Tarr — Origin of the Chert in the BitrWigton Limestone. 449 



9. The cracks in the chert and their tilling of limestone are 

 not due to replacement. 



10. Banding and mottling of chert is not due to replacement, 

 for such structures are not developed during replacement. 



11. The fossils in the chert are too well preserved to be 

 residuals from replaced limestone. 



12. Weathering produces a porous (where fossiliferous), frag- 

 mental, more or less decomposed chert. 



13. i^o evidence of growth at present time or since the 

 period of deposition. 



14. Presence of chert in conglomerates at the base of forma- 

 tions exactly like the chert in the underlying formation. 



lY. Application of the Colloidal Peecipitation Theory 

 TO Other Cherty Formations. 



It is of value and interest to determine to what extent the 

 theory as developed from the occurrence of the chert in the 

 Burlington formation is applicable to other occurrences of 

 chert and flint. 



The table on page 432 shows the extent to which chert occurs 

 in the limestone and dolomites in North America. No period 

 from the Cambrian to the Pennsylvanian is free from at least 

 some cherty formations. Two important areas have long been 

 noted for the abundance of chert they contain: the Mississippi 

 Valley and the Appalachian trough. Other areas, as the Great 

 Basin and Western Canada, also contain considerable chert. 

 On the whole, the chert occurs mainly in the lower formations 

 of the periods, although it may be found in the upper forma- 

 tions. 



Wherever found the published descriptions show that its 

 mode of occurrence is similar to that in the Burlington forma- 

 tion, nodules, lenticular bands, and beds being the chief forms. 

 In all occurrences distribution is essentially along planes. 



Fossils are common in chert, in fact in some formations 

 essentially all the fauna of the formation has been derived from 

 the chert, showing that fossils are best preserved in that 

 material. 



As to minor relationships, the published descriptions are 

 very meager, and it is these minor facts which are of so much 

 importance in determining the origin of the chert. 



Studies of the Cambrian and Ordovician formations in south- 

 eastern Missouri showed many of the same features found in 

 the Burlington formation. A detailed study of the chert in 

 these formations, however, was not so fertile in results as was 

 that of the much more recent Burlington formation. 



