THE ORIGIN OF THE PHENOCRYSTS 101 



of a medium, light to dark gray in color, according to the 

 amount of biotite present. The extreme coarseness of texture 

 renders the porphyritic structure less typically marked in this 

 than in finer grained porphyritic rocks. The porphyritically 

 developed mineral (feldspar) grades imperceptibly from very 

 large, irregular, sometimes stout, tabular phenocrysts, into the 

 smaller feldspars, making it difficult usually to distinguish 

 between groundmass and phenocryst feldspars, except in extreme 

 cases. 



The feldspar phenocrysts vary from extremely irregular 

 cleavable grains, anhedra, 30 by 3o mra to roughly idiomorphic 

 crystals; tabular, parallel to the clinopinacoid (010), and twinned 

 according to the Carlsbad law. Abundant inclusions of large 

 irregular plates of biotite are readily visible in all the pheno- 

 crysts to the unaided eye. The phenocrysts are prevailingly 

 allotriomorphic in outline and differ in this respect from those 

 of the other areas described below. The feldspars are white 

 in color, usually partially cloudy or opaque rather than limpid 

 in appearance. 



The biotite occurs as stout cleavable plates averaging 10- 

 15™ in size, occupying somewhat well-defined areas; is very 

 dark in color, and highly lustrous. The quartz is present in 

 large irregular colorless and smoky anheda, 4-5 mm in size, clearly 

 outlined against the feldspar and biotite, from which it is readily 

 distinguished. 



Microscopic study of thin sections of the porphyritic granite 

 confirms the macroscopic description, in affording no marked 

 differentiation between groundmass feldspar and a part of the 

 similar porphyritic constituent, or phenocryst. 



The feldspar constituent is composed of the potash varieties, 

 orthoclase and microcline, and a fair proportion of an acid 

 plagioclase, which from its optical properties is near oligoclase. 

 The presence of considerable lime in the analysis tends to cor- 

 roborate this inference. The orthoclase usually shows good 

 cleavage, and is separately intergrown with stringers of albite 

 and quartz, in the form of microperthitic and micropegmatitic 



