512 THE GEOLOGY OP MINNESOTA. 



[Diaboae. 



kaolinized feldspar and some aityitc. In places it shows aggregations of secondary 

 magnetite, with their interstitial portions filled with biotite, which is also an altera- 

 tion product. These forms simulate closely the appearance of a mineral partially 

 destroyed by the molten magma, instead of being what it actually is- a mineral 

 (biotite, holding magnetite) in the process of formation. The augite, when altered, 

 leaves dirty gray cloudy grains and masses resembling loicaj-cnc, while in the midst 

 of some of the quartz, chlorite, viridite, etc., are to be seen dark orange red and 

 yellow secondary granules resembling t!fni(c. They are translucent and anisotropic. 

 Acfimtlife and apatite needles are not uncommon." 



One section examined. 



At/i'. Cabotian. u- s - (i - 



No. 723. DIABASE (n-illt </i/<n-t:: nd lioritli/rixlfj. 



From near the bottom of the perpendicular part of the bluff. Same locality as No. 721. 

 Ref. Annual Report, ix, page 81; Annual Report, x, page 86. 



Mey. A fine-grained, dark, greenish-gray diabase, showing some pyrite and 

 chalcopyrite. 



Mic. The section shows a diabase more altered and finer grained than Nos. 

 721 and 722, but still of the same general typo and from the same rock mass. 



One section examined. 



Aye. Cabotian. r. s. o. 



No. 724. 



One foot lower in the bluff than No. 723. Same locality as No. 721. 

 Ref. Annual Report, ix, page 81; Annual Report, x, page 87. 



Mey. A fine-grained, very dark, greenish-gray diabasic rock. 



Mic, The section shows a rock'similar to. but much more altered than, No. 723. 

 Maynctife is abundant, and is in rods, feather-like and grating-like forms. I'i/ri1c is 

 common. 



One section examined. 



Age. Cabotian. r. s. o. 



No. 725. DIAHASK. 



Six inches lower than No. 724. Same locality as No. 721. 

 Ref. Annual Report, ix, page 81; Annual Report, x, page 87. 



Mey. Similar to, but finer grained than, No. 724. 



Mic. Another fine-grained, much altered diabase, in general similar to No. 724. 

 One section examined. 

 . \</c. Cabotian. 



Ili-nnn-k. This rock represents the liner grained lower part of the large diabase 

 sill which forms the ridge between (UinHint and Loon lakes, and of which Nos. 721 



