REPORT OF THE CHIEF ASTRONOMER 213 



SESSIONAL PAPER No. 25a 



mation was seen across the north slope of Phillips creek valley, about three 

 miles above the cascade. The field study gave the following result : — 



h 



Top, conformable base of Gateway formation. 



d. 60 feet — greenish-black amygdaloid. 



c. 40 " coarse basic breccia. 



b. 2CO " greenish-black amygdaloid with occasional large phenocrysts of labra- 



dorite. 

 ■J. 90 " porphyritic, non-vesicular, with abundant large phenocrysts of labra- 



dorite. 



390 feet. Base, conformable top of Siyeh formation. ; 



Mineralogically and chemically these rocks are similar to the correspond- 

 ing phases of the formation in the McGillivray range. Zone c appears to be 

 a true explosion-breccia but is apparently of quite local extent. In the other 

 sections it is replaced by an approximately equal thickness of the black amyg- 

 daloid. The conspicuous porphyritic phase is also replaced by the amygdaloid 

 in several sections made on the Kootenay valley slope, north and south of 

 Phillips creek. In each of these latter sections the formation is very homo- 

 geneous and massive, as if formed of a single great flow. The intercalation 

 of tuffaceous rock in zone c seems to show that zone d belongs to a later flow 

 distinct from that represented in zone h. There is no plane of separation 

 between zones a and &, which merge gradually into each other, being probably 

 phases of one erupted mass. 



At the summit of the Galton range the formation is cut off by a master 

 fault. To the east of the fault the lava has been completely eroded away and 

 it does not appear on the map of the belt covering the eastern half of the 

 Galton range and the whole of the MacDonald range. 



PURCELL LAVA IN THE CLARKE RANGE. 



The most westerly outcrop of the lava in the Clarke range occurs at the 

 head of Starvation creek, twenty-seven miles east of the summit fault of the 

 Galton range. From that point to the lake at the eastern extremity of the Com- 

 mission map the formation forms a conspicuous feature of the cliffs. Prom a 

 commanding point it can be seen contouring the mountains through several 

 miles of continuous exposure. In all, the Boundary map has twenty-five miles 

 of this outcrop. It rigidly preserves its conformable position between the Siyeh 

 and Sheppard formations and steadily holds a thickness of about 260 feet. 

 As in the western ranges it is, on account of its hardness, a strong cliff-maker, 

 often forming unscaleable precipices at cirque or canyon. 



Wherever examined the whole formation is a homogeneous, dark greenish- 

 gray to blackish amygdaloid, scoriaceous and of typical ropy structure at the 

 upper contact. White amygdules of quartz and calcite are there abundant 

 and often reach great size, even to six or eight inches in length. (Plate 23.) 

 The porphyritic phase and breccia of the western ranges are not associated with the 



