ARCHEAN GRANITES. 249 



The porpliyritic rocks are most common. Quartz is the usual phenocryst, 

 but it is sometimes accompanied by feldspar. In some of the porphyries 

 the phenocrysts are very abundant, but in others they are very scarce. The 

 quartz phenocrysts range in size from those that are scarcely separable from 

 the quartz of the groundmass up to crystals nearly an inch in diameter. 

 Two characteristic porphyries especially are of common occurrence in the 

 district. One contains a great number of small vitreous-looking quartz 

 phenocrysts; the other usually shows only a few very large phenocrysts. 

 The quartz phenocrysts differ greatly in number in different types of the 

 rock. In some of the rocks only a few are present; in others they occur 

 in great abundance. Moreover, the quartz shows considerable variation in 

 character. Usually it is clear and vitreous; less commonly it has a some- 

 what bluish tinge, but is still vitreous. Very commonly white, opaque, 

 porcelain-like phenocrysts appear, and some are found that are black and 

 opaque. Usually the quartz phenocrysts of the rock occuning at a single 

 exposure are all alike; that is, all are clear and colorless or all are black, 

 but sometimes these are found intermingled. 



Feldspar in white crystals appears also as phenocrysts in the 

 porphyries and, like the quartz, varies much in abundance. The dark 

 phenocrysts seen in these rocks are either mica or hornblende, or 

 occasionally the two together, but as a rule dark phenocrysts are scarce. 



The groundmass of these porphyries gives to them some of their 

 distinctive characters. In some the groundmass is dense and aphanitic; 

 in others it is distinctly granular; in still others it is coarse grained; 

 moreover, there are phases of the groundmass that show all gradations 

 between these different kinds. In some of the porphyries, as has been 

 said, the phenocrysts are practically wanting, and as these become reduced 

 in quantity the rocks gradually change to those which we would call 

 granites; and we find not only changes from porphyries into granites, 

 but gradational phases among the granites, varying from fine-grained 

 microgranites to normal coarse-grained granites. 



The granites and porphyries of Vermilion Lake occasionally include 

 fragments of the iron-bearing Soudan formation, both large and small, as 

 well as fragments of greenstones, but they contain no inclusions of a 

 recognizably sedimentary rock other than those derived from the 

 iron-bearinff formation. 



