PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 681 



Muscovadyte. Gabbro. Biderite.] 



Mic. Feldspar and diallage practically compose this rock, rather evenly and 

 finely granular, and cote mpo vary in origin. It is but rarely that can be seen a 

 feldspar encroaching on the boundary of a diallage. Yet occasionally a very small 

 round feldspar is wholly embraced in a diallage, and, rice versa, small diallages are 

 in the feldspars. The diallage shows an interesting instance of basal twinning. 

 Several lamellae are visible crossing the grain. Generally the diallagic lamination 

 parallel to 100 and this twinning are visible only in separate grains, giving, on first 

 view, the impression of separate and distinct minerals, but on careful search it can 

 be seen occasionally that they both occur in the same grain. They stand nearly at 

 right angles to each other. The lamellation parallel to 100 is also affected by the 

 interposition of some polarizing mineral, which renders the separation planes quite 

 light when the lamellae themselves are dark. 



The last mineral to form was maynetite, which is scattered in round small grains 

 sparsely, and in one instance in form of a large mass, which embraces small grains 

 both of feldspar and of the pyroxene. One section. 



Age. Cabotian (modified Keewatin). 



Remark. This is evidently petrographically a portion of the gabbro, but has a bed- 

 ding and dip resembling that of sedimentation. The reader may compare the chapter 

 on structural geology for a discussion of the origin of the gabbro and of muscovadyte. 



N. H. w. 

 No. 1288. GABBRO. (Iron-bearing. ) 



Near the same place as the last, but further west ; Mayhew lake. 

 Ref. Annual Report, xvi, page 120. 



Meg. Dark and heavy with iron ore. 



Mir. OH cine, magnetite, plagioclase essentially compose this rock. But there is 

 frequently a rim of brown hornblende between the magnetite and the plagioclase. 

 There seems to have been a powerful corrosion of the olivines and plagioclases, 

 leaving only remnants of their former sizes, sharp and angular, crescent shaped or 

 excavated in curvilinear contours, and the spaces filled by later magnetite. One section. 



Remark. This is another instance of the later date of the iron ore in the gabbro 

 rocks of the state. This subject is mentioned in connection with the description of 

 the rocks at Duluth (No. 1, etc.). N. H. w. 



No. 1289. SIDKRITE (iii r/lfifujy matrix). 



Boulder of the gray rock belonging near the bottom of the Animikie. 

 Ref. Annual Report, xvi, page 120. 



Meg. Gray, massive, finely crystalline, coated on weathered surfaces with a film 

 of limonite, on the fractured surfaces with glistening cleavages. 



Mic. The rock consists almost entirely of highly doubly refracting grains, which 

 are seldom so crowded that they do not show a tendency to idiomorphic rhombic out- 

 lines, and which, in form and cleavage, as well as in comparative refractive index, are 



