68 THE MARQUETTE IRON-BEARING DISTRICT. 



Huroulan districts in the Lake Superior region, and pi-oposes a new scheme, 

 whicli lie claims shows clearly the equivalency of the Penokee and the 

 Marquette series. 



Crosby, W. O. Oh a possible origin of petrosiliceons rocks. Read March 5, 

 1879. Proc. Boston Soc. Nat. Hist., Vol. XX, 1S78-S0, pages 160-169. 



Crosby compares the felsites and "petrosilexes" with the siliceous red 

 clays di-edg-ed Ijy the Clialleuger Expedition from the deep seas. With 

 respect to the Marquette jaspers and ores he says (p. IGS): 



One of the most interesting rocks in the Marquette iron district, in the Upi)er 

 Peninsula of Michigan, and the one jnost closely associated with the iron ore, is a 

 brownish or I'eddish jasi)er; it sometimes becomes cliloritic or micaceous, passing into 

 chlorite-schist, etc., but for the most part it is a distinct and beautiful stratified jasper. 

 This Lake Superior jasper, like all the petrosiliceous rocks so far as known, belongs 

 to the Huronian formation, and may, apparently, be fairly taken to represent the 

 petrosilex and felsite characterizing many other Huronian areas, but apparently 

 wanting here. Its association with the iron oi'e is usually very intimate; the two 

 substances being interlaminated in such a manner as to give rise to a banded structure 

 which matches in all important particulars the banding of the petrosilex of eastern 

 Massachusetts and other regions, the hematite simply taking the place of the feldspar. 

 The extreme irregularity of the banding in many cases makes it not only proper but 

 necessary for us to conclude that, as in the case of the petrosilex, it is largely the 

 result of a segregation process, the two constituents, hematite and jasper, having been 

 originally more intimately mixed 



With very few exceptions this JIarquette ore always contains some oxide of 

 manganese, usually from 1 to 3 per cent, though the ore ^roni one bed contains nearly 

 7 per cent. 



Here, then, so far as cin-mical composition is concerned, we have a formation 

 almost identical with some of the siliceous oozes of the deep sea; while the chief 

 structural distinction consists in the different forms of the segregated masses of the 

 iron and manganese oxides, lenticular layers taking the place of irregularly rounded 

 nodules, certainly a distinction of no great importance. 



The author does not claim to prove that the jaspers and ores origi- 

 nated from deep-sea oozes, but only to show the close analogy existing 

 between these ancient rocks and the deposits now lieiiig formed in the 

 ocean's depths. There are several misinterpretations in the descrijition 



