OUTLINE OF THIS MONOGRAPH. XXXI 



lenses form a lenticular deposit iu the voleauics ne;ir tlie top of the formation. They are insignifi- 

 cant in quantity. 



Under the Bone Lake crystalline schists there are included rocks of completely crystalline 

 character, but which by field and microscopical study have been connected with the voloanics and 

 are considered to have been derived from rocks similar iu nature to them. 



The rocks composing the Hemlock formation are little likely, owing to their somber color, to 

 be much used for building or ornamental purposes. Thej' offer, however, an inexhaustible supply of 

 the best quality of road-building material. 



Chapter V treats of the Upper Hurouiau series. This series is connected in the northern part 

 of the area described with the Upper Marquette series of the adjoining Marquette district, and is 

 considered to correspond stratigraphically to the Upper Marquette. Owing to lack of exposures and 

 to the intricacy of folding, the series could not be subdivided. It covers a great area surrounding 

 the Hemlock formation, and extends beyond the limits of the map. The exposures are scanty. It 

 influences the topography only in a very general way, being for the most part heavily drift covered. 

 Its thickness could not be estimated. The rocks of this series wrap around the subjacent Lower 

 Huronian series. The line between them is undulatory. The indentations in the Lower Huronian 

 represent minor cross synclines, and the protuberances represent minor cross anticlines. The most 

 prominent fold of the series is known as the Crystal Falls syncliue. The strike of the axis of this 

 syncline is in general to the south of west, and pitches in the same direction. The syncline is not 

 simple, but has minor rolls, as shown in various exposures. This folding has been productive 

 of extensive reibungsbreccias. The folding occurred immediately preceding the deposit of the 

 Keweenawan series in other parts of the Lake Superior region. The Upper Huronian is penetrated 

 by intrusive rocks of acid, intermediate, and basic composition. The rocks constituting the series 

 may be divided into those of sedimentary and those of igneous origin. The sedimentary rocks are 

 graywackes, ferruginous graywackes, micaceous, carbonaceous, and ferruginous clay slates and 

 their crystalline derivatives, and thinly laminated cherty siderite-slate, ferruginous chert, and iron 

 ores. In two places rocks of conglomeratic nature occur. The extensive folding which the series has 

 undergone, coupled with the intrusions of the igneous rocks, has produced crystalline schists from the 

 muds and grits. These are extensively developed in the southern portion of the district iu the vicinity 

 of tbe Paint and Michigamme rivers. The igneous rocks which have penetrated the Upper Huronian 

 subsequent to the folding which affected it are not described under the series. Interlaniinated with 

 the crystalline schists there are, however, certain rocks, now perfectly crystalline hornblende- 

 gneisses, which are presumed to have resulted from the metamorphism of either basic intrusive sheets 

 or interbedded flows. 



The economic development of the district followed that of the adjoining Menominee district. 

 The exploited ore deposits occur near Amasa, and in the vicinity of Crystal Falls. The ore is hematite 

 and limonite. The grade is non-Bessemer. The ore is associated with white and reddish chert, and 

 this formation lies between carbonaceous slates. The ore bodies in general pitch to the west at 

 varying angles, which correspond to the pitch of the axes of the synclines iu which they occur. These 

 minor folds in turn correspond to the western pitch of the main Crystal Falls synclinorium. The ore 

 bodies are concentrates iu synclinal troughs, as described by Van Hlse for other Hurouian districts. 

 The mining is now for the most part undergound, and is carried on in open stopes. The greatest 

 shipment of ore from the area, including the Lower Huronian Mansfield mine, was 586,970 tons in 

 1892. The total shipment for 1898 reached 325,814 long tons. 



Chaptbr VI treats of the intrusives. There is here included a varied assortment of rocks, 

 exhibiting in common intrusive relations to the sedimentary and igneous rocks. The term "intrusive" 

 is not to be interpreted as synonymous with the "dike rocks" of some authors. These rocks are 

 never found to penetrate the Cambrian rocks, and have not been affected by the folding which meta- 

 morphosed the Upper Huronian sediments. They are presumed to be of Keweenawan age. The intru- 

 sives have in some cases been injected along the axes of the folds, these representing the lines of 

 greatest shattering, and hence least resistance. 



