450 Van Tuyl — The Origin of Chert. 



no means intended to imply that all occurrences are of 

 this origin. Let us consider certain -facts which lend 

 support to this view. 



Evidences of Replacement. 



The following features may be listed as favoring the 

 replacement theory : 



(1) the occurrence of chert along fissures in limestone ; 

 (2) the very irregular shape of some chert nodules; (3) 

 the presence of irregular patches of limestone in some 

 chert masses ; (4) the association of silicified fossils and 

 chert in some limestones; (5) the presence of replaced 

 fossils in some cherts; (6) the preservation of structures 

 and textures in some cherts; (7) the failure of some 

 cherts to follow definite zones in limestone formations; 

 (8) the occurrence of silicified oolites formed by the 

 replacement of calcareous ones. 



Chert along fissures in limestone. — Chert with this 

 relationship has been noted by the writer in the St. Louis 

 limestone in the government quarry at Little Rock, Mis- 

 souri. In this limestone most of the chert is dark in 

 color, and occurs in the form of nodules and small lenses, 

 but a pinkish variety occurs in a bed 14 feet in thickness, 

 near the top of the formation This is usually in the form 

 of nodules, but it occasionally follows small fractures for 

 several feet. Frequently these are nearly vertical. 



Very similar relations are shown by the pink chert of 

 the St. Genevieve limestone in the exposures between 

 Little Rock and the town of St. Genevieve. 



In the Beekmantown dolomite at Ft. Ticonderoga, New 

 York, chert not only traverses the dolomite along 

 fracture lines and bedding planes, but also follows 

 fucoidal markings in such a way as to suggest selective 

 replacement. 



Irregular shape of some chert nodules. — Most chert 

 nodules are rounded or elliptical in shape, and more or 

 less symmetrically developed, but occasionally they are 

 extremely irregular. The most irregular nodule yet 

 encountered by the writer was collected from the Galena 

 dolomite near Galena, Illinois. This is composed of 

 branching and reuniting, rounded pipes of chert with the 

 interstices filled with brownish dolomite. That this was 

 formed by primary deposition seems improbable. 



