21g B. KOTO. 



The rest of the region represented in the section is entirely 

 composed of old nnd yoiing granites, which appear successively 

 aloner the lines of fault after the manner of ruin-marbles. Sometimes 

 aplitic dykes traverse the rocks irrespective of the nature of the granite. 

 The hornblende-granite assumes at times a gneissose structure in the 

 usual manner, as is indicated on the west end of the section G-H. From 

 Nakano a quartz-bearing augite-andesite covers the eroded edge of the 

 underlying rock. Near Shimo- Kawabe (z), fieiuiis of a hornblende-biotite 

 gneiss of the lower member of the Takanuki series are enclosed within 

 the mass of hornblende-biotite granite of schistose structure, and the 

 whole is traversed by coarse pegmatic dykes (Fig. 5, Plate XXIV). 



V. Petrography of the Laurentian Rocks. 



Generally speaking, the granites of the extensive region of Abu- 

 kuma may be classed in two categories ; the first group is a rather 

 greyish-looking, medium-grained variety, in whi(;h the long prisms of 

 hornblende (8mm.), and the broad, hexagonal scales of biotite could be 

 easily recognized within the admixture of the white ingredients of the 

 rock. It is hornblende-granite. To the second group belong those 

 common, coarse-granular granites of a lighter colour, built up of 

 a flesh-coloured orthoclase, and a grey, vitreous, allotriomorphic quartz, 

 mixed with black mica. 



These two groups differ greatly in many respects ; the grey rock 

 contains, as is usually the case, more of the plagioclases when com- 

 pared with the light-coloured rock, although none of the last named 

 rock is free from the presence of the striped feldspars. The most 

 common rock of this region is undoubtedly hor)il)lende-granite. It 

 may be seen all over the district wherever exposures of granites are to be 



