422 THE CRYSTAL PALLS IRON-BEARING DISTRICT. 



ill what form these constituents were present iu the original deposit. On this 

 question the microscopic structure seems to me to have a distinct bearing. 



Forms similar to the ovals in these rocks occur in the iron-bearing 

 formations of other districts in the Lake Supei'ior region. In the Gogebic 

 district of Michigan and Wisconsin, R, D. Irving and C. R. Van Hise^ have 

 supposed that such forms have resulted from processes of solution and 

 redeposition after the rock was formed, and are therefore concretionary. 

 They regard that portion of the formation — which they have named fer- 

 ruginous cherts — in which such forms occur, as an alteration product from 

 an original deposit of cherty carbonate of iron. On the other hand, J. E. 

 Spurr^ has shown that similar forms are exceedingly abundant throughout 

 the iron-bearing formation of the Mesabi range of Minnesota, and are there 

 original. In the least-altered stages Mr. Spurr has found that these oval 

 and roundish areas are filled with a green substance, which chemically is 

 a hydrous silicate of iron, in composition very close to glauconite, with 

 which it is also optically identical. The oval and rounded forms, moreover, 

 are those characteristic of glauconite in green sands of all geological ages. 

 Starting with this original substance, which is very unstable when exposed 

 to oxidizing and carbonated waters, Mr. Spurr has traced an interesting 

 series of changes, the final result of which along one line is the complete 

 oxidation of the iron to hematite or magnetite and the separation of the 

 silica as chalcedony and quartz. Throughout these changes the original 

 form of the glauconite grains is ■ preserved in the new minerals. Without 

 going into the details of these changes, and without accepting Mr. Spurr's 

 conclusions in their entirety as to the steps iuA^olved, he has clearly shown, 

 as I have satisfied myself from the study of the large number of Mesabi 

 slides in my own collection, that the green glauconitic substance is the 

 source of the iron and silica of the ferruginous cherts of the Mesabi range, 

 and that the peculiar spotted structure of these cherts is inherited from the 

 original foi-ms of the glauconite grains. 



Between the ferruginous quartzites of the Groveland formation and the 

 ferruginous cherts of the Mesabi range there is a very close resemblance, 

 especially in structure. The essential diiference is that the former contain 



' Loc. cit., pp. 254-257. 



" The ii'ou-bearing rocks of the Mesabi range in Minnesota, by .1. E. Spurr : Bull. Geol. and Nat. 

 Hist. Surv. of Minn., No. X, 1894, 259 pp., 12 pis. 



