496 THE GEOLOGY OF MINNESOTA. 



[Cumberlandyte. Gabbro. 



Mir. The section shows that the minerals other .than the magnetite are more 

 abundant than would be supposed by an examination of the hand specimen. This 

 mineral acts as a sponge inclosing the others. Its outlines are not sharply angular, but 

 are usually curved. The mineral next in abundance is olicinc, which occurs in more or 

 less rounded grains. It has altered some to a greenish-yellow sc>'j><-f<i>c, and at 

 times there is a narrow, green, serpentine band where the olivine borders on the 

 feldspar. The feldspar is apparently a basic plagioclase, although its species was 

 not determined; it crystallized at a later date than the magnetite and olivine. One 

 large grain of augite is seen, including rounded olivines. The augite contains numer- 

 ous dark, plate-like inclusions. 



Biotite is common in small flakes associated with magnetite, and it is almost 

 exclusively confined to areas of magnetite bordered by plagioclase. The biotite 

 sometimes, but not always, is arranged in plates radiating out from the magnetite. 

 Some of the magnetite areas are bordered by a very narrow, continuous, brightly 

 polarizing band which appeal's to be biotite. 



Associated with the magnetite are a few small areas of an opaque mineral which 

 has a metallic yellow color in reflected light. It is more yellow than ordinary pyrite, 

 and is perhaps chalcopyrite. One section examined. 



Age. Cabotian. 



Remarks. This specimen represents the titaniferous magnetite of the great 

 gabbro mass of northeastern Minnesota. 



An analysis by Prof. J. A. Dodge is as follows: 



SiO 2 20.90 



TiO 2 2.23 



A1 2 O, 1.75 



Fe 8 O 4 70.29 



FeO 2.01 



CaO trace 



MgO - 2.63 



P - none 



99.81 

 Metallic iron 52.46 



In the above analysis the amount of FeO required to form FeO-TiO 2 is computed and the rest of the iron 

 is given as Fe 3 O 4 . 



The following rock samples are from this same locality (or near it) and they are 



all very similar to No. 695: Nos. 1288, 1291, 1292, 791W, 792W,796W,442H, 177G, 179G. 



A determination of nickel in No. 177G, by Mr. F. F. Sharpless, gave' .41 of 



one per cent. u. s. o. 



No. 696. GABBRO. 



Same locality as No. 095. Embraced as nodules in No. 695. 



Ref. Annual Report, x, pages 81, 82; Bulletin ii, pages 69-71; Bulletin vi, page 135. 



Met/. A light-gray rock of coarse grain, composed largely of gray to flesh-colored 

 feldspar and some magnetite. 



