6 
MUSEUM BULLETIN NO. 2. 
glassy, a feature very rare in the basic rocks of East Kootenay. 
Under the microscope, the essential constituents of the gabbro 
are seen to be labradorite and pyroxene (Plate I, fig. 1). 
The labradorite occurs in lath-shaped individuals, which show 
carlsbad, pericline, and albite twinning. Filling the interstices 
between the labradorite crystals occurs the pyroxene which is of 
two varieties, hypersthene and augite. The augite is colourless 
and without pleochroism. Uralitization in all stages is common 
in this rock; in many cases crystals of hornblende occur with a 
core of augite. Where the secondary hornblende is in contact 
with the labradorite it has the characteristic pleochroism, 
greenish blue parallel to c, strong green parallel to b, and pale 
yellowish green parallel to a. This is worthy of notice as 
nearly all the hornblende in the hornblende gabbro is of this 
type and it also strongly supports the theory that all the horn- 
blende is secondary in origin. 
Calkins 1 , from a study of similar intrusives in Idaho, south 
of the International Boundary line, maintains that the horn- 
blende of the gabbro is secondary. An extract from his report 
may be quoted. 
“The rock now under consideration (provisionally called diorite) is, on the 
whole, richer in quartz than the diabase of the Coeur d’Alene district, but otherwise 
similar except in the absence of augite; and this may be accounted for by the probably 
lesser age of the latter rock, which cuts across beds as late as the Striped Peak, while 
the diorite sills here described were not seen in contact with rocks younger than the 
St. Regis. These considerations, together with the character of the hornblende, 
suggest that dioritic rocks of this region are uralitized diabases and gabbros, and 
although complete proof is wanting, I am inclined to believe that such is the case.’’ 
An occurrence of similar nature has been described by C. H. 
Warren 2 . The hypersthene of the Purcell Sills is about equal 
in amount to the augite and is characterized by a faint pink 
pleochroism parallel to c. It very frequently shows signs of 
alteration to hornblende. 
In one instance, the hypersthene appears to have changed to a 
fibrous hornblende, which in turn was changed to a compact 
hornblende. The accessory constituents are small in amount 
and consist of magnetite in irregular grains, and apatite in long, 
colourless, idiomorphic crystals. One allotriomorphic crystal 
1 Calkins, F. C., U. S. G. S„ Bull. 384, p. 49. 
2 Warren, C. H., Amer. Jour. Sci., Vol. 26, 1908, p. 469. 
