226 F. E. Wright — Schistosity by Crystallization. 



chemical standpoint is an undercooled liquid, and in it the 

 viscosity or internal friction at temperatures at which crystal- 

 lization may begin is sufficiently great to permit application of 

 unequal stresses. Certain glasses, as those from wollastonite, 

 diopside, anorthite and other minerals, crystallize at tempera- 

 tures far below the melting point of the mineral, and while 

 still in a state of fair rigidity. 



These minerals are, moreover, either prismatic or tabular in 

 habit and possess, therefore, a decided inclination to grow more 

 rapidly in one direction than in another. If made to crystal- 

 lize under unequal stress, they will, in consequence, tend to 

 develop most rapidly in the direction of least resistance and 

 their favored axis or plane of growth (prismatic or pinacoidal) 

 will be normal to the active stress ; while the resultant texture 

 will be comparable to those produced by the recrystallization 

 of a rock under stress. 



In our experiments about 50 grams of each of the above 

 minerals were first melted separately in a Fletcher furnace and 

 then chilled rapidly to glass by plunging the platinum crucible 

 containing the melt into water. Cubes of about l cm edge were 

 then cut from these glasses and subjected afterwards both to 

 heat and stress. Heat was applied by means of an air-gas 

 blast and concentrated by enclosing the preparation in a small 

 reverberatory hood of asbestos. Pressure was produced in two 

 different ways ; in the first, a cube of mineral glass, shielded 

 both above and below by thin disks of asbestos, was placed 

 between two short vertical steel rods held in position by a 

 suitable stand and gravity pressure obtained by weighting 

 down the upper rod. By this method the effects of stress in 

 one direction alone were studied, and the similarity of the 

 textures produced compared with those of rocks formed under 

 like conditions of stress. 



To obtain stresses acting along two directions normal to 

 each other and thus to imitate the pencilled texture of many 

 amphibolites, a device suggested by Dr. A. L. Day was 

 employed, consisting of a wide metal pipe ring of 12 cm diam- 

 eter, into the sides of which four screws were inserted at 

 intervals of 90°. Steel caps were fitted on the ends of these 

 screws and between them the cube was placed. Pressure was 

 applied by tightening the screws, while heat was derived as 

 above from an air-gas blast. 



After complete crystallization under stress, the cubes were 

 immersed in hot Canada balsam and afterwards embedded in 

 plaster of Paris. These precautions were found by experience 

 to be necessary, and were taken in order that satisfactory thin 

 slices could be cut from the cubes, which, after crystallization 

 are extremely brittle and fracture readily. Plates were then 



