28 THE CKYSTAL FALLS IRON-BEARING DISTRICT. 



associated with quartzites, graywackes, and small amounts of carbonate- 

 beds. The general character of the series is what one would expect in 

 rocks the detritus of which was from the Hemlock A^olcanics. It is in this 

 slate series that, with the exception of the Mansfield mine, the ore deposits 

 of the Crystal Falls district are found. The sedimentaries extend west 

 from the Hemlock volcanics to the limits of the district, underlying thus a 

 A'ery broad expanse of country. Where exposed, the}' show frequent 

 changes of character. This prevents the identification of individual beds 

 for any considerable distance. Owing to the imperfect exposures of the 

 beds and their close folding, it has been found impossible to subdivide this 

 series of rocks into distinct formations. 



The series has in places been highly metamorphosed, resulting in the 

 production of gneisses and mica-schists, iii places garnetiferous and staiiro- 

 litic. The series corresponds in a broad way stratigraphically and litho- 

 logically to the Michigamme formation of the Marquette district.^ Since, 

 however, it has been found impossible to subdivide this series, and because 

 it may possibly include more than the Michigamme formation of the Mar- 

 quette district, it is considered advisable to speak of it simply as the Upper 

 Huronian series. The generalized sections through the western half of the 

 Crystal Falls district, which are given on Pis. V and VI, will aid in the 

 comprehension of the structural and stratigraphical features thus briefly 

 outlined. 



Here and there in the Crystal Falls district isolated patches of Upper 

 Cambrian Lake Superior (Potsdam) sandstone are found. This occurs in 

 beds which are either horizontal or only a few degrees inclined from the 

 horizontal. They overlie unconformably the steeply inclined Huronian 

 strata. The great lapse of time represented by this unconformity is indi- 

 cated by the deposits of the Keweenawan and Lower and Middle Cambrian 

 time, found elsewhere. The Lake Superior sandstone grades from the very 

 coarse basal conglomerate below into a moderately coarse sandstone above. 

 The sandstone is of a reddish brown to gray color, and is not well indurated 

 as a rule, but is loosely cemented with ferruginous and in places calcareous- 

 material. As a result of this imperfect induration, the sandstone is not very 

 resistent to the agents of disintegration. Hence it is that only remnants- 

 have been found, but enough is present to indicate that the greater i^art, 



'Fifteenth Annual, cit., p.. 598, nii<l Moii. XXVIIT, cit.. p. 44-J. 



