262 THE GEOLOGY OF MINNESOTA. 



[Basalt. 



Mic. A fine diabase with plates of augite which include a number of feldspar 

 laths. The spaces between the augite plates, which are roughly circular in form, is 

 occupied by feldspar laths in a dark confused mass composed of magnetite, hematite 

 and greenish alteration products. No olivine is now present, but there may have 

 been some original olivine. This dark material between the augite plates perhaps 

 represents an original glassy matrix. Some of the augite plates are comparatively 

 free of feldspar laths. The general structure of the rock is similar to that repre- 

 sented on plate IX of the "Copper-Bearing Rocks of Lake Superior" (U. S. Geol. 

 Survey, Mon. v). 



One section. 



Age. Cabotian. u. s. G. 



No. 217. BASALT. ( AmygdaMdal. ) 



In the midst of a red beach, extending eastward from the easterly of the two little points west of Brule 

 river, are occasional exposures of this rock which resembles the Two Harbor rock. The specimen collected is 

 from a conspicuous outcrop within the bay between two streams. 



Kef. Annual Report, ix, page 55. 



MI-IJ. A very fine-grained, brownish-red rock which has irregular streaks and 

 blotches of a greenish color. There are a few rounded amygdules of quartz and 

 chalcedony, and also some of a soft, green chloritic material. 



Mic. The section shows plagioclase microliths in a background composed of 

 confused greenish alteration products, magnetite and an opaque brown substance. 

 This brown substance, which is bright red in reflected light, makes up the main mass 

 of the rock; it is supposed to represent original glassy material. Throughout the 

 section are more transparent areas where this brown substance is lacking. These 

 are, evidently, the greenish blotches seen in the hand specimen. They contain, 

 besides the plagioclase microliths, magnetite and greenish alteration products, small, 

 brightly-polarizing grains which are probably augite. 



One section. 



Age. Cabotian. u. s. G. 



No. 218. BASALT (with olivine). 



From the point near Brul4 river off which lie the principal islands. 

 Ref. Annual Report, ix, pages 55, 56. 



Meg. Brown, conchoidally fracturing rock, very fine grained, with small quartz- 

 lined geodes, weathering rough-angular, and black when not under friction. Back 

 from the beach friction, but on the beach, old weathered surfaces are brick red. 

 Similar to No. 217 and to No. 176. 



Mir. A fine-grained basalt. The feldspars are lath shaped, and sometimes 

 grouped in a radial manner, each consisting usually of two or three albite twins. 

 They do not conspicuously cut any of the other minerals, in the manner of a typical 



