REPORT OF THE CHIEF ASTRONOMER 333 



•SESSIONAL PAPER No. 25a 



Augite Andesite. — At two localities in the volcanic area, lavas belonging 

 to the common species, augite andesite, have been identified. This rock may 

 •occur at many other points but its macroscopic similarity to the augite latite 

 makes its discovery very uncertain. As already noted, the writer believes 

 that this type as well as the true basalts are subordinate to the latites in the 

 region covered by the Boundary survey map. 



A specimen belonging to what seems to be a massive flow was collected 

 near the Coryell syenite contact on the ridge running northward from Monu- 

 ment 171. It will be observed that this ridge is just west of the body of 

 enstatite-olivine rock which is mapped as harzburgite but may represent a 

 picrite, i.e., an extrusive form of the harzburgite magma. The augite andesite 

 has, notwithstanding its altered character, all the ear-marks of this species 

 of lava. The phenocrysts of augite and labradorite are embedded in a much 

 altered ground-mass in which microlites of those minerals can be detected as 

 the essentials. The alteration products are uralite, chlorite, and quartz and 

 thus differ essentially from those which are so characteristic of the latites. 

 The evidence is quite clear that the ground-mass is not rich in potash. 



True augite andesite was also found to compose most of the blocks in a 

 very coarse agglomerate capping the ridge lying between Monument 172 and 

 the confluence of Santa Rosa creek and (the western) Sheep creek. The larger 

 blocks are there from three to four feet in diameter. A few fragments in the 

 breccia are exceptional for this volcanic series in being of acid composition, a 

 biotite-quartz porphyry. 



Basalts. — A typical olivine basalt was discovered on Mt. Tamarac, the 

 broad divide between Malde creek and Little Sheep creek. It seems to form 

 there a very thick and massive flow interbedded . in specially voluminous basic 

 breccias. The phenocrysts are labradorite, augite, and olivine. The ground- 

 mass is the usual holocrystalline aggregate of augite and feldspar. The 

 feldspar is here much more altered than the femic minerals; this is just the con- 

 trary of the rule with the latites, in which the feldspars are almost always not so 

 badly altered as the augite, hornblende, or olivine. 



In the col on the trail southwest of Lake mountain an equally typical 

 olivine-free basalt forms at least two thick flows separated by a two-foot layer 

 ■of basic tuff. The contact-planes show here a strike of N. 10° E. and a dip of 

 75° to the westward; the series has evidently been greatly deformed at this 

 point. The distinction of this basalt from the augite latite is easily made, for 

 the fairly fresh ground-mass is the typical diabasic. A very similar rock, 

 though distinctly vesicular, was collected at the edge of the volcanic area on 

 the west side of Twelve-mile creek; it may, however, easily belong to the series 

 of lavas included in the Beaver Mountain group. 



Flow of Liparitic Obsidian? — Throughout the whole area covered by the 

 Boundary belt in the Bossland mountains, acid lavas are extremely rare. Frag- 

 ments of biotite-quartz porphyry, probably a liparite of extrusive origin, are. 

 as we have seen, enclosed in the coarse agglomerate at one point. The ?nly other 



