ACID INTEUSIVES. 191 



BIOTITE-GRANITE. 



The biotite-granites vary in color from light reddish-brown to dark- 

 gray and greenish rocks, and in grain from fine to coarse. The structure 

 ordinarily is that of a normal granite. In some of them the micropeg- 

 matitic intergrowth of quartz and feldspar may be observed in small quan- 

 tity. In others this forms the characteristic part of the rock, and these 

 rocks may be properly termed micropegmatitic granites. In most of the 

 sections the usual constituents in ordinary proportions occur. 



In all the rocks the main mass of the quartz forms irregular grains, 

 molded on the other constituents. Sometimes round areas of quartz are 

 included in the feldspars. The quartz contains very commonly, and usually 

 in great quantities, liquid inclusions with dancing as well as stationary 

 bubbles. 



The feldspar is nearly always of two kinds — orthoclase and plagio- 

 clase. Microcline was also observed, but in neglectable quantity. These 

 feldspars in the great majority of slides show fairly good rectangular out- 

 lines, and in some cases these are strikingly well developed. In some slides 

 the plagioclase is observed in rectangular crystals and the orthoclase is 

 found in large irregular plates which are molded on the plagioclase, show- 

 ing conclusively their relative age. The plagioclase is finely twinned 

 according to the albite law, and also in some slides exhibits pericline twin- 

 ning. A case was observed in which two crystals, one showing albite 

 twinning, the other both albite and pericline twinning, were grown together 

 so as to con-espond to the Carlsbad twins of orthoclase. The plagioclase 

 gives low extinction angles, which show it to be rather acid. The amount 

 of plagioclase in some of the sections — for example, in those of the numer- 

 ous small dikes cutting the schists near Norway portage in sec. 15, T. 42 N., 

 R. 31 W., and the one cutting the gabbro at the SE. corner of sec. 22, 

 T. 42 N., R. 31 W. — is very large, denoting an increase in soda and lime 

 and indicating a relationship to the diorites. The geological relations are 

 not such, however, as to enable this connection to be shown in default of 

 chemical analyses. 



The orthoclase is for the most part untwinned, or else shows simple 

 Carlsbad twinning. In some sections the feldspars are quite fresh, but in 

 others they are seen to be opaque, porcelain-like, and in still others the 



