226 JOSEPH P. IDDINGS 



The proportions in which these occur was determined by microscopi- 

 cal measurement to be approximately, in 62.8 per cent, of ground- 

 mass: 



Total 



Quartz, 23.9 34.6 



Feldspar, 29.2 - - - - - - 55.7 



Biotite, 8.6 ------ 8.6 



Fluorite, i.o- - - - - - i.o 



Apatite, 0.13 - - - - - - 0.13 



62.83 ' 100.03 



The fabric of the groundmass is uniformly heterogeneous, being 

 a mixture of automorphic granular and micrographic. It consists of 

 anhedrons of quartz, very free from inclusions, except some minute 

 gas cavities, with similarly shaped anhedrons of microcline slightly 

 clouded with alteration products, besides anhedrons of twinned 

 albite with an approach to automorphism. These anhedrons vary in 

 size from o.i to o.oi"^™ in diameter. Throughout the whole are 

 scattered at short intervals granular clusters of graphic intergrowth 

 of quartz and feldspar. The crystallization of the graphic parts was 

 almost contemporaneous with that of the anhedrons, as these are 

 developed in continuous orientation with the graphic clusters. 



The mica is xenomorphic in great part, and is in about the same 

 sized anhedrons as the quartz and feldspar. It appears to have been 

 almost contemporaneous in crystallization with these minerals. Its 

 color is green to brownish-green. 



Fluorite occurs in irregularly shaped anhedrons, xenomorphic 

 in form. It is colorless in thin sections, exhibits distinct cleavage, 

 and is characterized by its low refraction and isotropic behavior. It 

 is quite uniformly scattered through the groundmass. 



Apatite occurs in colorless microscopic prisms. Magnetite and 

 zircon both occur in anhedrons in such small quantities that they 

 were not measured. They appear to constitute a small fraction of 

 I per cent, of the rock. 



A careful study of the feldspars in the groundmass showed that 

 microcline and albite are present in nearly equal proportions, and 

 that they form separate and distinct crystals not perthitically inter- 

 grown. 



