2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER IX. 



PUKCELL LAVA FORMATION AND ASSOCIATED INTRTJSIVES. 



INTRODUCTION. 



Many of the higher peaks in the four ranges of the Rocky Moimtain 

 system, as well as in the McGillivray range, owe their special heights 

 to the strength of the Purcell Lava, which is even more resistant to the forces 

 of weathering than the massive Siyeh formation underlying. As above noted, 

 this lava formation is the faithful friend of the stratigrapher throughout 

 eighty miles of the transmontane section at the Eorty-ninth Parallel. Its dis- 

 covery in the McGillivray range is a fact of the first importance, since its 

 presence and relations have removed the last doubt as to the equivalence of the 

 Siyeh formation with the main body of the Kitchener. Therein we have a 

 main link in the correlation of the staple sedimentary rocks occurring in the 

 eastern third (150 miles) of the whole structure-section from the Pacific to the 

 Great Plains. The lava formation thus deserves a somewhat detailed descrip- 

 tion. For convenience certain associated dikes and flows will be treated in the 

 present section, which is to deal with the stratigraphy and petrography of the 

 Purcell formation proper. 



PURCELL LAVA OF THE MCGILLIVRAY RANGE. 



The formation is displayed with unusual perfection in three different areas 

 within the McGillivray range. The most westerly of these occurs at the strong 

 meridional ridge situated about six miles east of the main Yahk river valley 

 at the Boundary line. The great sheet there dips east-northeast at angles 

 varying from 42° to 50°. It also caps the ridge, three to five miles farther 

 eastward, where it reappears in the eastern limb of the broken and pitching 

 syncline at the summit of the range. Here the dip is from 20° to "28° north- 

 westerly. The third area of the lava as mapped is a small one, situated at the 

 edge of the drift-covered flat of the Kootenay river valley, where the dip is 35° 

 to the northeast and represents the attitude appropriate to the eastern limb of 

 the broad anticline that forms the main structural element in the Kootenay 

 slope of the McGillivray range. 



These localities were those at which the writer first encountered the 

 formation. Since it has its maximum known thickness in the McGillivray 

 division of the Purcell mountain system, the formation has been called the 

 'Purcell Lava.' 



One or more of the important vents must have been situated not many 

 miles from the western line of outcrops in this range. The lava seems never 



207 



