160 THE GEOLOGY OP MINNESOTA. 



[(Jniiiit... 



idiomorphic, and all gradations can be found from idiomorphic to granular plagio- 

 clases; and with these granular plagioclases and between the lath-shaped crystals 

 are grains of augite. Thus the augite is not in the usual plates common to diabases, 

 and in that respect this rock has gabbroid characters. It would seem that after the 

 usual lath-shaped plagioclases were formed there was still a considerable amount of 

 feldspathic material in the magma which crystallized at about the same time, or 

 perhaps slightly previous to the augite. A few areas of magnetite and chlorite are 

 present which perhaps represent old olivines. The augite, which is clear and 

 colorless when fresh, has altered in places to a more or less fibrous greenish substance 

 which often can be distinguished as hornblende. There are also some small areas of 

 yellowish or brownish pleochroic hornblende which may be the greenish grains cut 

 in. another direction, but these yellowish grains are not so distinctly fibrous as most 

 of the green ones. All of this hornblende is probably secondary after the augite. 



One section. 



A///'. Cabotian. u. s. G. 



No. 74. GRANITE. (lied.) 



Rock next east of No. 73, extending for some distance. 



Ref. Annual Report, ix, page 22; Annual Report, xiii, page 100 (No. 153), 102; Bulletin viii, page xxxiii. 



A brick-red, granitic rock of fine grain, composed of red feldspar and 

 quartz. A few cleavage faces of feldspar one-tenth of an inch across are seen scat- 

 tered through the rock, but the usual grains are much smaller than this. 



Mi/'. Rock composed of feldspar and ijmniz, about two-thirds feldspar, which 

 is of the usual red, almost opaque, variety common to the acid rocks of the Cabotian. 

 It .rarely shows any effect on polarized light. Judging from the analysis of the whole 

 rock, given below, we would expect the feldspar to be largely anorthoclase. Some of 

 the larger feldspars show evidence of an original zonal structure. The quartz is in 

 rather large areas embracing the feldspars poikilitically, and often penetrating and 

 replacing in part the feldspar grains. The rock furnishes a good example of the 

 coarser micropoikilitic structure. The feldspar enclosed in the quartz has frequently 

 a tendency to an idiomorphic development. A few areas of a dull brown, granular, 

 isotropic substance, with dark borders, are present. These may represent an old 

 ferro-magnesian constituent. Scattered abundantly through the rock in grains and 

 specks and stains is Jicnin/ifi', and the presence of this mineral is also clearly evident 

 from the analysis. 



One section. 



