ARCHEAN GRANITES. 251 



nature it is impossible on exposures consisting of intrusive rocks alone to 

 trace out the extent and character of this folding. Where the intrusives 

 are associated with younger sedimentary rocks the folding is clearly shown, 

 and is described on pages 288-291. 



STRUCTURAL FEATURES AND METAMORPHISM. 



As a rule the intrusives are broken up by a series of joints, whicK are 

 sometimes so close together that the rocks break up into small more or less 

 regular rhombs. The very general distribution of the jointing seems to 

 point to the fact that the rocks were not buried very deep, for had they 

 been so buried it is probable that they would have acted more as viscous 

 materials, and that schistosity would have been developed, instead of the 

 stress being relieved by the formation of joints, as is the case. As a rule, 

 however, the rocks composing the blocks between the fractures show no or 

 very slight indications of schi.stosity. On these jointed rocks it is not 

 uncommon to find a few shearing planes along which schistosity has been 

 developed. Moreover, the rock is sometimes schistose along the small 

 fracture planes themselves. Reference has casually been made to the 

 difficulty sometimes experienced, even under favorable circumstances, of 

 distinguishing between these igneous intrusives and some of the sedimentary 

 rocks derived directly from them. When schistosity has been developed 

 in the igneous rock, even though it be imperfectly developed, the difficulty 

 is much increased, whether the test applied be that of macroscopic or 

 microscopic examination, or, as in most cases, the two combined. 



Some very interesting occurrences of pseudo-conglomeratic rocks 

 derived from these acid rocks b^" orogenic movement may be seen at 

 Vermilion Lake. One of the best cases is shown on the flat island lying 

 just south of Ely Island, in the NE. J of the NE. J of sec. 30, T. 62 N., 

 R. 15 W. This island is composed chiefly of rhyolite-porphyry, with fine 

 felsitic groundmass, having large quartz phenocrysts scattered through it. 

 This porphyry is intersected by two sets of partings, which vary from big 

 joints down to very minute parting planes. One of these sets of fractures 

 strikes N. 16° E. and the otlier N. 80° W. These planes of fracture 

 separate the rock into numberless more or less regular rhombs. The large 

 joints, a foot or more distant from one another, break the rock into large 

 rhombs, which in their turn are subdivided into a great number of smaller 



