98 THE GEOLOGY OP MINNESOTA. 



[Gabbro. 



which extinction is 24. In the feldspars the plane of the optic axes is practically 

 in the plane 010, and gives an interference figure indicating the optic normal within 

 the field of the microscope. Comparing this with the " epures " of Michel Levy 

 (Mineraux des Roches, plate VI, the optic normal (,) is found situated between 

 30| and 20; practically at 24. This again indicates labradorite with the propor- 

 tions Ab,An 4 . All methods of examination which have been made of this feldspar, 

 /. e., the original gray feldspar of No. 1,) lead to labradorite. 



The red feldspar in No. 5 was subjected to the same test. Large cubes of fluo- 

 silicate of potash, some of them so rapidly formed as to be partially opaque, mono- 

 clinic crystals of lime and an occasional hexagonal rod of soda, pointed unmistakably 

 to orthoclase in which a part of the normal potash is replaced by soda. This shows 

 a mutual interchange between these feldspars, one giving up a little soda and receiving 

 potash and the other receiving soda in exchange for potash. This indicates a ten- 

 dency on the part of the orthoclase toward the composition of anorthoclase. In 

 general, the red feldspar is nearly or quite opaque, from some alteration, while in 

 immediate contact are the lath-shaped labradorites, with evident cleavage and twin- 

 ning bands. In the interior of the red feldspars are frequent grains of quartz. The 

 labradorites were formed earlier than both the quartz and the orthoclase. Not 

 infrequently a fringe or red border of orthoclase surrounds the labradorites, a kind 

 of reaction rim between the previously formed labradorite and the acidic elements 

 which subsequently enveloped it. Indeed, there is good reason to assume that, in 

 many cases, the red color simply marks such changed labradorites. This reddened 

 feldspar often constitutes a border round the clearer crystals of plagioclase, and in 

 such cases the border, or colored zone, seems to be a continuous part of the clearer, 

 central crystal. This can be explained on the supposition that the crystal is a zonal 

 one, the outer zone being less basic than the inner, and that it has consequently 

 undergone the reddish alteration to which the more acid feldspars of these rocks are 

 so subject. Or, on the other hand, this reddened zone may be simply a peripheral 

 altered portion of a practically homogeneous crystal. However this may be, the 

 reddened feldspar between the clearer crystals appears exactly similar to the simi- 

 larly colored feldspar of the acid granular rocks, which is known to be orthoclase 

 and anorthoclase. In these areas of reddened feldspar is considerable quartz in small 

 grains, and frequently these two minerals have grown together in the form of micro- 

 pegmatyte. Such parts of the section resemble sections of the acid red rocks- 

 granites and augite-syenytes (see Nos. IB, 3). 



The pyroxenic mineral appears much as in No. 1C, but is not so evidently dial- 

 lagic. It is much changed to chlorite and replaced by magnetite. Occasional grains 

 are cut nearly perpendicular to an optic axis, as shown by the interference figure. 



