290 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 

 PETROGRAPHY OF THE BATHOLITH. 



Within the part of the batholith covered by the ten-mile Boundary map (the 

 only part investigated), the granodiorite is a notably homogeneous rock of a 

 light-gray to pinkish-gray colour and a medium to fairly coarse grain. It is 

 essentially composed of quartz, microperthite, orthoclase, microcline, hornblende, 

 augite, and biotite. Crystals and aggregates of magnetite, well crystallized tita- 

 nite and apatite, a few small zircons, and rare idiomorphic crystals of allanite 

 are accessory constituents. Microperthite is the dominant feldspar ; it often has 

 the double lamellation of micfocline-microperthite. The orthoclase is probably 

 sodiferous. The soda-lime feldspar is of somewhat variable composition. Some 

 crystals (in Carlsbad-albite twins) have the extinction angles of andesine, 

 Ab E An s ; others are acid labradorite. Many of them are zoned, with cores 

 of labradorite, Ab„ An 4 , and outer rims of oligoclase, Ab ;1 An r The average 

 plagioclase has about the composition of basic andesine, Ab 5 An,. 



Next to the feldspars and quartz, hornblende is the most important consti- 

 tuent. It forms idiomorphic crystals, bounded by planes at the extremities as 

 well as in the prismatic zone. The colour scheme is: — 



Parallel to a — Strong yellowish green. 

 " b — Deep olive green. 



" c — Deep sea-green with bluish tinge. 



The absorption is strong: b>Oa. In sections parallel to (010) the extinc- 

 tion is 16° 30'; in sections parallel to (110), 20° 15'. These values show that 

 the optical angle is unusually small and near 50°.* The hornblende has proper- 

 ties somewhat similar to those of the variety ' philipstadite.'f 



The biotite is deep brown with powerful pleochroism; it is sensibly uniaxial. 

 The diopsidic augite is colourless to pale greenish in thin section and is not 

 noticeably pleochroic. It is quantitatively subordinate to the biotite but in all 

 the specimens collected must be ranked among the essentials. 



The other constituents need no special note. Though the rock is unusually 

 strong and fresh, a little secondary kaolin and yellow epidote may occasionally 

 be seen. 



The specific gravity of the rock varies from 2-743 to 2-785; the average for 

 five fresh specimens is 2-757. 



Mr. Connor has analyzed a typical specimen (No. 858) from the vicinity of 

 the Bayonne mine, with the following result: — 



* Cf E. A. Daly, Proc. Amer. Academy of Arts and Science, Vol. 34, 1899, p. 311. 

 t Proc. Amer. Academy of Arts and Science, Vol. 34, 1899, p. 433. 



