REPORT OF THE CHIEF ASTRONOMER 411 



SESSIONAL PAPER No. 25a 



so that the same magma which has intrusive relations on the south, found free 

 vent to the surface at the northern extremity of the chonolith. This lava, 

 often highly vesicular, composes the heights along the northern limit of the 

 map on the west side of Kettle river and forms a continuous mass for nearly 

 four miles measured east and west. It is not known how far it extends north- 

 ward, outside the five-mile Boundary belt. No safe clue could be found as to 

 the dip or as to the total thickness of the lava; the latter must, however, 

 measure many hundreds of feet. It is always massive and without such part- 

 ing's as would be expected if the mass were the result of many successive flows. 

 In any case the flows are thick and, apparently, are unaccompanied by pyro- 

 clastic materials. 



Petrographically this lava closely resembles the chilled phase of the chono- 

 lith. The colour of the rock varies from slate-gray to the more common 

 brownish-gray; in the more vesicular lava the tint becomes lighter and more 

 distinctly brown, changes doubtless incidental to simple weathering in a porous 

 rock. The vesicles are of all sizes up to those 2 cm. or more in length. They 

 are regularly filled with calcite, more rarely by isotropic silica, probably opal. 



The lava is always porphyritic; the phenocrysts are rhomb-feldspar 

 (' anorthoclase ' like that in the phenocrysts of the chonoliths), augite, and 

 biotite, though, in some slides, the biotite is quite rare among' the phenocrysts. 

 The ground-mass is mostly composed of the same minerals, along with ortho- 

 clase. All are in microlitic development and are embedded in an abundant 

 colourless to pale greenish or brownish glass. As in the intrusive phase the 

 feldspar of the ground-mass never shows rhombic sections. The accessories 

 are, here also, apatite, titaniferous magnetite, and probably titanite in minute 

 grains. The apatite occurs in unusually large prisms and is fairly abundant. 

 The glassy base is sometimes zeolitized and is then brownish and polarizes 

 faintly, as in the analyzed specimens of the chonolith. The glass is roughly 

 estimated to compose from 20 to 35 per cent or more of the whole volume. 

 These estimates agree with those which can be roughly made from the density 

 of the rock. For this purpose three fresh specimens of the non-vesicular lava 

 were specially chosen and their specific gravities determined; the values are, 

 respectively, 2-597, 2-602 and 2-624. In this respect as in the mineralogical 

 composition and general habit, the similarity of the lava to the chilled phase 

 of the chonolith is very manifest. So patent is this resemblance that no 

 chemical analysis of the lava seemed necessary. 



Analcitic 'Rhomb-porphyry (' shaokanite ') . 



North of Rock Creek. — As one climbs northward from Rock creek canyon 

 up the mountain lying immediately east of the north fork of the creek and 

 about five miles north of the Boundary line, he first passes over Tertiary 

 conglomerate, then over the most westerly of the three large bodies of intrusive 

 rhomb-porphyry, and, near the 4,000-foot contour, reaches the edge of the great 

 lava mass which has just been described. The first summit of the mountain 



