INTRODUCTION. 3 



It is plain from the foregoing that the district is one of exceeding 

 difficulty, and that it has been possible to unravel its intricacies onh^ by 

 patient and laborious work. 



The Algoukian of the ^lanpiette district is divisil)le into two series, 

 presumably the equivalents of the Lower Huroniau and the Upper Huro- 

 nian of other disti'icts of the Lake Superior region and of the Original 

 Huroniau of Canada. These two divisions are sepai'ated l)y an unconform- 

 ity. In this paper the lower clastic series of the Marquette area will be 

 called the Lower i\Iarquette series, and the upper the Upper Marc^uette 

 series. The Algonkian rocks are bounded on the north and on the south 

 by the Basement Complex, or Archean. The Archean consists of an intri- 

 cate mixture of gniiiite, gneisses, schists, and surface volcanics. ^Vll are 

 thoroughly crystalline. 



The Lower Marquette series covers the larger part of the area of 

 Algonkian rocks east of Ishpeming, and forms belts on the north and south 

 sides of the Algonkian area west of Ishpeming. The easternmost Upper 

 Marquette rocks a])pear at Negaunee and at Palmer. Here, however, they 

 are in patches, the east end of the main area appearing at Ishpeming. 

 From this place west the Upper Marquette rapidly Avidens. At Lake Michi- 

 gamme the Algonkian expands into a broad area, from which several arms 

 extend. In each of these arms the lower series occupies the outer borders 

 of the Algonkian belts, the upper series appearing in the centers. 



The area discussed in the present paper is limited on the west l)y the 

 east mile of R. 31 W., and on the south by T. 40 N., with the exception 

 that the southern extremity of the Republic tongue extends into T. 45 N. 

 The Algonkian rocks of tliis area, speaking broadly, are in a great syn- 

 clinorium. This synclinorium is of a peculiar and complicated character', 

 which will be fully considered later. It is sufficient here to say that in the 

 middle of the district the rocks in the outer borders of the Algonkian belt 

 are in a series of sharply overturned folds. The Algonkian rocks on either 

 side of the trough have moved over the more rigid Archean granite, and, as 

 a consequence, on each side of the Algonkian trough a series of overfolds 

 plunge steeply toward its center, producing a structure resembling in this 

 respect the composed fan structure of the Alps. There is, however, this 

 great difference between the Marquette structure and that of the Alps, that 



