120 THE MARQUETTE IKON-BEARING DISTRICT. 



Granitt' eruptions .succeeded tliose <it" the liasic rocks, as did also those 

 of serpentine. The age of the hitter with respect to tlie g-ranite is not 

 determined. 



The i)rincipal de})osits of chemical origin in the district are the .sott ores. 

 The jaspiHtes anil their associated ores and rocks have been leached l)y per- 

 colating, nsually hot, waters, and their iron oxides dissolved and deposited 

 elsewhere along the channels through which the waters flowed, or the silica 

 has been removed and the ore left behind or locally concentrated. (_)n top 

 of tlie Azoic rocks and unconformable with them lies the Potsdam sandstone. 



The two important points to be noted are that the author had not, np 

 to this time, changed his views as to the age of the granite, which he still 

 believed to be younger than the ore l>eds, nor as to the origin of the ores 

 associated with the jaspilites, both of which latter rocks are still regarded 

 as eruptive. 



1S91. 



Ibvino, R. D. Explanatory ami historical uote. The greenstonescbist areas of 

 the Menominee and Marquette regions of Michigan. Bull. U. S. Geol. Survey No. G2, 

 Wasliiugton, ISOl, pages 11-L't. With nia[>. 



In 18!)1 appeared the paper by G. H. Williams ()n the greenstone- 

 schist areas of the Menominee and Marquette regions, in which it was 

 shown that the l)andeil green schists (which had l)een regarded by mo.st 

 geologists as sedimentar\' and had been placed by them in the iron series, 

 and which Irving had separated from the Huronian and placed in the 

 Basement ( 'omplex) are fragmental volcanic rocks and lavas. 



As an iutroductiou to the discussion, Irving gives an account of the 

 general relations of the schists to the granite and to the fragmental rocks 

 associated with them, and explains in more detail than had been done 

 hitherto his reasons for sejjarating them from the Huronian series and 

 placing them with the underlying granites and gneisses. 



Besides occurring here and there more or less confusedly mingled with masses 

 of granite and other rocks, these greenish schists occur also in large continuous areas, 

 which they entirely occupy, except for certain relatively unimportant basic and acid 

 intrusives. » * * The bulk of these areas is composed of nondescript fine-grained 

 greenish schists, which appear to grade into the more massive greeustoue-like forms 



