IOWA ACADEMY OF SCIENCES. 105 



Considering in this light the cherts of the Missouri min- 

 eral belt in the southwestern part of the state similar con- 

 ditions appear to have prevailed. The calcium carbonate 

 of the fossils is found entirely replaced, one end only 

 replaced, or only a small part replaced, by chert. A simi- 

 lar replacement also takes place in which iron sulphide is 

 substituted for the original material of the fossils. Like- 

 wise zinc sulphide, the principal ore of the district, is 

 found composing fossils as perfectly as in the original, 

 every structure being perfectly preserved. 



From these facts, and many others, no other conclusion 

 can be reached but that the cherts were formed long after 

 the terranes were laid down; that the cherts were formed 

 under the same conditions as the ores of the region, and 

 that, like the ores, the cherts were formed at a compara- 

 tively recent date; that, as in the case of the ores, the 

 cherts were formed by the displacement of the original 

 limstone molecule by molecule. 



The formation of the ores of the region is going on rap- 

 idly at the present time. The last uprising of the Ozark 

 dome is not l)elieved to be yet finished. The formation of 

 ore and chert is manifestly very recent, geologically speak- 

 ing. It is not impossible, but not probable, that the whole 

 transformation may have taken place within the memory 

 of man. 



COMPARATIVE VALUES OF DIFFERENT METHODS 



OF GEOLOGIC CORRELATION IN THE 



MISSISSIPPI BASIN. 



BY CHARLES R. KEYES. 



(Abstract.) 



The need of a number of independent criteria in the 

 broader work of geologic correlation has never been more 

 appare.:it than at the present time. Yet for necessary 



8 I AS 



