﻿Iron Ores of the Lake Superior region. 259 



cement to a brecciated mass of fragments of the interbanded 

 materials. All theories of a formation of these ferruginous 

 rocks by metamorphism, or recrystallization in situ, from some 

 sort of sedimentary deposit, seem to regard the jaspery or cherty 

 material as representative of a fragmental siliceous ingredient 

 in the original deposit — either a quartz sand or a fine " siliceous 

 silt." On these theories this substance has been recrystallized 

 from the fragmental condition. The facts above cited refute any 

 such explanation, and prove incontestably the chemical origin 

 of the silica. We may cite in addition, moreover, the fact 

 that in these siliceous iron rocks themselves there is at times 

 an admixture of fragmental quartz, which in the thin sec- 

 tion is always sharply defined as an original ingredient and 

 easily distinguishable from the water-deposited silica. The 

 latter is frequently identical with that cherty material which 

 has often taken the place of large bodies of limestone among 

 the unaltered formations. 



These considerations leading to the rejection of a meta- 

 morphic origin for any of these materials, as others had previ- 

 ously led to the rejection of an igneous origin, it followed that 

 we were thus restricted to some theory which should account 

 for the precipitation of most of them essentially in their present 

 conditions, with perhaps some slight internal rearrangement ; 

 or to one in which the production, from some form of sedi- 

 mentary deposit, of the conditions now obtaining, should be 

 assigned to metasomatic processes, carried out, in part at least, 

 at a very remote period. Lack of any definite evidence as to 

 what might have been the original condition of these de- 

 posits led at first to a preference for the former of these 

 views, though no such theory was ever held more than tenta- 

 tively. One great difficulty with any such view seemed to be 

 the want of any modern phenomena from which might be 

 derived the data necessary to form any clear conception of the 

 process. While we have, perhaps, in the deposition from some 

 modern siliceous springs, a slight analogy to the interstratifica- 

 tion of iron sesquioxide and silica, this analogy is, after all, but 

 slight, and any theory of deposition from springs fails entirely 

 to secure in its support any modern analogues for the various 

 magnetitic and actinolitic schists whose production must be ex- 

 plained. Again, there is nothing in the structure of these 

 deposits to indicate spring deposition, and everything to indi- 

 cate deposition in bodies of water. But of the formation of 

 such deposits by chemical deposition in bodies of water we cer- 

 tainly have no modern instances. Yet another difficulty seems 

 to lie in the necessity, under this theory, of separating the 

 jaspery and cherty silica into two parts, the main part having 

 been deposited chemically with the original sediment, and a 



