Van Tuyl — The Origin of Chert. 453 



south side of Eighth Street, a short distance above Bluff 

 Street, at Dubuque, Iowa ; in the Fort Atkinson dolomite 

 of Maquokata age, at Fort Atkinson, Iowa; and in the 

 Niagaran chert at Hopkinton, Iowa. 



In his description of the Longfellow limestone, of Ordi- 

 vician age, in the Clifton-Morenci district of Arizona, 

 Lindgren- describes microscopic grains of calcite which 

 show a similar relationship. He remarks on the occur- 

 rence of chert in that formation as follows : 



"The chert occurs in irregular bands or nodules, 

 which under the microscope appear as an aggregate of 

 greatly varying grain. Some of it consists of irregular 

 quartz grains, while other parts contain much crypto- 

 crystalline and fibrous chalcedonic material. Ragged 

 calcite grains lie embedded in this mass, giving distinct 

 evidence of the metasomatic origin of the chert by 

 replacement of calcite by siliceous waters. The normal 

 rocks contain no pyrite, nor other sulphides. The Long- 

 fellow limestone, is, as a rule, too siliceous to be used for 

 quicklime or smelting flux." 



Association of silicified fossils and cherts. — The 

 presence of an abundance of silicified fossils in the lime- 

 stone layers associated with chert would appear to 

 furnish strong evidence of the replacement origin of such 

 chert. Such an association has been observed in several 

 Paleozoic formations. For example, it is common in the 

 cherty horizons of the Niagaran dolomite of Iowa. Cal- 

 vin, long ago, mentioned the relationship of chert and 

 silicified corals in this formation, in Delaware county. 3 

 In discussing the fauna of the formation and the favor- 

 able localities for collecting, he says : 



' ' One of the best known localities in the county is that 

 along Prairie creek, in section 28, of Coffins Grove town- 

 ship. Residual clays and cherts have in places a thick- 

 ness of several feet, and these are rich in beautifully 

 preserved specimens of silicified corals." 



Again, in his discussion of the cherty phase of the same 

 formation in the vicinity of the town of Hopkinton in the 

 same county, Calvin makes the following statement : 4 



"While there are few rock exposures that do not show 

 more or less of chert, the most remarkable beds of this 



2 U. S. Geol. Survey, Prof. Paper 43, pp. 62-66, 1905. 

 •"Iowa Geological Survey, vol. 8, p. 156, 1898. 

 4 Ibid., p. 159. 



