218 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



led to similar results except that the untwinned feldspar, which is present in 

 large amount, has been determined as orthoclase, probably bearing soda. The 

 other constituents, both primary and secondary, are the same as in the Oil 

 creek and Kintla canyon sills. Augite is as important as the hornblende and 

 micropegmatite is again a prominent accessory. 



On account of the striking predominance of the bisilicates compared to 

 the feldspar, this rock can scarcely be called a true diorite. Its systematic 

 position is better recognised by calling it a somewhat acidified, abnormal gabbro. 

 It constitutes both the sill and the dike at the Swift Current Pass. The specific 

 gravity of a typical specimen from the dike is 3-055, a value almost identical 

 with those found for the Kintla canyon and Oil creek sills. 



RELATION OF THE SILLS AND DIKES TO THE PURCELL EXTRUSIVE. 



The Kintla canyon sill and dikes crop out twelve miles or more to the west 

 of the Oil creek sill, while the Swift Current Pass locality is about twenty miles 

 from either of the other two. Thus, at each of three widely distributed localities, 

 we have a constant association of an extrusive basaltic lava resting on the top 

 bed of the Siyeh formation and an intrusive gabbroid sill-rock thrust into the 

 Siyeh itself. Though the vertical dikes, either feeding the visible sills or appar- 

 ently independent of them, are relatively numerous in the Siyeh, no dike or sill 

 has yet been observed in the admirably exposed Sheppard formation. These facts, 

 of themselves, afford good presumptive evidence that the Purcell Lava proper is 

 genetically connected with the sills and dikes. This conclusion is amply corro- 

 borated by microscopic study, which, even in face of the great alteration of all 

 the rocks, goes to show an essential identity of the principal minerals respec- 

 tively occurring in intrusive and extrusive. 



The main difference of chemical composition consists in the presence of the 

 silica and potash represented in the primary orthoclase, micropegmatite, and 

 quartz which are so abundant in the sills while entirely absent in the surface 

 flows. The marked rarity of secondary quartz in the altered lava seems to 

 indicate that these acid materials were not originally dissolved in the glassy 

 base of the amygdaloids. Neither quartz nor orthoclase were appreciable con- 

 stituents of the noncrystalline phases of the lava. It appears, therefore, highly 

 probable that they enter into the composition of the intrusives because of a 

 special modification of the magma when in purely intrusive relation. The 

 simplest cause for the appearance of the acid constituents is to be found in 

 the absorption of a small amount of the invaded metargillites along the contact- 

 surfaces, and this the writer believes to be the true cause. If the sills had been 

 considerably thicker, their greater heat-supply would have led to yet more 

 pronounced acidification; as in the case of the Moyie sills (described in the 

 following chapter), a facies of granitic acidity might have been developed, 

 preferably at the top of such a sill. 



On the other hand, the amygdaloid was not so acidified because of the 

 manifest speed with which the extrusive magma must have passed through the 



