46 THE GEOLOGY OP MINNESOTA. 



[The Grand Portage graywacke. 



t 



T. 58-17, and doubtfully on the shores of Gunflint lake. Boulder masses have been 

 found, however, at one or two intermediate points, which are supposed to have been 

 derived from this stratum. The thickness of this limestone does not probably exceed 

 twenty feet, varying from place to place. It is essentially only the bottom phase of 

 the black slates. Tt exhibits a fragments! structure, except when it has been sub- 

 jected to the alteration incident to the formation of the iron ore deposits. 



The upper or slaty member, with its siliceous variations, constitutes the bulk of 

 the Animikie, and is probably several thousand feet in thickness. It has been more 

 fully studied in the region of Gunflint lake, where it presents abundant outcrops in 

 the perpendicular cliffs facing toward the north. Dr. Grant has there divided it into 

 two parts: the lower part is composed largely of black slates, often very fissile and 

 apparently carbonaceous, the lower portion being flinty, having a total thickness of 

 1,050 feet, of which about 100 feet represent the diabase sills; the upper part, or 

 graywacke slate member, more firm, often a quartzyte, having a total thickness of 

 1,900 feet, of which about 250 feet are composed of diabase sills. 



The Grand Portage graywacke. In the Indian Reservation at Grand Portage is 

 a more fragile member of the Animikie. It outcrops at various places on the Grand 

 Portage trail, especially toward the western end of the trail. This rock is greenish, 

 gritty, rough and unevenly bedded, having a coarse conchoidal manner of disintegrating 

 under frost and sun. It has suffered great denudation. It probably once rose to 

 the summits of the great dikes that form prominent hill ranges crossing the country, 

 but has been removed, leaving those dikes standing with sheer walls rising from 50 feet 

 to 150 feet above the talus. This graywacke is supposed to overlie the quartzyte and 

 slate of the black slate member, but its extent and stratigraphic position have not 

 been satisfactorily established. 



The summit of the Animikie has never been seen so as to be identified. Whether 

 the fragmentals of the uppei'most, graywacke member, probably represented by the 

 Grand Portage graywacke, gave place suddenly to the next higher fragmental stratum, 

 which is the Puckwunge conglomerate, the fragmental base of the Keweenawan, 

 is not known. But as there was a period of great disturbance which witnessed 

 the upheaval and the profound metamorphism, as well as the fusion of the Ani- 

 mikie before the accumulation of that conglomerate, it is reasonable to suppose 

 that there were other rocks formed before the commencement of the conglom- 

 erate. The facts that are mentioned in the chapters devoted to the Duluth 

 and the Pigeon River plates, and referred to in connection with the Lake County 

 plate, show that there was more or less of eruptive activity prior to that con- 

 glomerate, and hence that there were probably agitated seas and coarse rapid 

 detrital accumulation. These significant facts are as follows: (1) An amygda- 



