566 THE MAEQUETTE IRON-BEARING DISTRICT. 



was instrumental in reproducing iron carbonate from the precipitated 

 iron oxide. This ferruginous and carbonaceous shale was very similar 

 to some of the Paleozoic shales of the Appalachians, and argues sim- 

 ilar conditions of deposition. Subsidence must have steadily continued 

 during the deposition of the shale, for it is of considerable thickness. 

 The sediments varied in coarseness, as shown by the fact that the rocks 

 now found include fine-grained slates, graywackes, and even rocks which 

 approach a quartzite. These rocks indicate waves and cm-rents of varj-ing 

 streng'th or water of varying depth, or both. The shale and graywacke 

 have been modified over extensive areas into mica-slates, mica-schists, or 

 mica-gneisses. 



FOLDING OF THE BASEMENT COMPLEX, LOWER JUARQUETTE SERIES, 

 AND UPPER MARQUETTE SERIES. 



The Marquette district had been an area of deposition since the begin- 

 ning of Upper Marquette time, and sediments of great thickness had accumu- 

 lated. A physical revolution next occurred, as a consequence of which this 

 district was raised above the sea and was folded in a complicated manner. 

 (See Atlas Sheet IV.) Whether there was an epeirogeuic moven:ient which 

 raised the plateau above tlie sea before the orogenic movements, and whether 

 the main folds now found were formed simultaneously or successively, have 

 not as yet been determined. In general, the directions of folding are 

 approximately east-west and north-south. The only important exception 

 to this is in the southwest part of the distnct, where the Republic arm 

 swings away from the main area of Algonkian in a southeast direction. 



The largest but least conspicuous fold of the district is an anticline hav- 

 ing a north-south axis, running through Marquette. This fold has a gentle 

 dip, but a breadth of man}' miles gives it a great amplitude. Its effect upon 

 the minor but more conspicuous east-west folds is to give them a westward 

 pitch. It follows that in going west from Lake Superior the area of the Mar- 

 quette rocks becomes broader and broader, and higher and higher members 

 appear in successive eastward-pointing U's, the ends being, however, often 

 crenulated, due to the folds of the second and third orders. This great 

 fold is by no means simple in its character, but has, especially near its 



