Il6 THOMAS L. WATSON 



one rock facies into the other was not entirely clearly defined in 

 all of the granite masses, owing to lack of exposures of the 

 fresh rock, but could be easily traced L in many. The feldspar 

 phenocrysts are irregularly distributed through the coarse- 

 grained groundmass without definite arrangement or orientation. 

 The fluxion primary structure was not entirely evident in any 

 one of the areas studied. 



Microscopic features. — Microscopically, the rocks are as nearly 

 identical as is possible for separate areas to be. They contain, in 

 every case, the same minerals, both essential and accessory, in 

 nearly the same proportions. They are composed of admixtures 

 of the feldspars and quartz, in which lie stout plates of biotite. 

 The relative amounts of the component minerals may be 

 expressed as follows : feldspar, including all species present, 

 >quartz>biotite. Biotite is the chief accessory and varies 

 somewhat in quantity for the individual areas. In a number of 

 the sections the biotite is intergrown with occasional foils of 

 muscovite. The potash feldspar varieties of the groundmass 

 predominate and are prevailingly allotriomorphic in crystal out- 

 line. Both orthoclase and microcline occur with the former in 

 excess. The plagioclase crystals are roughly lath-shaped in out- 

 line, and, as a rule, afford small extinction angles in basal 

 sections, which indicates an acid feldspar near oligoclase. The 

 presence of considerable lime and soda in the analyses corrobo- 

 rates the inference. The orthoclase feldspar shows microperthitic 

 intergrowths with a second feldspar, probably albite. In all the 

 sections some of the feldspar crystals show a micropegmatitic 

 intergrowth with quartz, which takes the form of rounded disks 

 or ovals, and are not of the arborescent or radiate growth type. 1 

 There can be little doubt that this structure is primary in the 

 porphyritic granites as a whole, affording evidence of simultane- 

 ous crystallization of the quartz and feldspar. The quartz 

 occurs in irregular interstitial grains of varying size, and is very 

 common in drop-like inclusions in the feldspars. Prismatic 



'See Romberg in N. J. B. B. — B., 1892, Vol. VIII. Mathews, E. B.: Jour. 

 Geol., 1900, Vol. VIII, p. 231. 



