90 THE GEOLOGY OP MINNESOTA. 



[Granite. 



No. IB'. GRANITE. 



Duluth. Probably N. W. ^ sec. 34, T. 50-14. 



Mi'!/. A rather fine-grained, reddish to flesh-colored granitic rock composed of 

 quartz, a pinkish feldspar and a little epidote. A number of the feldspar grains are 

 considerably larger than the other constituents and are surrounded by a finer ground- 

 mass of quartz and feldspar, but these larger grains are not sufficiently distinctly 

 marked off from the rest of the rock to give it a porphyritic appearance. 



.]//>. The section shows a granitic groundmass of feldspar and quartz, the 

 former in larger quantity than the latter. In this groundmass, and not very sharply 

 separated from it, are larger areas of feldspar with which quartz is plentifully inter- 

 grown in most beautiful micropegmatyte; the feldspar of the groundmass does not 

 show this feature. Frequently these areas of feldspar contain more than one crystal 

 and occasionally a core which is not filled with quartz, but the feldspar does not 

 show idiomorphic outlines. The feldspar is much altered and is cloudy, reddened 

 and sometimes almost isotropic; the cleavage is poorly developed and good cleavage 

 flakes on which to make determinations of the extinction angles are almost impos- 

 sible to obtain. In some crystals traces of polysynthetic twinning can be observed, 

 but most of the grains are too highly altered to show this. As the feldspar is the 

 only mineral of any importance besides the quartz, the analysis of the whole rock, 

 which is given below, will give a good index of the composition of the feldspar. The 

 amount of soda is very high in comparison with the potash and lime, and a consid- 

 erable part of the feldspar would thus seem to be albite, but anorthoclase, and per- 

 haps some orthoclase, are also probably present. 



Reddish iron oxide is quite common in minute flakes scattered throughout the 

 section. A few small areas of chlorite and of epidote are also present. If the chlorite 

 represents an original ferro-magnesian constituent, nothing can now be said as to 

 what it was. 



Chemical analysis. The following analysis of the whole rock was made by Prof. J. A. Docile, and first 

 published in the tenth annual report, page 204 (Chemical series 73). 



SiO, 75. 7S 



A1 2 8 11.09 



Fe 2 0. 2.09 



CaO .86 



MgO .65 



K a O 1.06 



Na a O iU:i 



HJO l.s-J 



99.78 



Remarks. One of the "red rock" series. See under IB. 



Age. Cabotian of the Keweenawan. i . s. G. 



