3IO SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



ciated with quartz, producing the micrographic structure. While 

 much of the feldspar shows albite twinning, a larger portion shows 

 no such phenomenon, and it is evident from the frequency of the 

 parallel extinctions that much orthoclase must be present. It does 

 not appear to be so susceptible to kaolinization as the more basic 

 plagioclase. The latter is -generally twinned according to the albite 

 law, and its symmetrical extinctions, ranging as high as 18.7°, 

 indicate a feldspar intermediate between andesine and acid labradorite, 

 more nearly the former than the latter. Perthitic intergrowths were 

 noticed; also a curious instance of secondary feldspar twinned 

 according to the albite law, enclosed in a feldspar of less refrac- 

 tive power. This is not true microperthite, and its development 

 is undoubtedly the result of pressure, the enclosing feldspars showing 

 the effect of strain in their wavy extinctions. 



The more basic feldspars, it has been noted, are the more kaolin- 

 ized. They contain areas of brownish, cloud-like masses, non-re- 

 active between crossed nicols. Other inclusions consist of little tufts 

 or fibers, which polarize brightly; these are often arranged along 

 cracks or cleavage planes and are to be referred to the mineral 

 sericite. Fluid inclusions are present, together with minerals of 

 earlier crystallization, iron oxide, apatite, and zircon. 



The colored constituents include the light-green monoclinic pyrox- 

 ene, which, from the low extinctions and presence of characteristic 

 partings, is to be referred to the mineral diallage. This monoclinic 

 variety is generally associated with hypersthene, the two minerals 

 being frequently intergrown. Many inclusions of iron oxide, both 

 primary and secondary, are present, the hypersthene being appar- 

 ently very susceptible to change. The secondary iron oxide is 

 often present in masses simulating skeleton forms, which, co- 

 alescing, form larger masses often seamed with dark green 

 chloritic matter. Epidote often occurs along the fissures or cleavage 

 planes. 



In but few sections studied was amphibole observed. It occurs in 

 rather large patches (one inch and less) in a light-colored segregation 

 of the country rock at an elevation of about 2,400 feet. It is the 

 usual brown variety, strongly pleochroic, as follows : C, brown ; 

 A, brownish yellow ; and B, chestnut brown with absorption scheme 

 B^C > A. This is rather unusual, but the difference between the 

 absorption parallel to B and C is very slight. 



The presence of primary quartz is of interest in this relatively 

 basic rock. It occurs in large isolated masses and is also intergrown 

 with the feldspar. Iron oxides (both hydrous and anhydrous), 



