DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 507 



ends of the specimens where this loss of transparency is most pro- 

 nounced. 



It is thus evident that under the differential pressure the diopside 

 crystals became slightly twisted, and then, as the pressure increased, 

 changed their shape somewhat by the development of a series of poly- 

 synthetic twin lamellae, finally breaking along certain lines, with the 

 development of cataclastic structure. In a paper published in 1886, 

 Mugge' describes some experiments which he carried out on the behav- 

 ior of diopside under pressure. He inclosed clear and untwinned crys- 

 tals of this mineral in lead and then squeezed the mass down by means 

 of a powerful screw press. In some cases he found that the mineral was 

 reduced to a powder, and in other cases the crystal survived, but, even 

 after repeated trials, he was unable to induce any twinning in it. In 

 some few cases, in the partially crushed crystals, he found what was 

 apparently a twinning parallel to the base. He states, however, that 

 he could very rarely get a section of the twinned material so thin and 

 with the lamellae so broad that the individual lamellae showed their 

 own optical orientation, their existence being indicated merely by the 

 fact that between crossed Nicols the extinction was never complete. 

 It is possible, however, as above described, by employing Kick's 

 process, to obtain in diopside a perfect twinning, in which clear, 

 well-defined lamellae extend through the whole individual and are 

 identical in character with those seen in the twinned diopsides found 

 in crystalline limestones which have been subjected to orogenic 

 movements. 



Limonite. — A cube, pseudomorph after pyrite, from a locality in 

 Virginia was selected. It was treated in precisely the same manner 

 and in a tube of the same thickness as in the case of the apatite and 

 diopside. On dissolving away the alum, the cube of limonite was 

 not found to present any distinct evidence of plastic flow. Its lower 

 surface was intact except for the presence of a minute crack. The 

 upper surface was traversed by many minute open fissures which 

 crossed one another, giving rise to a rudely rectangular pattern. 

 Each rectangle formed the base of a wedge of the material which was 

 driven downward, causing the sides of the cube to slant outward. 



I "Ueber kiinstliche Zwillingsbildung durch Druck am Antimon, Wismuth und 

 Diopsid," Neues Jahrbuch filr Mineralogie (1886), I, 183. 



