PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 321 



Granite. Dioryte.] 



Mic. The rather thick section only permits the determination of the essential 

 minerals present, which are quartz, feldspar, chlorite and hornblende. 

 One section. 

 Age. Archean (igneous). N. H. w. 



No. 331. GRANITE (with hornblende ). 



"The rock of the country at Basswood lake, taken from an island two miles northwest of the eastern 

 extremity." Perhaps in sec. 34, T. 65-9 W. 

 Ref. Annual Report, ix, page 88. 



Meg. A gray granite of medium grain, composed of quartz, white feldspar 

 which rarely shows twinning strias and hornblende. 



Mic. The section is composed of quartz, feldspar, hornblende and a little epidote. 

 The quartz shows undulatory extinction. The feldspar is apparently orthoclase, 

 microcline and plagioclase which seems to be near oligoclase. 



One section. 



Age. Archean. u. s. G. 



No. 332. GRANITE (witli hornblende). 



"Near the portage landing, west end, in a low exposure; a fine chloritic (?) gneiss, the bedded structure 

 sloping south at a high angle." Bassimenan (Basswood lake); west end of portage from the main lake to a narrow 

 arm; sec. 5, T. 64-10 W. 



Ref. Annual Report, ix, page 88. 



Meg. Fine-grained, gray granite, composed of quartz, white feldspar and 

 hornblende. 



Mic. The most noticeable feature of the section is the large crystals of feldspar, 

 which, however, are not idiomorphic. These feldspars are in a groundmass of finer 

 grained quartz and feldspar. The feldspar of the rock is frequently considerably 

 altered, but much of it is comparatively fresh. Some of it appears to be orthoclase, 

 but much of it is plagioclase with low extinction angles probably oligoclase. One 

 grain, which was cut almost perpendicular to the positive bisectrix, gave an 

 extinction on the cleavage of about 4; this indicates oligoclase or andesine-aligoclase. 

 (M. Fouque, Bulletin de la Societe Mineralogique de France, tome xvii, 1894.) The 

 large feldspars frequently show a zonal structure. There are some small green 

 hornblendes in the slide; also a few small sphenes and a little epidote. 



One section. 



Age. Archean (igneous). u. s. o. 



No. 333. DIORYTE. 



Same place as No. 332. 



Ref. Annual Report, ix, page 88. 



Meg. The abundance of hornblende in this rock gives it a dark aspect. 



Mic. This only differs from No. 332, so far as can be seen, in the relative 



amount of hornblende it contains. The sphenes are sometimes large for microscopic 

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