CRYSTALLOBLASTIC ORDER AND MINERALS 509 



resulted in the thinning of the protruded part of the ilmenite and in 

 its being bent into parallelism with the adjacent schistosity. Since 

 the ilmenite reveals no sign of fracture at its "elbow of deforma- 

 tion," its distortion was probably accomplished by gliding or by less 

 regular molecular readjustment. In either case the facility of 

 adjustment in the ilmenite must have been at least as great as the 

 ease of accommodation of the groundmass; i.e., deformation of the 

 ilmenite must have kept pace with development of the schistosity. 



Fig. II shows a similar instance of an ilmenite crystal partly 

 inclosed by garnet, but here the projecting portion of the ilmenite is 

 less bent and less thinned than in the preceding example. A small 

 re-entrant at the elbow of deformation, now filled with secondary 

 quartz, suggests that the shearing produced a small fracture, sub- 

 sequently perhaps more or less restored, or — and this is more 

 probable — ^that it locally increased the tendency toward molecular 

 readjustment at that place, making the ilmenite susceptible to 

 replacement by quartz.' 



Garnet. — -Garnet metacrysts are found in rocks assigned to 

 Stages C and D. In all cases there is clear evidence that this 

 mineral originated before shearing entirely ceased.^ Figs. 9 to 1 2 

 bring out this feature very well. The structure of the groundmass, 

 particularly in Figs. 9 and 10, indicates rotation of the garnet 

 crystal, and a similar relation appears in Figs. 13 and 14. Fig. 13 

 shows also the so-called "tails" of quartz, light areas that extend in 

 opposite directions a short distance out from the metacryst, parallel 

 to the schistosity. Such "tails" are without doubt in process of 

 formation during the shearing of the rock. Consequently, their 

 pecuUar association with the garnet metacrysts is another fact 

 pointing to the conclusion that the garnet was formed before the 

 cessation of mechanical deformation. 



Biotite. — ^As would naturally be expected, in rocks with no flow 

 cleavage, biotite plates, if present, have no definite orientation. 



^"The experimental work of Barus and Hambeuchen together has completely 

 demonstrated that a state of strain in substances is favorable to chemical action." — 

 Van Hise, Treatise on Metamorphism, p. 691. 



^ Cf. this statement by Leith: Garnet, staurolite, andalusite, etc., "in many if not 

 in most cases crystallized out later than the principal cleavage-making minerals. 

 . . ."— C. K. Leith, "Rock Cleavage," U.S.G.S., Btdl. 239 (1905), 93-94. 



