358 F. II. Lahee — Metamorphism and Geological Structure. 



small ilmenite crystals. After treating a crushed knot with 

 dilute hydrochloric acid, the residue was found to contain 

 minute, nicely shaped crystals and reticulated aggregates of 

 rutile about ilmenite plates. 



On the whole the facts indicate that these knots represent 

 the positions of once existing crystals of some mineral which 

 decomposed into a pseudomorphic form and was then modified 

 by addition of material, by recrystallization, and by mechani- 

 cal rearrangement. 



The metacrysts, in pelites of Stage C, named in the order of 

 decreasing frequency, are, ilmenite, biotite, garnet, and ottre- 

 lite. They are not all found in all specimens. Generally one 

 or two are conspicuous and the others are scarce or absent. 

 These four minerals were formed contemporaneous with, or 

 later than, the schistosity. Only one among them, the ilmen- 

 ite, acquires parallel orientation of its crystals in Stage C (see 

 lig. 2). Sometimes biotite has a linear parallelism such that 

 the length of its plates are parallel, but the widths lie in all 

 positions perpendicular to this common direction. Although 

 isometric and therefore equidimensional, garnet is distinctly a 

 mineral which develops under dynamic and not under static 

 conditions. Ottrelite, which is very rare, is wholly unrelated 

 in size, shape, and orientation, to the schistosity, and is conse- 

 quently of late anamorphic derivation under static stress. 

 The infrequent lack of dimensional parallelism in the ilmenite 

 and the more common absence of the same in the biotite are 

 due to the fact that these minerals, too, under such circum- 

 stances, originated under static pressure. 



The most highly metamorphosed pelites (Stage D) are char- 

 acterized by as large a percentage of white mica (muscovite or 

 sericite) as the composition of the rock will permit ; by a very 

 high sheen on the fracture surfaces ; and by an excellent cleav- 

 age. As seen in thin sections, the quartz grains are commonly 

 elongate (fig. 12) and may be grouped in ribbon-like aggre- 

 gates. Their shape, their clearness, their freedom from strain- 

 shadows, and their relations to the adjacent minerals, point to 

 the conclusion that they are secondary or recrystallized quartz. 

 Both as single grains and as aggregates, they are oriented with 

 their longest dimensions parallel. Together with the similarly 

 disposed mica, they are the chief cause for the very good 

 cleavage. 



Among the metacrysts, ilmenite occurs in small plates par- 

 allel to the schistosity (fig. 3) ; biotite here acquires elongate 

 habit in single plates and as aggregates, which lie with their 

 lengths parallel to the cleavage (fig. 17) ; and garnet and ottre- 

 lite possess the same characters which they had in Stage C (figs. 

 18 and 20). 



