498 THE GEOLOGY OF MINNESOTA. 



[Gabbro. 



plane is at right angles to the elongation and the cleavage. The mineral thus agrees 

 with epidote, although colorless epidote with such a marked elongation has not been 

 noticed in the other rocks of this region. 



Two sections examined. 



Age. Cabotian. '. s. G. 



No. 698. GABBRO. (Oraiwlith-.) 



Same locality as No. 095. "This is distributed through No. (595 in rounded lumps like some masses of 

 No. 696. Some masses of No. 698 are very large, where they occur on the north side of the swamp." 

 Ref. Annual Report, x, page 81; Bulletin ii, pages 93, 94; Bulletin vi, page 136. 



A fine-grained, granular, yellowish-gray rock, looking somewhat like a 

 gray sandstone. It appears to be composed of feldspar and a dark mineral. 



Mic. The section shows a fine-grained, almost completely granular, aggregate 

 of plagioclase and pyroxene with some magnetite. Small flakes of biotite, usually 

 in connection with the magnetite, are seen, and there are some indistinct greenish and 

 yellowish alteration products. The pyroxene frequently shows somewhat rounded 

 outlines and part of it is clearly earlier than the feldspar. 



The feldspar is sometimes twinned according to the albite law, but is frequently 

 untwinned. The twinned and untwinned grains appear to be of the same species. 

 One section cut nearly normal to a gave an extinction angle of 65, while another 

 almost exactly normal to this axis gave 60. Tn sections cut normal to 010 equal 

 extinction angles as high as 32 occur. A cleavage fragment parallel to 010 gave an 

 extinction angle of 19. All these determinations point to a plagioclase near acid 

 1/ilirndorUe. The feldspar is commonly fresh, but shows a little alteration to kaolin'tlr. 



There appear to be two species of pyroxene present in the section, although their 

 separation is not always certain. One is of a very light cinnamon color and closely 

 resembles in habit and color the ordinary uuyifv of the associated rocks. It frequently 

 has the dark plate-like inclusions so common in augite. Very fine polysynthetic 

 twinning is rather common, and a typical cross section was found showing the 

 prismatic cleavages and the parting parallel to 100 and also parallel to 010. This 

 section in converged polarized light gave an eye, and the optic plane was determined 

 as normal to 100. This section showed fine polysynthetic twinning parallel to Hid. 

 The mineral is augite with the d'uilhtyic parting common to the augite of the 

 gabbros. 



The other pyroxene appears to be orthorhombic. Tt has a fine columnar 

 structure (or twinning) parallel to the vertical axis, and some sections show a very 

 slight pleochroi sin, changing from a very faint greenish to a weak tint of pinkish. 

 This mineral is probably enstatite, or perhaps hypersthene. Some of its sections 

 show bands crossing the columnar structure. These bands are sometimes of a 



