182 THE GEOLOGY OP MINNESOTA. 



[Mass of anorthosyte. Diabase. 



()/ id lie is present in two grains of considerable size, one of which shows an 

 optic axis and one bisectrix in the field, and the other a quick, dim, dark cross as an 

 interference figure, which maneuvers like the figure of the optic normal. Hence 

 the section is parallel to 001. Its fissures are filled with serpentinous (?) growths of 

 a yellowish brown color. Cleavage not apparent, but irregular fissures. There is, 

 however, an irregular, scant distribution of minute, elongated inclusions or 

 impurities, parallel to which extinction takes place, thus revealing the orientation, 

 and the cleavage parallel to 010. These grains, taken together, are tolerably fresh, 

 and would furnish good sections on which to measure the double refraction. 



One section examined. 



Age. Cabotian. Beaver Bay diabase. N. H. w. 



No. 115A. A MASS OP ANORTHOSYTE ROCK. 



Embraced in No. 115 at Splitrock point. 



Ref. Annual Report, ix, page 30; Bulletin, ii, page 104. 



Meg. A mass of granular, well preserved plagioclase feldspar, with a very little 

 of the ferro-magnesian silicates. 



Mir. Albite and pericline macles are common. There is also a banded 

 extinction which crosses some of the larger grains which is neither albite nor 

 pericline. These bands are not sharp, but fade into each other, and sometimes they 

 experience a progressive, shadow which runs over several in conjunction. These 

 bands are also curved, as if resulting from a zonal growth in the crystal. Many of 

 the grains are cut singularly perpendicular to an optic axis. The section being 

 rather thick this indefiniteness in the outlines of the bands may be due to super- 

 position, and their curvature to a distortion caused by the rock No. 115 during the 

 period of transport as a foreign mass. The generally shattered condition of the 

 feldspars points to the latter cause. Indeed, the curvature in one case is accompanied 

 by a breaking, which shows it is attributable to some cause later than the formation 

 of the bands. 



Augite and maym-titc are rare, the former in small grains that are very fresh, 

 showing not a particle of secondary matter. There are also greenish chloritic masses 

 that have been produced by an alteration of some mineral, perhaps from the broken 

 feldspar. 



One section examined. 



Age. Cabotian. N. H. w. 



No. 11 5B. DIABASE. 



Phase of No. 115, at the mouth of Splitrock river. 

 Ref. Annual Report, ix, page 30. 



A finer-grained, vein-like belt runs through No. 115 and seems to be associated 

 with very coarse plagioclases, as if derived from some of the Cabotian anorthosytes. 



N. H. W. 



