438 THE GEOLOGY OP MINNESOTA. 



[Copper. Diabase. 



Remark. This mineral appears to be the same as that noted by J. D.Whitney* 

 at the Copper falls and Douglass Houghton mines on Keweenaw point, where it occurs 

 implanted on quartz and copper, and which gave on analysis the following com- 

 position : 



Silica, 65.88 



Alumina, 17.35 

 Oxide of iron, .57 



Potash and soda. 16.20 (by loss) 



100.00 



He made no direct determination of the alkalies, not having been able to obtain 

 enough of the substance for such analysis. 



r In still another section it appears that in crystals cut perpen- 



/ dicular to n v there are four sectors which, while extinguishing 

 approximately in unison have partial extinctions somewhat out of 

 unison, and also exhibit an imperfect microcline structure. The 

 mineral has too small an optic angle for microcline. The figure 

 FIG. 29. ADULARIA, attached shows the grain cut perpendicular to n v . There is no 



OUT PERPENDICU- 

 LAR TO p. evidence of twinning on the Baveno plan, as the whole section 



has n s in the same direction. These variations of extinction must be attributed to 

 irregularities of development. 



Samples in the General Museum are Museum Nos. 1900, 1389 and 1370, where 

 it is implanted on epidote, lining irregular cavities. N. H. w. 



No. 584. COPPER. 



Minong mine, Isle Royale. 



Ref. Annual Report, x, page 54. 



Battered thin scales, from half an inch to two inches in greatest dimension. 

 Found in the mines about the large masses worked by the ancients. N. H. w. 



No. 585. DIABASE (with olivine). 



Minong mine, Isle Royale. 



Ref, Annual Report, x, page 54; Popular Science Monthly, vol. xix, page 001. 



In the form of rounded beach-stones, used as hammers by the ancient miners. 

 Probably transported from the north mainland, where they are common. They 

 evidently got their form by the action of the waves on the beach. They are not 

 withed. They are found in the debris about the old mines. 



Three sections. N. H. w. 



*Rcport on the Lake Superior Land District, part ii, p. 102, 1851. 



