METAMOKPHISM IN THE MARQUETTE DISTKICT. 575 



sandstones; slates, gravwackes, mica-slates, nuca-scliists, or mica-gneisses 

 in place of the shales and arkoses; and the pecnliar phases of rocks of the 

 iron-bearing' formation in place of the sideritic slates. 



In so far as the rocks have a slaty or schistose strncture it is believed 

 that the metamorphism was contemporaneous with the folding, but during 

 the long period of quiescence which has subsequently occurred further 

 extensive metasomatic and weathering changes have taken place. These 

 appear to have been particularly potent in the iron-bearing formation, l)ut 

 they have also doubtless produced important changes in other rocks. In 

 this time of quiescence must have occurred the tinal enrichment of the 

 ore bodies and the extensive impregnation of the various rocks with the 

 granular hematite and magnetite. Finally, during this period of quies- 

 cence it is believed that there developed many of the crystals of hornblende, 

 gai'net, staurolite, chloritoid, and andalusite, and much of the secondary 

 feldspar of the mica-schists and mica-gneisses. 



The metamorphism is more nearly complete in the western [^art of the 

 district than in the central and eastern parts. In the western part crystal- 

 line schists ai-e the rule for all the formations, while in the central and 

 eastern parts of the district, excluding localities of exceptional readjustments, 

 the rocks are semicrystalline. The varying metamorphism con-esponds 

 with the closeness of folding. In the western part of the district the folds 

 are closer upon the average than farther to the east. 



COREELATION. 



Reasons have been given in previous publications for regarding the 

 Upper Marquette and Lower Marquette series together as the equivalent of 

 the Huronian of the north shore of Lake Huron. These will not here be 

 repeated. Nor will the argument be repeated for placing the Upper Mar- 

 quette and Lower Marquette as the equivalent of the Upper Huronian and 

 Lower Huronian of the other parts of the Lake Supei'ior region.^ Accejjt- 

 ing these conclusions, this implies that the Lo^Acr Marquette series is to be 

 equated with the Lower Felch Mountain and Lower Menominee series. 



' Correlation papers, Archean and Algonkian, by C. E. Van Hise : Bull. U. S. Geol. Survey No. 86, 

 1892, pp. 156-199. Principles of North American pre-Cambrian geology, by C. R. Van Hise: Sixteenth 

 Ann. Kept. U. S. Geol. Survey, Part 1, 1896, pp. 780-807. 



