REPORT OF THE CHIEF ASTRONOMER 239 



SESSJONAL PAPER No. 25a 



Flat Position of Quartzite at Epoch of Intrusion. — Since the granophyre- 

 granite zone of sill C is known to have a tolerably constant thickness through- 

 out an exposure of at least three miles along the outcrop, the hypothesis 

 involves the assumption that that sill and the adjacent ones lay much more 

 nearly horizontal at the time of intrusion than they do now. This assumption 

 is favoured by all the pertinent facts determined during field work, though it 

 cannot be claimed that they furnish absolute proof. 



In the first place, it is probable that the majority of the faulting and 

 upturning suffered by the Purcell sedimentary series was brought about at one 

 orogenic period. The intrusive sills are themselves profoundly faulted and 

 their outcrops are repeated by faulting in such a way as to indicate throws of 

 thousands of feet. If this extensive disturbance of the sills had followed their 

 intrusion, which itself followed earlier important dislocations of the intruded 

 sediments, we might reasonably expect that the detailed structures of the twice- 

 faulted sediments would show some evidence of the history. As a matter of fact, 

 the cleavage often fully developed in the quartzites apparently belongs to one 

 orogenic period and to one only. It was developed after the intrusion of the 

 sills, for the gabbro itself is occasionally cleaved with its planes of cleavage 

 parallel to those in the adjacent quartzites. The sediments must, of course, 

 have been slightly disturbed as the intrusive bodies were injected, but true 

 mountain-building seems to have been postponed until long after the solidifica- 

 tion of the magmas. The repetition of sill outcrops by faulting is most easily 

 understood if it be believed that the dips have been greatly increased by the 

 relatively late disturbance. If the strata had been well faulted, tilted, and 

 cleaved before the intrusions took place, the injected bodies should. show much 

 greater irregularity of form than they now actually show; most of them would 

 be in the relation of dikes or chonoliths (injected bodies of irregular form), 

 following faults and other secondary planes of weakness, rather than in the 

 relation of sills following bedding-planes. 



A second argument is to be derived from the fact that sills and dikes of 

 hornblendic gabbro, mineralogically and chemically very similar to these sills 

 of the Yahk and Moyie ranges, cut the Kitchener formation and equivalent 

 Siyeh formation of the McGillivray, Clarke, and Lewis ranges at horizons 

 immediately below the Purcell Lava, and, almost without question, represent 

 feeders or offshoots of the magma represented in the widespread lava flood. 

 That these eastern dikes and sills do thus represent the contemporaneous intru- 

 sive facies of the lava is suggested, as above remarked, not only by the litholo- 

 gical consanguinity, but also by the fact that none of the formations overlying 

 the Purcell Lava horizon, i.e., the Sheppard, Kintla, Gateway, Phillips, Roos- 

 ville, or Moyie, is known to have been cut by dikes or sills which are younger 

 than the older beds of the Kintla formation. Granting, further, the contem- 

 poraneity of all these Purcell mountain-system sills with the (Middle Cam- 

 brian?) Purcell Lava, which is rigidly conformable to the geosynclinal sedi- 

 ments, it follows that the intrusions took place when the strata lay flat and, in 

 the eastern ranges, were covered at the end of Kitchener (Siyeh) time, by the 

 great flows of the extrusive, post-Siyeh lava. 



