REARRANGEMENT OF CHERT. 851 



instances the process of silicification of the carbonate formations is largely 

 that of segregation of the originally dispersed silica of the limestone 

 formations themselves. 



No better illustrations of extensive cherts are known to me than those 

 furnished by the iron-bearing' formations of the Lake Superior region. 

 Here silica in its various forms and iron oxide constitute almost the entire 

 weathered belt of the cherty iron-carbonate formations. Locally in this 

 region the chert is segregated almost entirely free from iron oxide. Such 

 occurrences are illustrated by the chert near Amasa, in the Crystal Falls 

 district of Michigan." Within the limestones extensive chert beds are also 

 found at many localities. Some of the best known in this country are those 

 of the Boone chert formation, of Lower Carboniferous age, in the lower 

 Mississippi Valley, and the thick bed of chert in the Carboniferous limestone 

 of southwestern Missouri. b 



In none of these formations are organic remains found from which 

 the chert is derived. Where the rearrangement has gone so far that the 

 siliceous organisms, if they once existed, would have been destroyed, it 

 becomes difficult or impossible to certainly discriminate between chert 

 deposits which are mainly organic and those which are mainly chemical. 

 It is natural to suppose that where chert beds are somewhat persistent they 

 are largely organic, although doubtless they may have been extensively 

 rearranged and have had great additions of silica. On the other hand, 

 where the chert masses are small, nodular, and bunchy, and especially 

 where they are exceedingly irregular, it may be supposed that they are 

 mainly substitution products. But reasoning' along either of these lines is 

 of very doubtful value. 



For instance, the rather persistent nature of the chert formations above 

 mentioned in the Lake Superior region might be taken as evidence that 

 they were mainly organic deposits in their present position. However, a 

 close stud}- of this district leads to the conclusion that these formations by 

 substitution for carbonate now occupy the place of cherty iron-bearing 

 carbonates and silicates, and that they are mainly explained by segregation 



"Clements, J. Morgan, and Smyth, H. L., with Bayley, W. S., and Van Hise, C. JR., TheCiystal 

 Falls iron-bearing district of Michigan: Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 62, 177. 



&Bain, H. F., Van Hise, C. R., and Adams, Geo. I., Preliminary report on the lead and zinc 

 deposits of the Ozark region: Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2, 1901, pp. 86-87, 129. 



