THE KEWEENAW AN. 403 



which have rather a fine g-rain, these two grading into each other. In 

 places the gabbro becomes so feldspathic that it can be correctly spoken of 

 as anorthosite. These anorthosite masses usualh^ weather white, and being 

 more resistant than the more basic gabbro stand out as bare, white, 

 conspicuous knobs. In examining these anorthosite masses, which are 

 beautifully exposed in numerous places on the islands and west and 

 southwest shores of this lake, one finds scattered through them irregular and 

 roundish areas of what appears to be normal gabbro. This grades directly 

 into the anorthosite. Furthermore there are also seen finer-grained facies 

 of the gabbro in small, rounded areas occurring in the midst of the 

 anorthosite and grading into the surrounding anorthosite. It thus appears 

 that the anorthosite grades both into the normal gabbro of coarse grain 

 and also into the normal gabbro of fine grain, thus showing both a 

 mineralogic and textural gradation. The more basic areas which are 

 scattered through the anorthosite range in size from IJ inches in diameter 

 to 4 or 6 inches in diameter. Between these basic masses lies the 

 anorthosite, which makes up the greater portion of the rock, covering 

 much larger areas than are occupied by the basic parts. The basic 

 portions weather more readily than the anorthosite, producing a pitted 

 surface upon the exposures. When disintegration proceeds much farther, 

 the anorthosite is apt to break down into rounded bowlder-like masses. 



In many places, and especially near the northern contact, the gabbro 

 is found to be very friable, and this character seems to be due to a 

 considerable extent to some character of the rock dependent upon its 

 contact with the adjacent formations, for specimens taken farther within 

 the mass were uniformly fresher and harder. 



The exposures normally show a rock of dark color, either dark-reddish 

 brown, or black, varying, as is stated above, to the anorthosite, which is of 

 rather rare occurrence, and has a gray to white color. The other extreme 

 in the variation from the anorthosite is represented by masses consisting 

 essentially of titaniferous magnetite, such as is well developed at Mayhew 

 Lake" and especially at Iron Lake.'' 



The chief components, plagioclase, feldspar, pyroxene, olivine, titan- 

 iferous magnetite, are clearly recognized in hand specimens. With these 



«W. M. Chauvenet, TJ. S. Geol. Survey, manuscript notes. 



t>Geol. and Nat. Hist. Survey of Minnesota, Final Eept., Vol. IV, 1899, p. 489. 



