192 NEW YORK STATE MUSEUM 



it is strictly analogous to the fossil ores of the Clinton and points 

 clearly to the coating and replacement of the organic fragments of 

 the latter during, and immediately after, their accumulation on the 

 sea bottom and before the deposition of the overlying sediments. 



The results of the analyses give an affirmative answer to the 

 question as to the presence of ferrous iron, but leave in doubt the 

 matter of the ferrous silicate for, unlike the Clinton ores, these 

 lake ores show no approximation to constancy of proportion between 

 ferrous iron and soluble silica. It is evident that, in every case, 

 there is an excess of silica over that required to form the ferrous 

 silicate, which suggests the presence of the latter. 



Assuming the presence of silicate, however, the question remains 

 as to the mode of occurrence of the remaining silica. The tendency 

 of collodial ferric hydroxide to adsorb various substances, to which 

 Van Bemmelen 1 has called attention, strongly suggests that this 

 is the correct explanation in the present instance, while Sosman 2 

 says: " The intimate association of silica with many ores, amounting 

 even to chemical combination, as in some of the siliceous ores of 

 the Lake Superior region, may represent a simultaneous 

 agglomeration of colloidal silica and ferric oxide, or an adsorption 

 of ferrous and ferric iron by agglomerated silica." It is quite possible 

 that a ferric silicate is present, though there is no definite evidence 

 that such is the case. 



In the Clinton ores, where all the silica is regarded as forming 

 ferrous silicate, the absence of excess of silica, like that of the lake 

 ores, may be an original character or, may be due to either a 

 leaching out of silica or its conversion to the insoluble form. In 

 addition to the amount of silica present, an obvious controlling 

 factor in the formation of ferrous silicate is the quantity of oxygen 

 available to precipitate the iron as ferric hydroxide. The chamosite 

 layer points clearly to a deficiency of oxygen, while in the ores the 

 oxidation was much more nearly complete. That silica adsorbed 

 by ferric hydroxide, under strongly oxidizing conditions, might, 

 under subsequent reducing conditions, combine with ferrous iron 

 to form a silicate, as suggested in discussing the origin of the gray 

 oolite, does not admit of a doubt, and this may be a frequent method 

 of formation of the silicate. 



1 Van Bemmelen, J. M., Ueber das Vorkommen, die Zusammensetzung tmd 

 die Bildung von Eisenanhaufungen in und unter Mooren: Zeits. f. Anorg. Chem. 

 XXII, p. 378, 1899. 



2 Sosman, R. B., Some Problems of the Oxides of Iron: Jour. Washington 

 Acad. Sci., VII, p. 71, 1917. 



