pressure in the usual manner adopted in Kick's method. 

 The copper tube was 0.877 inch (22.29 mm.) high, but otherwise of 

 the same dimensions as that employed in the case of apatite — the 

 experiment being carried out in the same manner. The diopside 

 crystals were quite transparent and showed both pinacoids, the prisms, 

 two sets of domes, and one set of pyramidal faces. One of these 

 crystals was placed in the tube so as to lie upon its orthopinacoid, the 

 direction of the pressure being consequently at right angles to this 

 face; while the other was placed so that the pressure would be exerted 

 upon it in a direction as nearly as possible at right angles to the base. 

 The pressure was continued until the copper tube commenced to show 



