REPORT OF THE CHIEF ASTRONOMER. 243 



SESSIONAL PAPER No. 25a 



independent, twinned grains of small size seems certainly determined by vari- 

 ous optical tests. Innumerable, minute grains of zoisite and epidote occur as 

 dust clouding the feldspars, micropegmatitic intergrowths, and even the quartz. 

 Scattered anhedra of magnetite and small crystals of anatase and apatite are 

 rather rare constituents. 



The chemical analysis of this highly metamorphosed quartzite is entered 

 in Col. 3, Table XIV. In the preliminary study of the sill it was considered 

 as probable that the quartzite had been somewhat feldspathized during the 

 metamorphism, but the critical analyses seem hardly to bear out any certain 

 conclusion on that point. The analysis shows that in several respects the 

 metamorphosed rock is intermediate in composition between the granite of 

 the sill and the unaltered quartzite. However, there is a perfectly sharp line of 

 contact between the granite and this metamorphosed zone of the quartzite. The 

 former rock has been in complete fusion j the latter rock still preserves its bedded 

 structure. 



The net result of the foregoing mineralogical and chemical comparisons 

 affords good grounds for believing that the striking similarity of granite and 

 quartzite is really due to a kind of consanguinity; that the igneous rock is 

 due to the fusion of the sediment. 



Comparison with Other Sills in the Purcell Range. — The assimilation theory 

 assumes sufficient heat to perform the work of fusion. It is, hence, an indication 

 of great value that there is some acidification of the respective upper-contact 

 zones in all of eight different sill-outcrops optically studied in the 60-mile stretch 

 from Porthill to Gateway ; yet that this acidification is, in general, in a direct pro- 

 portion to the thickness of the sills. The closely associated Moyie sills A, B, C, 

 and D together have about three times the thickness of any other of the 

 intrusive bodies. Presumably, therefore, the total store of heat in the Moyip 

 group was a local maximum and the capacity for energetic contact-action was 

 there much the largest. As a matter of fact, the Moyie sills are the only sills 

 bearing the truly granitic phase. The other sills are also somewhat more acid 

 at their upper contacts than at their respective lower contacts, but the rock 

 throughout is of gabbroid habit. The acidification in these cases has, as we 

 have seen, led to the development of abundant interstitial and poikilitic quartz, 

 abundant biotite, and less abundant alkaline feldspar in the hornblende-plagio- 

 clase rock. The rock of the acidified zones is here very similar to, if not 

 identical with, the intermediate rock of the Moyie sills. The acidification is 

 relatively slight because these sills have been more rapidly chilled than the 

 huge Moyie complex. This point is based on deductive reasoning but it is no 

 le3s positively in favour of the assimilation theory than the testimony of 

 chemical comparison between the acid zone and the sediments. 



Evidence of Xenoliths. — There is, finally, direct field evidence that the 

 gabbro has actually digested some of the quartzite. Along both the lower and 

 upper contacts and, less often, within the main mass of the sill, fragments of 

 the quartzite are to be found. These rocks have, as a rule, sharp contacts with 



25a — vol. ii — 16i 



