288 THE GEOLOGY OP MINNESOTA. 



[Gabbro. 



Chemical Analysis. The following analysis of this rock was made by Prof. C. F. Sidener, and first 

 published in the Thirteenth Annual Report, page 100 (No. 163): 



SiO 2 81 - 86 



A1 2 3 9 - 87 



FeO 



CaO - -* 6 



MgO 



K 2 - 45 



Na 2 O 



H 2 



1-43 



Total - 10 .29 



In comparison with the other analyses of quartzytes, both altered and unaltered, 

 published by Bayley (U. S. Geol. Survey, Bulletin cix) from Pigeon point, this rock 

 is seen to be especially low in the amount of alkalies present. 



Age. Animikie. u. s. G. 



No. 263. GABBRO (with lurrnblende ). 



N. E. % sec. 30, T. 64-6 E.; east side of Wauswaugoning bay. Lies between sedimentary sheets, in the form 

 of a sill. It forms the straight high coast which makes a sharp angle in Wauswaugoning bay. When followed 

 toward Birch island it becomes overlain by layers of quartzyte which are curved and twisted as if by heat from 

 below. Angular pieces from this quartzyte are enclosed in No. 263, changing the weathering color and the com- 

 position in spots, reminding one of the " red rock" embraced in thegabbro at Duluth. 



Ref. Annual Report, ix, page 63; Annual Report, x, page 142; Bulletin ii, page 81, plate VII, figure 2, 

 plate IX, figure 1; American Association for the Advancement of Science, xxx, page 164. 



Meg. Has the appearance of No. 5, at Duluth, showing a striated feldspar, quartz, 

 magnetite and a red feldspathic or siliceous ingredient irregularly disseminated. The 

 rock is therefore spotted with red, although in general a dark basic eruptive. Some 

 of the dark mineral has the elongated habit of hornblende. Small amounts of pyrite 

 and of calcite are visible. 



Mic. Dr. Wadsworth has carefully and fully described this rock and illus- 

 trated the same by three figures. (Figure 2, plate VII, and figures 1 and 2, plate IX; 

 Bulletin ii, page 81, 1887.) His description is as follows: 



" A dark grayish and reddish brown crystalline rock, composed macroscopically 

 of reddish and grayish feldspar, pyroxene, hornblende, biotite, magnetite, quartz and 

 calcite. 



"The section has its pyroxene largely altered to biotite, hornblende and viridite. 

 Much secondary feldspar of a plagioclase type occurs, but the primary feldspar 

 appears to have been largely, if not entirely, replaced by the graphic or eozoon quartz 

 and fibrous kaolinized feldspathic material. Secondary quartz in irregular grains, 

 besides the graphic form, is quite abundant, while both the quartz and the feldspar 

 are filled with microliths (apatite ?). 



" Plate VII, figure 2, shows the structure of one of the altered diallage crystals. 

 The diallage is in the form of a core surrounded and penetrated by a greenish viridite 

 which traverses the irregular cracks of the diallage. The viridite passes on its outer 



