442 THE GEOLOGY OF MINNESOTA. 



[Breccia. Huntilite. 



The olivine has quite disappeared, as such, and its spaces are filled with more 





 or less indeterminate ferruginous substances, and by biotite. 



The pyroxenic element is nearly intact, but ferruginous opaque substances have 

 gathered about it, and in its basal fissures. It is orthorhombic, and hence is either 

 enstatite or hypersthene. Its high double refraction seems to indicate hypersthene, 

 but its very slight pleochroism is nearer to that of enstatite. At the same time the 

 acute angle of the optic axes seems to embrace ,,, which stands perpendicular to the 

 face 100. This mineral, in the same slide, having been determined as enstatite by 

 Wadsworth, we prefer to accept his decision provisionally, until better material is at 

 hand. 



One section. 



Age. Cabotian dike in the Animikie. 



Remark. This rock shows to what extent, under favorable conditions of access 

 of heated waters, the massive rocks may be affected by alteration, this being taken 

 from about 730 feet below lake Superior. 



On having made another section, thinner than that examined by Dr. Wadsworth, 

 it is apparent that the pyroxene above is hypersthene. This is indicated by the high 

 double refraction in a section perpendicular to m , which reaches red of the first 

 order, in a section not over, but rather less than, 0.03 millimeter in. thickness. This 

 hypersthene is older than most of the feldspars, and it was frequently encased in 

 augite, at least in a highly doubly refracting rim which is much altered and which has 

 the fibration of a diallagic alteration of augite. This rim, as well as other isolated 

 augites, was generated, for the most part, later than the feldspars, and approximately 

 cotemporary with the zoning of the latter. N. H. w. 



No. 596. BRECCIA (cemented bycalcite). 



"Stamp ore," Silver Islet. 



Ref. Annual Report, x, page 55. 



Meg. The fragments are partly from the slate and partly from the dike rock. 

 There seems to be but little reason for using such rock as stamp ore. 



Mic. The slide consists of the elements of the dike rock, and of the slate. But 

 of the former the feldspars only remain identifiable. Quartz, hematite and chloritic 

 substances have arisen as products of decay, while veinings of calcite also cross the 

 slide. 



One section. 



Age. Vein in Animikie rocks. N. H. w. 



No. 596A. HUNTILITE. 



From the Silver Islet mine. 



Ref. Annual Report, x, page 55; Engineering and Mining Journal, vol. xxvii, page 55, 1879. 



