766 THE GEOLOGY OP MINNESOTA. 



[Gabbro. Magnetite. Hypersthene. 



Mic. The associated minerals constitute the most of the slide. Besides quartz 

 and green hornblende there is much microcline and rusty chlorite(?) or fibrous actino- 

 lite. One section. 



Age. Archean. N. H. w. 



No. 1710. GABBRO (with fiyperstJiene and bastite). 



Sec. 23, T. 61-12, Spellman's, north side of Birch lake. ' Associated with the iron ore about 200 feet from 

 the granite. 



Ref. Annual Report, xxi, page 156. 



Meg. Rather coarse grained, pyroxenic. 



Mir. The slide is principally occupied by bastite, an alteration product probably 

 of bronzite or enstatite. It has parallel extinction, but its axial plane is perpendicular 

 to the evident cleavage. It might easily be mistaken for an orthorhombic pyroxene. 

 It has low refraction and low double refraction. Mingled with bastite is a pyroxene 

 having higher refractive index, and two systems of rectangular cleavage, with one of 

 which its extinction is parallel, which is apparently hijpersthene. The slide also 

 contains biotite, labradorite, magnetite, and a multiple-twinned colorless amphibole 

 having high double refraction and n e nearer parallel with its elongation, apparently 

 grttnerite. The magnetite is in small grains distributed through the pyroxenes. 



Age. Cabotian. 



Remark. In another section this rock appears largely hornblendic, twinned on 

 the face 100, apparently grunerite. N. H. w. 



No. 1711. MAGNETITE. (Ore.) 



From the same place as No. 1710. 

 Ref. Annual Report, xxi, page 156. 



Meg. Black ore, but with impurities. 



Mic. The appearance is that of a gabbro gradually eaten into and replaced by 

 nitttjnetite in all directions, leaving only remnants of the original minerals, which 

 remnants are bounded by curvilinear sides, with sharply angular corners. The 

 minerals are labradorite, diallage( ?) broini hornblende. One section. 



Age. Probably Keewatin jaspilyte modified by the gabbro. 



Remark. The ore encountered at this pit shows another phase of the ore known 

 about a mile further east forming a ridge a short distance inland from the shore 

 (Nos. 960 and 1138). This rock is illustrated by figure 12 of plate III. N. H. w. 



No. 1712. HYPERSTHENE. 



Same place as the last. 



Ref. Annual Report, xxi, page 156. (Compare No. 960.) 



Meg. Appearing hornblendic. 



