REPORT OF THE CHIEF ASTRONOMER 



219 



SESSIONAL PAPER No. 25a 



dike fissures to form the highly fluid and hence widespread floods of lava. 

 In its rapid mounting there was not time enough for the basic magma to 

 dissolve an appreciable amount from the walls of the fissures. The Purcell 

 Lava is, in this view, to be considered as representing the pure, original magma 

 that, at the end of Siyeh time, underlay the Rocky Mountain and Purcell 

 mountain system at the Forty-ninth Parallel. Some of the feeding dikes are 

 composed of the same material, chemically considered, while others, like the 

 sills, are made of the magma which has been enriched in silica and potash by 

 slight but appreciable assimilation of the invaded quartzitic and metargillitic 

 strata.* The uniformity of the extrusive lava through the ninety miles of 

 distance between the Swift Current Pass and summit of the IVIcGillivray range, 

 is matched by the uniformity in the lithology of the intrusive bodies wherever 

 discovered in the Clarke and Lewis ranges. 



The further correlation of these sills, dikes and flows with the huge sills 

 of the Moyie and Yahk ranges will be discussed in chapter X. 



SUMMARY. 



The variations in thickness, field-habit and associations of the Purcell Lava 

 may be conveniently shown in the form of the following table: — 



Purcell Range. 



Galton Range. 



Clarke Range. 



Lewis Range (Swift 

 Current Pass). 



Local 20-foot flow of rhy- 







35' of amygdaloid in 



olite about 50 feet 







Sheppard and 40' of 



above /. 







amygdaloid in Kintla. 



/'. 65', amygdaloid. 



d. 60', amygdaloid. 



260', amygdaloid. 



18', massive, amygdal- 



e. 4" argillite. 



c. 40', coarse breccia. 





oidal flow. 



d. 150', amygdaloid. 



b. 1:00', amygdaloid with 





40', ropy flow passing 



c. 200', porphyry. 



phenocrysts. 





below into pillcw lava . 



h. 10', compact lava. 



a. 90', non-v esicular 







a. 40', "aa" lava. 



porphyry. 







Total lava, 465'. 



Total lava, 390'. 



Total lava, 260'. 



Total lava, 58'. 



Dikes and sills cutting 





Dikes and sills cutting 



Dikes and sill cutting 



Kitchener (Siyeh) im- 





Siyeh immediately 



Siyeh imm ediately 



mediately below. 





below. 



below. 



Locally, 220' below a, a 









50-foot flow of amygda- 









loid. 









The lavas are everywhere thoroughly conformable to the overlying and 

 underlying sediments. 



Excepting the local rhyolite the lavas are petrographically similar and 

 belong to the basaltic family. The more unusual characters include the local 



* It is conceivable that the local rhyolite flow overlying the main sheet of Purcell 

 lava, represents a product of differentiation following the acidification of a large, 

 though invisible body of the gabbroid magma. 



