22G THE GEOLOGY OP MINNESOTA. 



[Granite. Diabase. 



cleavage faces. The second is considerably finer grained and redder; it is composed 

 of pink and white feldspars and smaller grains of black minerals. A few long acic- 

 ular feldspars are also seen. 



Mic. The section from the first hand sample shows a coarse diabase. The feld- 

 spars are mostly kaolinized; one of the fresher ones gave a negative bisectrix almost 

 exactly perpendicular and an extinction angle of 61, indicating lalradorite. The 

 feldspars are usually in short lath-shaped forms, and frequently several of these are 

 grouped together with their long axes parallel. The tutyite is brownish, is in plates 

 of considerable size, later than the feldspars, and is altering to an opaque mass com- 

 posed largely of magnetite and Jtcinatitc. The section from the second hand specimen 

 shows a similar rock, with the feldspars more nearly allotriomorphic, much altered 

 and reddened, and with less but more highly altered augite. 



Three sections. 



Chemical analysis. The following analysis was made by professors J. A. Dodge and O. F. Sidener: 



SiO 2 50.86 



A1 2 O 3 15.72 



Fe 2 O 3 - . 9.77 



FeO 2.48 



CaO 10.52 



MgO 3.55 



Na 2 O 3.89 



K 2 O .90 



H,O - 2.53 



Total 100.22 



Age. Manitou (?) u. s. G. 



No. 157. GRANITE. 



Near the western side of the broad, shallow bay on sec. 30, T. 57-6 W. Forms a high bluff for ten rods, 

 and is terminated by a dike. (See No. 636.) 



Ref. Annual Report, ix, page 40; Annual Report, x, page 64. 



Mr//. A medium-grained granitic rock, composed of quartz and pinkish feldspar. 

 Passing through the specimens are some small streaks of a fine-grained brown 

 material which contains minute quartz grains. 



Mic. The section shows quartz, cloudy felki>ur. -tnai/netitc and hematite. 



One section, which is too thick for study. 



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



No. 158. DIABASE. ( 'Amygdaloidal. ) 



The rock tirst west of Little Marais; trap and amygdaloid, the latter having saponite (?) as a coating for 

 the other amygdaloidal minerals, which are calcite, stilbite and apparently scolecite, as well as heulandite. 

 Ref. Annual Report, ix, page 41. 



Meg. There are two hand samples. One is a fine-grained, dark-brown rock, 

 looking like a diabase. It is permeated by a green substance, and is not amygdal- 

 oidal. The other sample is a fine-grained, almost aphanitic brown rock, containing 



