810 THE GEOLOGY OP MINNESOTA. 



[Quartz schist. Diabase. 



mesh of a spongy appearance is filled with a reciprocal mesh of this greenish chloritic 

 material, but most usually the greenish material predominates largely over the rusty. 

 These two, and a little hornblende, and numerous sub-angular quartz grains, compose 

 the rock. One section. 



Age. "Upper Huronian," i. e., Taconic. N. H. w. 



No. 1940. QUARTZ SCHIST. 



South of the Aurora mine, Gogebic iron range, Michigan. Contact of the granite. 

 Ref. Annual Report, xxii, page 15. 



Meg. Siliceous, slaty. 



Mic. Interlocking quartzes, with a few feldspars (microcline), compose the most 

 of this rock, but between the quartzes, running in more or less continuous sheets, 

 and sometimes not in sheets, is an actinolitic schist which gives a prevalent structure 

 to the rock. As an inclusion in the granite, or as a schist at an intrusive granite 

 contact, it is perfectly explicable, but as the bottom of the upper iron-bearing series 

 at the place where it occurs it would be an anomaly. One section. 



Age. Archean (Keewatin). 



Remark. The bottom of the iron-bearing series at the Aurora mine is a rather 

 crumbling sandstone, which forms the foot-wall of the mine (or runs through the 

 mine east and west), which is but a few feet thick. The hard, grayish-green, siliceous 

 slates lying to the south of the mine, having a thickness of several hundred feet 

 (Twenty-sixth Annual Report, page 58), belong to the Keewatin; and it is on these 

 slates that the granite forms an intrusive contact, as in many places along the 

 Mesabi range in Minnesota. Still, these green slates have been included frequently 

 in the Penokee-Gogebic series. N. H. w. 



No. 1942. DIABASE. 



Short Line park, St.' Paul and Duluth railroad. At the east end of the first cut in the gabbro. 

 Ref. Annual Report, xxi, page 15. 



Meg. Some of the most coarsely crystalline portions of the "gabbro" as seen 

 at this place. Evidently ophitic. 



Mic. The augite, which formerly was in very large crystals, has been weathered 

 to a uralite, and then almost wholly chloritized, but the ophitic structure is perfectly 

 preserved. One section. 



Age. Cabotian (Beaver Bay diabase). N. H. w. 



No. 1949. DIABASE. (Amygdaloiddl.) 



From a detached amygdaloidal mass, lying on the ancient flood plain of the St. Louis, between the depot 

 and the river at Cloquet. 



Ref. Annual Report, xxii, page 15. 



Meg. Coarsely crystalline, amygdaloidal, and spotted with red, as if by orthoclase 

 derived from contact on the elastics as at Duluth, 



