384 • THE CRYSTAL FALLS IKON-BEAEING DISTEIOT. 



truncated by tlie deep Cambrian denudation. Our studies have dealt with 

 the Archean o'nlj in narrow marginal zones, and have included little inore 

 than the location of its outer boundaries, except when it was necessary to 

 go deeper in order to complete the work over a full section. Consequently 

 no attempt at classification can be made upon the map. 



The rocks, chiefly of sedimentary origin, Avhicli are intermediate in age 

 between the Archean below and the Paleozoic above, and therefore fall within 

 the system to which the name Algonkian has been given by this Survey, 

 occupy a narrow strip nowhere more than a mile and a half and usually 

 less than a mile wide, which as a whole runs almost exactly east and west 

 for a distance of over 13 miles. This strip constitutes the Felch Mountain 

 range. On the north and south it is bordered, by the older Archean. The 

 lowest member of the Algonkian occupies parallel zones next to the Archean 

 both on the north and south, and is succeeded toward the interior of the 

 strip by the younger members. While the general structure, therefore, is 

 synclinal, a single fold of simple type has nowhere been found to occupy 

 the whole cross section of the Algonkian formations, but usually two or 

 more synclines occur, separated by anticlines, which may have different 

 degrees and directions of pitch and different strikes, or may be sunk to 

 different depths, and complicated besides both by subordinate folds and by 

 faults. 



Among the Algonkian rocks we distinguish two main divisions or series, 

 which are probably separated from each other by an unconformity. Owing 

 mainly to the peculiar lithological and weak physical character of the 

 younger of these two series, actual contacts between them have not been 

 found, and the evidence of unconformability consequently consists not so 

 much in observed discordance of structure as in an inferred discordance 

 base.d upon their relative surface distribution. This evidence will be fully 

 stated hereafter. 



In the lower of these two series are included four formations which 

 clearly appear to be identical in lithological character and order of super- 

 position with the four formations that, so far as is known, make up the lower 

 iron-bearing series along the Menominee River. These are, reckoning from 

 the base upward, (1) the Sturgeon quartzite, (2) the Randville dolomite, (3) 

 the Mansfield schists, (4) the Groveland iron formation. 



Above this series follows the younger series, which lithologically and in 



