W. S. Bayley — Spotted Rocks from Minnesota. 389 



bright red drusy granite, whose relations to the gabbro have 

 not yet been satisfactorily determined. To the south of these 

 eruptive rocks, on the Lake Superior side of the point; is a 

 narrow strip of slates and quartzites, which dip to the southeast 

 at an angle of 15°-20°, and are cut by numerous diabase dykes. 

 According to Irving* these slates and quartzites belong to Hunt's 

 Animike series and are Huronian in age. Upon their contact 

 with the eruptives these fragmental rocks lose all traces of 

 sedimentary origin. Under the microscope they are seen 

 to have undergone extreme metamorphism. Whether this 

 consists in a mere recrystallization of the material already 

 existing in them, or in a recrystallization with the addition of 

 new substance derived from the eruptive rocks, or in a com- 

 plete solution of their fragments in the latter must be left for 

 further study to decide. 



The object of the present paper is to describe certain pecu- 

 liar spots noted on the quartzites in a few localities and to dis- 

 cuss briefly their origin 



The quartzites in their unaltered forms comprise evenly 

 bedded light gray, pinkish and black varieties. These are all 

 very compact and hard, and have the vitreous appearance 

 characteristic of indurated quartzites. They are cut by joint 

 cracks running in a north and south direction and standing 

 nearly vertical. The sides of these cracks are usually covered 

 with little quartz crystals. Under the microscope the darker 

 varieties are seen to consist mainly of round and angular pieces 

 of quartz, in a groun.dmass of interlocking silica, which under 

 crossed nicols is resolved into small areas optically continuous 

 with the original grains to which they are attached. f De- 

 composed orthoclase, colored red by minute plates of hematite 

 and iron hydroxide, a very little plagioclase, sometimes fresh 

 but more frequently much altered, little flakes of brown bio- 

 tite, chlorite, and grains of magnetite and earthy iron minerals 

 constitute the balance of the rock. In the lighter varieties 

 the plagioclase is more abundant and the chlorite much less so. 

 In the pinkish varieties reddened orthoclase and plagioclase 

 make up about half of the entire rock. 



In certain restricted areas on the south shore of the point, 

 notably in the western half of section 25 and in the southeast 

 quarter of section 32, curious circular spots are developed on 

 the surface of the quartzite. These spots vary in size from less 

 than a quarter of an inch to over two inches in diameter. On a 

 weathered surface they appear as slight depressions surrounded 

 by a raised rim of a lightish brick -red color. Their distribution 



* This Journal, xxxiv, 1887, p. 262. 



f Cf. Irving and Van Hise : Bulletin of the IT. S. G-eol. Survey, No. 8, Wash- 

 ington, 1884. 



