206 THE GEOLOGY OP MINNESOTA. 



[Granite. Apobsidian . 



No. 134. GRANITE. (Red. ) 



Third island below Beaver bay. 



Ref. Annual Report, ix, pages 34, 35; Annual Report, x, page 141; Annual Report, xiii, pages 100 (No. 157), 

 103; Proceedings American Association for the Advancement of Science, vol. xxx, page 164; Bulletin ii, page 127; 

 Bulletin, viii, page xxxiii. 



A medium-grained granitic rock composed essentially of red feldspar and 

 quartz. In places it has whitish blotches, as if from the decay of the feldspar. 



Mir. An ordinary example of the granular red rocks of the Cabotian, composed 

 of <i(niz and/M/.s-^/y; the latter is probably mostly orthoclase, but is reddened and 

 almost opaque, and has almost no effect on polarized light. Mti</n>i/fe occurs in 

 small amount. In places the quartz and feldspar have intergrown to form micro- 

 pegmatyte. 



One section. 



< 'lie in ii-iil . I uti/i/xin. The following analysis was made by Prof. J. A. Dodge, and was h'rst published in the 

 Thirteenth Annual Report, page 100 (No. 157). 



SiO 2 73.28 



A1 2 O 3 11.83 



Pe 2 O 3 4.61 



FeO .50 



CaO 1.04 



MgO .36 



K 2 O 4.50 



Na 2 O 1.66 



H 2 O 1.82 



Total 99.66 



Age. Cabotian; red-rock series. u. s. G. 



No. 135. APOBSIDIA:N 



Opposite the fifth island east of Beaver Bay. Forms a bluff twenty to sixty feet high. 

 Ref. Annual Report, ix, page 34. 



Meg. A reddish-brown, compact, aphanitic rock. Throughout the rock are 

 minute, rarely as much as one-sixteenth of an inch in diameter, rounded spots of a 

 whitish material. Frequently these spots have a centre like the mass of the rock, 

 and thus consist only of a narrow ring of the white material. 



Mir. The mass of the rock is essentially like No. 127, and need not be rede- 

 scribed. Throughout the section are small circular areas which are much clearer 

 than the mass of the rock. These areas contain many minute black specks which 

 seem to be magnetite; in some cases these are arranged in roughly concentric lines. 

 The rest of each circular area is composed of a transparent fibrous mineral radially 

 arranged. In the thinner part of the section these fibres have almost no effect on 

 polarized light. Where the section is thicker an indistinct, fibrous, radial, or fan- 

 like, structure is seen when viewed in polarized light. The extinction is practically 

 parallel to the elongation of the fibres, which are optically positive. 



The exact nature of this mineral in the circular areas is not known. Sometimes 

 the quartz of the mass of the rock runs into these circular areas. It seems quite 



