CRYSTALLOBLASTIC ORDER AND MINERALS 511 



This is illustrated in Fig. 15. The biotite is here mottled lighter 

 and darker. The light portions were once quartz grains of the 

 groundmass, which were absorbed or replaced by the mica in its 

 growth. The dark areas contain a large proportion of opaque 

 carbonaceous matter which was also a part of the groundmass and 

 which was included, but not absorbed nor replaced. For this 

 reason the structure of the groundmass is nearly as distinct within 

 these biotite crystals as it is outside. Their edges, like those of the 

 ilmenite plates, are irregular because of their growth against con- 

 stitu tents of the groundmass, and the white inclusions and re- 

 entrants are unabsorbed quartz grains. 



In schists belonging to Stage C, in which ilmenite has already 

 acquired parallel orientation and much of the quartz of the 

 groundmass bears evidence of recrystallization, biotite still lacks 

 dimensional and crystallographic parallelism. The biotite crystal 

 photographed in Fig. 4 was cut where a quartz band passes quite 

 through it. Its cleavage is nearly perpendicular to the schistosity 

 and its length is only very little greater than its width. 



Under conditions of extreme metamorphism (Stage D), biotite 

 acquires dimensional parallelism (Fig. 16). At this stage the quartz 

 of the rock is of secondary origin and the sericite, the earliest new 

 mineral to appear at the inception of metamorphism, has given 

 place to muscovite. .Crystals of these three minerals are roughly 

 of the same size. The quartz and white mica have become rela- 

 tively larger, and the biotite has become relatively smaller. 



Several slides show that the biotite was subsequent to the 

 ilmenite in respect to its origin. In Figs. 2 and 17 biotite crystals 

 have partly inclosed adjacent plates of ilmenite. The projecting 

 ends of the ilmenite in Fig. 17 (see also Fig. 18) have quartz borders 

 like those illustrated in Fig. i. This quartz border, once entirely 

 surrounding the ilmenite, was absorbed or replaced by the biotite 

 in just the same way as the quartz grains of the groundmass, as 

 shown in Fig. 15. This is the explanation of the light halo that 

 encircles the included portion of the ilmenite.^ 



^ Professor Wolff describes a rock in which ilmenite plates are coated with chlorite 

 except where they are included in ottrelite (J. E. Wolff, "On Some Occurrences of 

 Ottrelite and Ilmenite Schist in New England, Btill., Harv. Mas. Conip. Zool., XVI, 8 

 [1890], p. 162). 



