558 THE GEOLOGY OP MINNESOTA. 



[Anorthosyte. Diabase. 



Mic. The section shows a rock with a very fine-grained background, which is 

 composed of minute grains of quartz, cloudy feldspar, chlorite, epidote, magnetite and 

 hematite. Throughout the section are larger, more or less angular grains of quartz 

 and feldspar, and also many sub-angular and rounded grains of a colorless mineral. 

 The grains of this mineral are usually stained with hematite along the borders and 

 sometimes the hematite stain and an incipient cloudiness penetrate the grains. 

 This mineral is biaxial, positive, has a double refraction a little weaker than quartz 

 and an index of refraction higher than quartz, although the appearance of a higher 

 index of refraction may be due to the red borders of the grains. The species of this 

 mineral was not determined, but it may be hetilandite. 



One section. 



Age. Cabotian. u. s. G. 



No. 818. ANORTHOSYTE. 



East side of Splitrock point, north shore of lake Superior. One of many pieces embraced in the dark 

 diabase on the east side of the point. 



Ref. Annual Report, x, page 115; Bulletin ii, page 95. 



Meg. A coarse-grained rock composed almost entirely of plagioclase which is 

 generally gray in color. There are small areas of reddish feldspar and along the 

 weathered surface and joint planes the feldspar is entirely reddened. 



Mic. The section shows plagioclase, more or less altered and stained reddish, 

 and some earthy and greenish alteration products. 



One poor section. 



Age. Cabotian. u. s. G. 



No. 819. DIABASE (with olivine). 



"Prom the top of the high hill back of Silver creek; the highest hill near the mouth of the creek, yet on 

 the west side, rising 415 feet above the lake, though a mile further back it rises perhaps 150 feet higher." 

 Ref. Annual Report, x, pages 64, 115, 116; Bulletin ii, page 98. 



Meg. A rather coarse-grained diabase of 'a yellowish-black color. 



Mic. M. E. Wadsworth's description of this rock is as follows:* 

 " Is a grayish-brown rock having a resinous lustre. The section is composed of 

 lath-shaped plagioclases cutting through irregular masses of augite, also olivine grains 

 and magnetite. The feldspars are quite clear, but are in places somewhat kaolinized, 

 and contain a viriditic product as well as the remains of inclusions of the globulitic 

 base. Trains of vapor cavities extend in the feldspar and are continued with and 

 through the augite. . This mineral is quite clear, of a brownish color, and shows its 

 characteristic irregular cleavage. 



"The olivine is partially inclosed in the augite and is much fissured, with the 

 border of the fissures formed by greenish, yellowish and brownish serpentine, which 



"Bulletin U, p. 98. 



