1918] Davis: The Radiolarian Cherts of the Franciscan Group 339 



in small exposures, and it would appear that a large part of the beds 

 are only rather short lenses which do not persist for any great dis- 

 tance. Some specimens of chert show a fine lamination 04 within the 

 separate beds. 



Some of the earlier geologists believed that these iron ores and 

 jaspers were eruptive rocks. Others, impressed with their banded 

 nature, regarded them as sediments of some sort, believing that the 

 silica of the jaspers was originally in clastic grains and later became 

 metamorphosed to its present condition. The iron oxide was regarded 

 by some as originally bog iron ore, by some as lake iron ore, while 

 others thought that the bands of iron oxide might have originally 

 been laid down as magnetite sands. 



R. D. Irving 03 showed that the silica of the jaspers was often 

 chalcedonic or amorphous, which varieties are known only as deposits 

 from solutions. He pointed out that, for the most part, the jaspers 

 showed no trace of fragmental origin. 



He considered the hypothesis that the jaspers and iron ores were 

 deposited from siliceous springs but rejected it principally because 

 of the association of the iron formations with magnetitic and amphi- 

 bolitic schists, which he regarded as having a common origin with 

 the jaspers and iron ores : 



While we have, perhaps, in the deposition from some modern siliceous 

 springs, a slight analogy to the interstratifieation 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 explained. 

 Again there is nothing in the structure of these deposits to indicate spring- 

 deposition, and everything to indicate deposition in bodies of water. But of 

 the formation of such deposits by chemical deposition in bodies of water we 

 certainly have no modern instances. 



From the occurrence of ferruginous carbonates in association with 

 iron ores and jaspers, and the fact that the least altered rocks of the 

 iron formation contain considerable amounts of such carbonates, he 

 was led to the hypothesis that the iron formations were once thin 

 bedded ferruginous and magnesian carbonate rocks associated with 

 carbonaceous shales. These shales frequently contained some admix- 

 ture of carbonate. This material was later silicified and, in the pro- 

 cess of silicification, the iron was leached out in whole or in part. 

 The iron remaining formed the coloring matter of the jaspers; that 



9* Clements, J. M., U. S. Geol. Surv. Monograph 45, p. 181, 1903. 

 »5 Origin of the Ferruginous Schists and Iron Ores of the Lake Superior 

 Eegion, Am. Jour. Sci., vol. 132, p. 255, 1886. 



