)4 THE GEOLOGY OP MINNESOTA. 



[Diabase. Granite. 



No. 2. DIABASE. 



Duluth. Dike running N. 30 W., separating No. 1A from No. 3. 

 Ref. Annual Report, ix, page 11; Annual Report, x, page 41. 



Mr;/. A dark gray, compact rock of rather fine grain. Composed of plagioclase, 

 a greenish mineral looking like an alteration product of augite and magnetite. The 

 feldspar weathers whitish to pinkish. 



Mir. A rather fine grained diabase, more or less altered. The ophitic structure 

 is very distinct, and the plagioclase is usually little altered. In the angles between 

 the plagioclase, and often penetrating it along the cleavage cracks and twinning planes, 

 is a dirty-greenish alteration product of the augite. In some places fresh augite still 

 remains; it has a purplish tinge similar to that of a large proportion of the diabasic 

 augite of these rocks. Mrif/itrfitr is quite abundant, but is confined largely to the areas 

 of altered augite; it is in irregular grains, and especially in rod-like bodies, which 

 often exist in small groups, in each of which the rods lie parallel. Sometimes one 

 rod will have several smaller ones branching off from it at right angles, thus 

 resembling the skeleton crystals of magnetite found in certain glassy rocks.* But in 

 the rock under consideration, the rods are several times larger than these skeleton 

 crystals. 



One section examined. 



Age. Probably a dike of Manitou. 



Remarks. "A finer-grained rock of t 1 ^ came general character as No. 1A, and 

 running in the form of a dike, N. 30 W., and separating No. 1A from No. 3."f 



No. 3. GRANITE. (Fine, red, liornblt-iidic.) 



Duluth-. Fifth avenue east and Seventh street, occurs as patches and veins in No. 1. 

 Ref. Annual Report ix, pages 11, 12, 17. Annual Report, x, pages 41, 140. American Association for the 

 Advancement of Science, vol. xxx, page 163, 1882. 



Met/. This is a fine grained, granitic rock of a reddish-brown color. The hand 

 sample varies somewhat in shade, one side being brown and the other reddish, the 

 change occurring gradually along the centre of the specimen. The mass of the rock 

 is quite fine grained, but seems to be made of reddish feldspar and a darker horn- 

 blendic mineral in ill-defined grains, blotches, and some distinct crystals. A few 

 large, but dark colored feldspar crystals two to three millimeters long are seen. These 

 and the larger crystals of hornblende give the rock a sub-porphyritic appearance. 

 The feldspars do not show striation. A weathered surface is lighter red and filled 

 with small cavities due to the more rapid decay of the hornblende. 



Mir. In thin section the rock is seen to be sparingly porphyritic with feldspars. 

 While these are quite small, the largest one in the section being less than two milli- 



* Compart- H . Ro.senbusch, Mikrosk. Physiog., Bd. I, Taf . H, figure 5, 1892. 

 fN. H. WINCHKI.I.. Ninth Annual Report, p. 11. 



