REPORT OF THE CHIEF ASTRONOMER 215 



SESSIONAL PAPER No. 25a 



At the same time there are many points of lithological resemblance between 

 the Kintla canyon sill and the amygdaloid of the Purcell formation. It is 

 clear that the extremely abundant chlorite of the amygdaloid could have been 

 derived from a dominant original pyroxene identical with that in the under- 

 lying sill. The feldspars of sill and lava seem to be of exactly the same species, 

 while the list of important accessories, excepting the micropegmatite and quartz, 

 is common to both. The existing differences in mineralogical and chemical 

 composition are to be explained by the contrasted conditions of crystallization, 

 as well as by a slight acidification of the sill magma. The latter was thrust 

 into a zone of silicious metargillites ; a relatively slight absorption of the 

 invaded rock would lead to the generation of interstitial quartz and micropeg- 

 matite, as in the Moyie and other of the western sills. The significance of these 

 parallels will be noted in the discussion of the latter intrusives. At present 

 it may suffice to observe that the Kintla canyon sill seems to belong to the 

 same eruptive period as the Purcell Lava and that both are probably contempor- 

 aneous with the great sills west of the Yahk river. 



Another sill, fifty feet thick, cuts the Siyeh formation on the eastern slope 

 of the Clarke range. It is well exposed on both sides of Oil creek, about two 

 miles upstream from the derricks at Oil City. The intrusive has split a zone 

 of silicious metargillites at a horizon roughly estimated to be 1,000 feet above 

 the base of the Siyeh. 



The rock is essentially a fine-grained duplicate of the Kintla canyon sill- 

 rock but there is here a considerably greater amount of freely crystallized 

 sodiferous, microperthitic orthoclase, which replaces some of the labradorite 

 and becomes a major constituent. Micropegmatite is an abundant interstitial 

 accessory. The rock is badly altered with the generation of epidote, chlorite, 

 kaolin, sericite, saussurite, and limonite, but it is certain that at least half the 

 volume of the rock was originally composed of bisilicate. Through most of 

 the sill, the same green, idiomorphically developed hornblende which was found 

 in the Kintla canyon sill, is an abundant essential along with the- colourless 

 augite. 



A specimen taken at a point five feet from the upper contact and thus 

 representing the contact-zones, bears no hornblende, but the bisilicate is entirely 

 augite, crystallized, as usual, in apparently two generations. The hornblende, 

 here, as in the other sill, has every evidence of being a primary constituent. 

 It seems to have been able to crystallize only in the interior part of the sill, 

 while augite monopolized the contact zones. These contrasted, augitic and 

 hornblendic, phases of the sill are homologous to the similar phases found in the 

 fifty-foot dike near the summit of the HcGillivray range. This dike has been 

 noted as most probably one feeder of the Purcell Lava flood. The specific 

 gravity of the augitic phase is 3-005; that of the normal hornblende-bearing 

 phase, 3-048. These values further show the similarity of this sill to the 

 Kintla canyon sill (sp. gr., 3-057). 



