Van Tuyl—The Origin of Chert. 449 



Art. XXX.— The Origin of Chert;* bv Francis M. 



Van Tuyl. 



Introduction. 



In a recent issue of this Journal 1 there appeared a 

 paper by W. A. Tarr, entitled "Origin of the Chert in the 

 Burlington Limestone." This was based on a careful 

 study of the exposures of this chert in the vicinity of 

 Columbia, Missouri. In this paper Tarr summarizes 

 briefly the prevalent theories of chert formation and con- 

 cludes that these are inadequate to explain the Burling- 

 ton chert, the occurrence of which is described in detail. 

 He then introduces a new primary theory of collodial 

 deposition of this chert, and presents a large body of 

 data in its support. Finally he discusses the possibility 

 of the application of his theoiy to other cherts and con- 

 cludes as follows : 



' ' Though the theory advanced to explain the origin of 

 the chert in the Burlington formation has many points in 

 its favor, and would appear to have a wide application, it 

 cannot be taken as explaining all occurrences of chert. 

 More detailed studies of each formation which contains 

 chert are necessary to determine its more general appli- 

 cation. The evidence, as gathered from the literature, 

 appears to be favorable to the theory, however. 



The notable features of the theory are its explanation 

 of the minor features of the chert as it occurs in the Bur- 

 lington limestone and its application to the larger occur- 

 rences of chert as a rock in the formations from the 

 Cambrian to the Cretaceous. The theory, if its applica- 

 tion to those points is correct, should be found applicable 

 to other deposits of chert. ' ' 



On pages 443 to 449, Tarr presents an array of evi- 

 dence opposed to the replacement theory of the origin 

 of the Burlington chert, and it would appear that he 

 attributes little importance to this possible mode of chert 

 formation in general. 



The writer's experience with cherts has led him to 

 believe that many of them may be most satisfactorily 

 explained upon the basis of replacement, although it is by 



* Published with the permission of the Director of the Iowa Geological 

 Survey. 



1 Tliis Journal, vol. 44, pp. 409-452, December, 1917. 



