504 



FREDERIC H. LAHEE 



and 3).^ When examined under the microscope they are quite 

 opaque. Occasionally they contain small inclosures of quartz and 

 sericite, the principal constituents of the groundmass, and their 

 edges are more or less serrate where they have grown against these 

 minerals. Since ilmenite is not a silicate, it evidently made room 

 for itself by replacing the minerals of the groundmass and not by 

 absorbing them. Its efficiency of replacement must be high for it 

 contains relatively few inclusions.^ 



Where the groundmass has no parallel arrangement of its con- 

 stituents and even where it does possess a flow cleavage that is 

 •p^^„,g ^u --1.^^ not too well marked, the 



ilmenite plates lack definite 

 orientation. Fig. 2 illus- 

 trates a portion of a rock 

 slide in which the ilmenite 

 crystals were scattered at 

 random and the schistosity 

 curved round them. It is 

 evident here either (i) that 

 they developed before the 

 flow cleavage and that, 

 in this case, the shearing 

 of the rock was not suf- 

 ficiently rapid or strong to 

 bend or break them; or (2) 

 that they grew pari passu 

 with the schistosity, but 

 that the shearing force was not enough to prevent their random 

 orientation, that is, that their molecular forces were greater than 

 the exterior stress. In either event they may have suffered some 

 rotation. 



Fig. 3. — Outline sketch of Fig. 2. Ilmenite 

 crystal partly inclosed in biotite. Note curv- 

 ing of the schistosity. I, ilmenite. B, biotite. 



' The photomicrographs used for this article were taken by Professor E.G. Jeffrey 

 of Harvard University. The writer is happy to express his deep gratitude to Professor 

 Jeffrey for this compliment and for valuable suggestions in preparing the illustrations. 



^ Cf. Van Hise, "In proportion as minerals are unable to absorb, they are able to 

 enclose." — Treatise on Metamorphism, U.S.G.S. Moiiog., XL VII (1904), 700. 



