190 NEW YORK STATE MUSEUM 



oxide with adsorbed silica and subsequently changed to silicate, 

 under reducing conditions due to organic matter on the sea bottom, 

 is possible, as suggested by Leith 1 in his first discussion of the 

 Mesabi ores ; and certain facts derived from examination of modern 

 lake ores, to be discussed later, support this view. But direct 

 precipitation as ferrous silicate is thought more probable. 



In this connection, the artificial precipitation 2 of greenalite is of 

 great interest but, while the laboratory conditions involved, with 

 their high temperatures, approximate those indicated for the natural 

 precipitation of greenalite, they are quite unlike the conditions 

 under which the Clinton silicate was formed. In spite of this, 

 however, they indicate the readiness with which ferrous silicate 

 may be formed, while the difference in conditions may account 

 for the difference in the minerals produced. 



In the case of the Clinton ores, there seems to have been a delicate 

 equilibrium between silicate and hydroxide and, while the latter 

 was formed in larger amount, there was still, as a rule, a certain 

 amount of the former. A slight shift in one direction, that" of lack 

 of oxygen, would increase, while a similar shift in the opposite 

 direction would decrease, the amount of silicate. 



It appears, then, that primary precipitates of ferrous silicate 

 and ferric hydroxide, the latter easily dehydrated, may form 

 simultaneously, and that, with varying conditions, either one may 

 predominate in a given case, though as a rule both are present. 

 That subsequent oxidation, under new geological conditions, may 

 convert the silicate to oxide is undoubted; and it is almost equally 

 certain that reducing conditions may, in the presence of sufficient 

 silica, convert the oxide to silicate, reference being made to alteration 

 under moderate cover, in the region of cementation, not to deeper 

 seated conditions, where any silicate formed would be of the type 

 characteristic of the crystalline schists. 



Stapff 3 long ago pointed out that modern lake and bog ores, when 

 treated with acid, leave a residue similar to that left by the Clinton 

 and other ores, and concluded that there was iron silicate present. 



The writer has examined a large number of samples of modern 



1 Leith, C. K., The Mesabi Iron Bearing District; U. S. Geol. Survey, Mon. 

 XLIII, p. 256, 1903. 



2 Van Hise, C. R. & Leith, C. K., Geology of the Lake Superior Region; U. S. 

 Geol. Survey, Mon. LII, p. 521-22, 191 1. 



3 Stapff, F. M., Ueber die Entstehung der Seeerze; Zeits. d. Deutsch. Geol. Ges. 

 18, p. no and 166, 1866. 



