STRUCTURAL GEOLOGY. 41 



Oldest quartz-porphyry . ] 



potassium was stored up in the Archean sedimentary rocks, in the form of alkaline 

 silicates, where it remains to the present. 



The oldest quartz-porphyry. This oceanic source may be applied to the earliest 

 quartz-porphyry. Not enough examination has been given to this quartz-porphyry 

 to warrant positive conclusions as to its source, but a few facts may be recalled here 

 which seem to be in consonance with this idea. 



The oldest quartz-porphyry known is that which is in the Lower Keewatin, 

 forming a large mass southwestwardly from Snowbank lake. It is described in 

 general terms in the twenty-fourth annual report (page 69). The following leading 

 facts may be mentioned : 



1. It lies between two greenstones, the older one being the massive portion of 

 the Kawishiwin and the later one the fragmental portion, and is perhaps 2,000 feet 

 across. 



2. Between it and the older, however, is a curious conglomerate, made up 

 entirely of greenstone debris, but with a few fragments of jaspilyte. 



3. It does not cut, so far as seen, the older greenstone, and at no place has any 

 quartz-porphyry been known to cut a greenstone of that date. 



4. It has supplied many large fragments to the later greenstone, and its finer 

 debris is scattered abundantly through the same (plate Z, vol. iv). 



5. This quartz-porphyry varies. It does not generally show any sedimentary 

 structure, but occasionally it presents characters that are usually accredited to 

 oceanic origin. For instance, it becomes roughly schistose, in places, and ridged 

 with interrupted finer belts resembling siliceous argillyte. It holds pieces of green- 

 stone and of slaty greenstone, varying in size from ten inches (rounded) downward 

 to half an inch, also pieces of jaspilyte and rounded quartz. The slaty greenstone 

 is like argillyte and runs usually with the structure, standing on edge. The rock 

 contains much quartz in grains usually less than a pea in size, but also as large as an 

 inch in diameter, the last being very rare, while other quartzes as if phenocrysts of 

 quartz-porphyry, are abundantly disseminated. 



6. In some places the bulk of the whole rock consists of more or less rounded 

 fragments of orthoclase and quartz lying in a pellucid matrix which appears to be 

 essentially quartz of a kind like the so-called chalcedonic quartz seen in the 

 jaspilyte, mingled with equally fine feldspar. 



7. This mass of quartz-porphyry occupies about the position of the great 

 jaspilyte lodes, /. e., it is subsequent to the massive greenstone of the Kawishiwin. 



It may be inferred, from the fact that it succeeds immediately after the green- 

 stone conglomerate lying on the agglomerated condition of the Kawishiwin, that, if 

 it resulted from oceanic precipitation, it was formed at a time when the ocean was 



