410 THE GEOLOGY OP MINNESOTA, 



[Diabase. Aporhyolyte. Basalt. 



Mic. The feldspar shows its plagioclastic character in many striations of albite 

 twinning. The augite is both earlier and later than the feldspars. The earlier 

 crystals, which are the most abundant, are small, and interfere with the outlines of 

 the feldspars, but the later are fresher and embrace several of the feldspars ophitic- 

 ally. But two or three augites of this later date can be seen in the two sections 

 examined. They are much larger than the earlier augites. The olivine is for the 

 most part altered to a yellowish brown mineral which represents boiclingite, although 

 most of it is not perfectly developed as bowlingite. Olivine was generated before 

 the feldspar, but later than the augite. Magnetite in small amount and a small 

 quantity of the magmatic residuum may be seen in the slide. 



Two sections. 



Age. Cabotian. N. H. w. 



No. 537. DIABASE. (Luster-mottled.) 



West side of Cow's Tongue point; S. W. J sec. 10, T. 61-2 E.; same as No. 211. 

 Bef. Annual Report, x, page 43. 



Meg. A fine-grained, brownish diabasic rock showing, especially on weathered 

 surfaces, luster-mottling. 



Mic. The section shows small feldspar laths embedded in augite plates and in 

 a dark opaque mass which is almost entirely hematite and magnetite. The iron ores 

 fill in the spaces between the augite plates. 



One section. 



Age. Cabotian. u. s. G. 



No. 538. APORHYOLYTE. 



The first rock in the bay east of Cow's Tongue point. 

 Ref. Annual Report, x, page 43. 



Meg. This rock, both macroscopically and microscopically, is similar to No. 

 212, which is from the same locality. 

 One section. 



Age. Cabotian. u. s. G. 



No. 539. BASALT. 



Short distance west of the mouth of Brul<5 river (see No. 220). 

 Ref. Annual Report, ix, page 56; Annual Report, x, page 43. 



Meg. Brown, rather fine grained, with small irregular shaped mottlings of 

 green. 



Mic. The interstitial matter is a brown devitrified glass, which embraces now 

 the feldspars and the augites, as well as certain altered ferruginous crystals that 

 were probably olivine originally. It also contains many crystalliths and opaque 

 grains, the latter probably of magnetite. The green spots are formed by irregular 



