400 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The augite-biotite andesite occurs at various other points both north and 

 south of the river — on the slope southwest of Midway, on the hills south of 

 Myer's creek, and on the slopes of the Kettle river valley opposite Rock creek. 



Biotite andesite was found at only two points, cropping out in the wall of 

 the box-canyon on Myer's creek three miles from the main river, and again in 

 the blocks of a volcanic agglomerate found on the left bank of the river three 

 miles north of the first locality. In both cases the rock is highly vesicular and 

 varies in colour from a dark greenish-gray to a more ashy hue. Dark brown 

 biotite and basic andesine form the phenocrysts. The ground-mass is chiefly 

 glass in which minute plagioclase microlites are embedded. Chemically this 

 rock may be equivalent to the augite-biotite andesite. 



The biotite-trachyte is extensively developed on the heights north of the 

 Kettle river and east of Rock Creek post-office. It also crops out in the box- 

 canyon of Myer's creek five miles above the confluence with the river. A some- 

 what doubtful and certainly local body of it was noted a few hundred yards 

 northeast of the bold cliff at the elbow of the Kettle river four miles west of 

 Midway. At all the localities the trachyte is notably uniform in field-habit 

 and in composition. Chemically it is unquestionably very similar to the 

 analyzed syenite porphyry to be noted as forming the great sills and dikes 

 northeast of the Kettle river bridge. One can scarcely doubt that the trachyte 

 and syenite porphyry belong to the same eruptive period. 



The trachyte is a brownish-gray, commonly vesicular rock with conspicu- 

 ous phenocrysts of feldspar and biotite, though the latter mineral is not always 

 macroscopically visible. In one thin section augite was seen to occur among 

 the phenocrysts. The feldspars range from one to three millimetres in diameter; 

 the biotites, from 0-5 mm. to 1-5 mm. in diameter of foils. In each case the 

 average and maximum sizes are much smaller than in the intrusive, syenite- 

 porphyry phase of the same magma. The phenocrystic feldspars were found 

 to be quite variable in composition. They may be made up of labradorite 

 wholly surrounded by thick shells of orthoclase or soda-orthoclase ; or of true 

 microperthite and soda-orthoclase without associated plagioclase; or of ortho- 

 clase and andesine occurring together but not intergrown. The biotite is deep 

 brown and intensely pleochroic. 



The ground-mass is generally microcrystalline and is then composed of 

 minute feldspars of tabular form. These are almost invariably murky with 

 alteration-products, so that their determination is not easy. Most of them are 

 untwinned and are probably sodiferous orthoclase. A little interstitial quartz 

 is often visible. Magnetite and apatite are the well individualized accessories. 

 Some brownish glass is occasionally present; it is never abundant. 



The rock is to be classed among the alkaline biotite-trachytes, the effusive 

 form of a typical pulaskite. 



