MINERALOGY AND PETROLOGY. 977 



Quartz-porphyry.) 



cooling and a gradual crystallization such as to form the granitic structure, but 

 nowhere was a granitic structure observed in this mass, except where it had obviously 

 been regenerated by a later dynamic epoch (No. 2239). Its feldspars are greatly 

 decayed, same as those of graywackes (noted above) and are permeated by calcite, 

 muscovite, epidote, etc. This decay cannot have taken place since the consolidation 

 of the rock, since it is seen uniformly in the rock at all depths and is the same as 

 that often noted in the feldspars of graywackes, and of granites and gneisses resulting 

 from the recrystallization of acid debris (page 942). It is not at the centres of the 

 feldspar grains, but is disseminated uniformly and is much greater than that of the 

 more changeable older greenstones underlying (No. 2227). 



The quartz "phenocrysts" are sometimes an inch in diameter, and are rounded, 

 and are sparsely associated with other fragments, such as greenstone, fine slaty 

 greenstone, and of jaspilyte. 



The feldspar "phenocrysts" are not wholly of orthoclase, but many of them are 

 of some plagioclase, a fact which is not in keeping with the idea of a great magmatic 

 mass which would have attained a stable and identical composition such as would 

 give origin, on cooling, to a single feldspar as first consolidation. 



The rock does not consist wholly of the usual minerals of a quartz-porphyry, 

 but embraces a little biotite, sphene and hornblende (No. 2229). 



There is a very striking resemblance in the amount and kind of alteration of 

 the Lower Keewatin quartz-porphyry (No. 2229) to the debris of the same. This is 

 very noticeable at Vermilion lake, where the pebbles of the Stuntz conglomerate can 

 hardly be distinguished in thin section from the finer debris in which they lie. It is 

 an indication that the elements of the oldest quartz-porphyry had already suffered 

 a period of exposure and alteration analogous to that suffered by the debris of the 

 Stuntz conglomerate. It was often remarked that the general aspect, aside from a 

 difference of schistosity, of the thin sections of the original porphyry was so closely 

 like that of those taken from the matrix of the conglomerate, that they could with 

 difficulty be distinguished. There is, indeed, so far as known, no such thing as a 

 fresh quartz-porphyry in the Archean. There is much fresh granite and dioryte and 

 allied coarsely crystalline rock, but, in the quartz-porphyry referred to, the orthoclase, 

 so far as observed, is much altered and crowded with sericites. These alteration 

 products are uniformly distributed in the orthoclases except where the rock has 

 been metamorphosed; then they are grouped centrally in the manner described for 

 the orthoclase of granite. 



In order to adjust the foregoing facts with others that show that this rock orig- 

 inated under the action of some great force which was widespread and probably of 

 long duration, the hypothesis was entertained that this rock was due to oceanic sed- 



G3 



