364 



DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



ohonolith will be specially described, a& it also represents the type of the 

 porphyry where exposed in the many dikes between Sophie mountain and the 

 water-divide eight miles to the westward. 



At distances of fifty feet or more from a contact the porphyry of the chono- 

 lith is a rather light gray rock having abundant phenocrysts of soda-orthoclase, 

 andesine (near Ab. An 3 ), biotite, and augite, embedded in a fine-grained, f eld- 

 spathic base. The feldspar phenocrysts are in thick-tabular Carlsbad twins and 

 are characteristically glassy, like sanidine; they have lengths varying from 5 

 mm. to 15 mm. The lustrous, highly idiomorphic, black foils of biotite 

 measure from 1 mm. to 2 mm. in diameter; the likewise idiomorphic, diopsidic 

 augite stands out in stout prisms % mm. to 3 mm. in length. The 

 andesine phenocrysts are often surrounded by a thick shell of orthoclase, the 

 two feldspars then having common basal cleavage. The ground-mass is a fine- 

 grained hypidiomorphic-granular aggregate of orthoclase individuals, associated 

 with a little oligoclase and considerable interstitial quartz. The accessories, 

 titanite, titaniferous magnetite, and apatite, also form part of the ground-mass. 

 A fresh specimen (No. 409) of this phase has been chemically analyzed by Mr. 

 Connor with result as here noted: — 



Analysis of syenite porphyry. 



SiO„ 



TiOo 



AJriOs 



Fe 2 0, 



FeO. 



MnO 



MgO 



CaO 



SrO 



BaO 



Na,0 



K,0 



H\0 at 110° C. , 

 H 2 above 110°C. 

 P.O. 



Mol. 



60-51 1 



008 



•60 



008 



16-71 



164 



1-72 



011 



3-34 



046 



•10 



001 



2-53 



063 



3-62 



064 



■12 



•001 



•10 



•001 



4-64 



075 



5-20 



•055 



•03 





•27 





•16 



•001 



99-65 





2-667 





Sp.gr 



In the Norm classification the rock enters the sodipotassic subrang, monzo- 

 nose, of the domalkalic rang, monzonase, in the dosalane order, germanare. The 

 norm is as follows : 



Quartz.. .. 

 Orthoclase. 

 Albite.. .. 



Anorthite.. . 

 Hypersthene. 

 Diopside.. .. 

 Magnetite. . 

 Ilmenite.. .. 

 Apatite.. .. 

 H.0 



2-40 



30-58 



39-30 



9-45 



6-81 



6-55 



2.55 



1-21 



•31 



•30 



99-46 



