54 IF. 8. Bayley — Rocks of Pigeon Point, Minnesota. 



Art. VI. — A Quartz-Keratophyre from Pigeon Point and 

 Irving 's Augite- Syenites • by W. S. Bayley. 



(Published by permission of the Director of the U. S. Geological Survey.) 



(I.) Introductory. 



One of the most striking features in the geology of Pigeon 

 Point,* Minnesota, is the occurrence there of a bright red rock 

 along the borders of the large mass of olivine-gabbro, which 

 forms the main portion of the point. This rock is best seen 

 along the south or Lake Superior side of the point near its 

 eastern extremity, where its brilliant color when moistened by 

 the water, forms a beautiful contrast to the dark gray of the 

 gabbro with which it is in contact. 



The first mention of the rock was made by Dr. JSforwood,f 

 Assistant IT. S. Geologist, in 1851, who described it as a red- 

 dish colored syenitic rock,, containing but a small amount of 

 quartz. About thirty years later Professor K". H. Winch ell, 

 of the Minnesota State Survey, saw a red rock:]: associated 

 with gabbro near the extremity of Pigeon Point, and a rock§ 

 red with orthoclase, from its north shore about a mile from its 

 eastern extremity, which latter he mentions as having probably 

 originated by the fusion and recrystallization of the sedi- 

 mentary beds through which the gabbro cuts. A microscop- 

 ical examination of this rock has very recently been made by 

 Dr. M. E. Wadsworth,|| who regards it as an altered phase of 

 some eruptive, the original nature of which he is unable to de- 

 cide from the single section at his command. Professor P. D. 

 Irving^f in 1881, described a red rock from Brick Island, one 

 of the smaller of the Lucille group of islands, about a mile 

 south of Pigeon Point. He says : " Its thin section reveals a 

 rock very close to those red rocks of the Keweenaw Series 

 which I have described under the names of augite-syenite and 

 granitic porphyry." On Pigeon Point also Professor Irving 

 found a red rock which " resembles in every particular the 

 rock from Brick Island." 



Similar red rocks have been observed by several geologists 

 at various other places in the Lake Superior region, but no 

 careful study has been made of any one of them. 



*For exact location see this Journal. June, 1888, p. 388. 



f Report of a Geological^Survey of Wisconsin, Iowa and Minnesota. By D. D. 

 Owen, U. S. Geologist, Philad., 1852, p. 399. 



|Geol. and Nat. "Hist. Survey of Minnesota for 1880, p. 70. 



§Ib. for 1881, p. 57. 



|| G eol. and Nat. Hist. Survey of Minnesota, Bulletin 2. Preliminary De- 

 scription of the Peridotytes, Gabbros. Diabases, etc., of Minnesota, 1887, p. 81. 



1 Copper Bearing Rocks, etc. Monog. V., U. S. G. S., 1883, p. 369. 



