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upon it. Besides these small segregations of feldspar there are 

 irregular dike-like masses of similar, light-colored rock and a few 

 fairly distinct dikes, all of small size and varying from one-half 

 inch to a foot in width. These dikes are rather fine grained and 

 in thin section show the following characters. The texture is 

 usually micropegmatitic and one section is composed of about 

 60 per cent feldspar, 30 per cent quartz, 10 per cent hornblende, 

 and a little magnetite and hematite. The feldspar is chiefly 

 orthoclase with a little albite and the rock is a granite. Another 

 section contains very little hornblende and a little epidote, the 

 rock being composed almost entirely of feldspar in the proportions 

 of 65 parts orthoclase to 35 parts plagioclase. This rock is a 

 syenite grading toward a monzonite. Still another section is 

 from a dike which might be regarded as a monzonite. It contains 

 a little enstatite, epidote, and titanite, while the greater portion 

 of the rock is feldspar and in the proportions of about 66 per cent 

 albite and oligoclase and 34 per cent orthoclase. A fourth section 

 is from an augite-syenite dike in which the orthoclase makes up 

 75 per cent and the sodic variety 25 per cent of the feldspar. There 

 are a good many small augite crystals and the micropegmatitic 

 texture is well developed. A section of a dike from near "Two 

 Mountain" Island is also an augite-syenite. 



The most interesting dikes in the region are those occurring 

 on Flat Rock Portage near the south end of Lake Nipigon. At 

 this point a large mass of rock, described by Coleman as a sill of 

 epi-basalt or fine-grained diabase-porphyrite, is cut by a pegmatite 

 dike varying in width from three inches to one foot. Where the 

 diabase has suffered columnar jointing the fissures have filled 

 with acid rock similar to the flesh-colored or pink dikes described 

 above (Fig. 4) . The surface of this sill is flat and fine grained, and 

 when the glacier passed over it interesting chatter-marks were 

 left. The pegmatite dike is medium coarse grained, flesh colored, 

 and appears to be composed very largely of feldspar. Under the 

 microscope the pegmatitic intergrowth of various minerals and the 

 graphic intergrowth of quartz and feldspar are well developed. 

 The rock consists of quartz in proportion of 10 per cent, epidote 

 10 per cent, and sodium-calcium feldspar 80 per cent. The 



