PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 867 



Quartz-porphyry.] 



Throughout the rock is much fine, granular epidote and a few grains of quartz. 

 One section. 



Age. Dike cutting quartz-porphyry of the Lower Keewatin. N. H. w. 



No. 2237. QUARTZ-PORPHYRY. 



At 670 paces west from the same corner post (i. e., northeast corner of sec. 8, T. 63-9), four rods west of 

 the above dike. 



Ref. Annual Report, xxiv, page 70. 



Meg. The specimen from which the section was made is a gray quartz-porphyry, 

 with the grain of the matrix graduating in size to that of the phenocrysts. The 

 point at which it was collected is in the midst of the general quartz-porphyry mass, 

 and its chief interest consists in its field relations. " At this place the porphyry, so 

 called, presents fragmental characters. It is roughly schistose in a direction E. 25 

 N., and ridged with interrupted finer belts resembling siliceous argillyte. It holds 

 pieces of greenstone and of slaty greenstone, varying in size from ten inches down- 

 ward (rounded) 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 less than a pea in size, but also as 

 large as an inch in diameter, the last being very rare, while other quartzes, as of 

 phenocrysts in quartz-porphyry, are abundantly disseminated. Indeed, 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 quartz, essentially, sufficiently 

 abundant to keep the quartz and orthoclase grains from interlocking, but apparently 

 allowing them to come loosely into contact. * * * In other places near by 

 are other variations in this porphyry. It may hold distinct crystals of orthoclase in 

 abundance, or none at all. It also varies to a fine-grained gray rock with no appar- 

 ent quartz nor feldspar as crystals, but yet on close examination it is seen that fine 

 quartz grains are still present. In other cases it holds vitreous quartz grains 

 surrounded by a mesh of quartz which is like that of the jaspilyte, *. e., very finely 

 granular and interlocking (rock No. 2238). This serves as a matrix for vitreous 

 quartz grains. Such characters are seen in the porphyry ridge that extends north- 

 ward along the west side of the dike represented by rock No. 2236. 



Mic. The phenocrysts are orthoclase and plagioclase, with large bipyramidal 

 quartz. The feldspars are much eaten into by decay, the resulting micro-granulitic 

 substance being calcite (abundant), muscovite, secondary feldspar, epidote, the same 

 feature mentioned already in several instances. Some of the feldspars are wholly 

 obliterated, only their forms remaining, and all of them are more or less changed. 



The matrix consists of calcite, muscovite, epidote, little feldspars and quartzes, 

 generally coarser than the structure seen in the micro-granulitized feldspars. Still, 



