5o6 FRANK D. ADAMS 



signs of rupture, the time occupied by the experiment being about one 

 hour, and the load finally rising to 48,000 pounds (21,773 kilos). 

 On removing the test from the press, it was found that the end of one 

 crystal of pyroxene was protruding slightly from the alum and had 

 sunk into the brass plate, leaving a distinct impression in it; and, on 

 dissolving away the alum, it was found that both pyroxenes displayed 

 a twinning parallel to the base which had been developed in them 

 by the pressure; this being strikingly seen in the case of the crystal 

 which was compressed approximately in a vertical direction. The 

 twinning in the case of this crystal appeared as a series of little parallel 

 lines crossing the lateral faces in the direction of the base, and in 

 appearance resembling closely the basal parting so frequently seen 

 in pyroxenes which occur in the limestones of the Grenville series 

 and other rocks which have been submitted to great compressive 

 stresses. 



A section was then cut through this crystal in a vertical direction. 

 Under the microscope, between crossed Nicols, this was found to be 

 so orientated as to intersect the crystal in a direction between the 

 orthopinacoid and a prismatic face. The prismatic cleavages were 

 well seen and a second set of interrupted cleavages crossed these at 

 right angles or nearly so, being parallel to the base. Parallel to these 

 latter was a beautiful series of clear, sharply defined, polysynthetic 

 twin lamellae, which had been developed by the pressure. The sec- 

 tion was 14 millimeters wide, and in this width displayed 140 twin 

 lamellae, each of which was wide enough to show clearly its individual 

 character, the series being spaced at nearly equal distances across 

 the crystal. Their appearance under the microscope is shown in 

 Plate III, Fig. h, while Plate III, Fig. a, shows the apearance of a 

 section of the original pyroxene. Apart from the twin lamellae, the' 

 mineral shows a slightly undulating extinction, and the section is 

 crossed in one or two places by narrow lines of granulated material, 

 along which, under the pressure, the mineral has broken with the 

 development of a cataclastic structure. A study of the section shows 

 that probably the twinning was first developed and that the mineral 

 under further pressure broke along certain lines. The partial loss of 

 transparency observed in the deformed crystal is largely due to the 

 development of similar lines of broken material, especially at the 



