CRYSTALLOBLASTIC ORDER AND MINERALS 



507 



the facility of solution and crystallization varies in different crys- 

 tallographic directions and for different crystal forms of the same 

 substance;^ but, while this property might be suggested to explain 

 the replacement of sericite, it could not account for the replacement 

 of quartz, because quartz does not have crystallographic parallelism. 

 Quartz plates would not tend to dissolve more readily at their edges 

 than on their broad surfaces. If the rule applied to sericite only, 

 and not to quartz, we should expect to find evidence of selective 

 replacement by the ilmenite — substitution of sericite and not 

 of quartz. However, no such 

 evidence is apparent. The 

 necessary conclusion is that 

 the ihnenite plates, like the 

 sericite and quartz, were 

 formed during the operation 

 of the shearing stress.^ 



The relations of the growth 

 of ilmenite to the development 

 of the schistosity are brought 

 out in Figs. 9 to 12. Figure 9 

 exhibits a garnet grain (about 

 1/16 inch in diameter) which 

 partly inclosed an ilmenite 

 crystal. The portion of the 

 ilmenite outside the garnet is 

 about half as thick as that 

 inside and lies parallel to the 

 schistosity which wraps round 



the garnet. The events illustrated by the photograph seem to 

 have been as follows: (i) growth of a relatively thick plate of 

 ilmenite; (2) inclosure of part of this plate by a crystal of 

 garnet; (3) shearing of the rock, which produced, in the ground- 

 mass, a schistose structure that curves round the garnet, and 



' See writings by Goldschmidt and others. 



2 Van Hise states that no minerals show crystallographic orientation under mass- 

 static conditions {op. ciL, p. 689). When minerals are crystallographically, as well as 

 dimensionally, oriented, the suggestion derived from Goldschmidt's work should 

 receive careful consideration. 



Fig. 8.— Similar to Fig. 6. An ilmenite 

 lath may be seen near the top of the figure. . 

 The flow cleavage has been somewhat folded, 

 thus showing a tendency toward the forma- 

 tion of a false cleavage. 20 diameters. 



