CRYSTALLOBLASTIC ORDER AND MINERALS 505 



In many of the finer mud schists the plates of ilmenite are coated 

 with quartz^ (see Fig. i). The correct explanation for this phe- 

 nomenon is not certain. The suggestion has been made that the 

 quartz-ilmenite aggregate occupies a space which was formerly an 

 actual cavity or a potential one. This does not seem to be true, for 

 the space has not the shape of a rent, and neither the ilmenite nor 

 the quartz granules of the rim reveal any evidence of having grown 

 inward from the walls of a cavity. The form of the present quartz- 

 ilmenite aggregate indicates that it is a replacement of an earlier 

 larger crystal of ilmenite or of some other flat mineral. 



In rocks with a good flow cleavage, produced both by thin plates 

 of recrystallized quartz and by sericite (Stages C and D), the 

 ilmenite crystals are parallel to the schistosity (see Figs. 4 to 8). 

 It is not at all likely that the ilmenite here originated before shear- 

 ing, for rotation could not account for the nearly exact parallelism 

 of all the plates in the rock. Two possibilities remain, then: that 

 the building of its crystals was either contemporaneous with, or 

 subsequent to, the origin of the schistosity. If the first condition 

 was the actual one, the ilmenite as well as the quartz and sericite 

 contributed to the accommodation of the rock to the stress, the 

 accommodation being brought about by crystallization and 

 recrystallization, i.e., by chemical processes. If, on the other hand, 

 the second condition was the real one, the ilmenite plates did not 

 contribute toward the accommodation, but acquired their parallel- 

 ism on account of a property of the groundmass constituents 

 (principally quartz and sericite) to dissolve more readily parallel to 

 the schistosity than in any other direction. Now, it is known that 



' Ilmenite has been described by Wolff and Pumpelly as bordered by chlorite; by 

 Renard as bordered with sericite; and by Williams as coated by biotite. See the 

 following : 



A. Renard, "Recherches sur la composition et la structure des phyllades 

 ardennais," Bull. Mtis. R. His. Nat. Belg., I, 212; II, 127; III, 84, 230 (1884). 



G. H. Williams, "The Greenstone Schist Areas of the Menominee and Marquette 

 Districts," Michigan U.S.G.S., Bull. 61 (1890), 200. 



J. E. Wolff, "On Some Occurrences of Ottrelite and Ilmenite Schists in New 

 England," Bull. Harv. Mus. Comp. Zool., XVI, 8 (1890), 162. 



R. Pumpelly, J. E. Wolff, and T. N. Dale, "Geology of the Green Mountains in 

 Massachusetts," U.S.G.S. Monog., XXIII (1894), 183. 



