450 Tarr — Origin of the Chert in the Burlington Limestone. 



Chert is often found in the lower limestone and dolomite 

 members of the various formations of the periods. These 

 formations were laid down in a sea which was advancing upon 

 a more or less peneplained area and as a result were receiv^ing 

 the drainage from low-ljing lands, a condition favorable to the 

 removal of large amounts of silica, which would become a part 

 of the formations being deposited. As the sea advanced and 

 the land areas became reduced in size, smaller quantities of 

 silica would be brought to the sea and less chert deposited. 

 This relationship between the chert and the events of the larger 

 periods of time appears to be significant and in accord with the 

 theory here advanced. 



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 can not be 

 taken as explaining all occurrences of chert. More detailed 

 studies of each formation which contains chert are necessarj^ 

 to determine its more general application. The evidence as 

 gathered from the literature appears to be favorable to the 

 theory, however. 



The notable features of the theor}^ are its explanation of the 

 minor features of the chert as it occurs in the Burlington lime- 

 stone and its application to the larger occurrences of chert as a 

 rock in the formations from the Cambrian to the Cretaceous. 

 The theory, if its application to those points is correct, should 

 be found applicable to other deposits of chert. 



Y. Summary and Conclusion. 



The widespread occurrence of chert in the Burlington forma- 

 tion of Mississippian age has made a careful study of it possi- 

 ble. It is believed that this chert was formed from colloidal 

 silica, derived in most part from streams which contributed it 

 to the sea. This colloidal silica was widely dispersed through 

 the sea water, and finally, after sufficient concentration, was 

 deposited through the action of electrolytes. Upon coagula- 

 tion the silica is believed to have aggregated into more or less 

 globrJar masses which eventually assumed the more common 

 elliptical foi'm which they now possess. This elliptical form 

 of the chert is attributed to the flattening of the colloidal mass 

 under its own weight and later under the weight of the over- 

 lying sediments. The various structural features of the chert, 

 its relationship to the enclosing limestone, its physical charac- 

 teristics, and the abundance of calcareous fossils within the 

 chert, are all readily explained by the theory. 



The evidence that these same features cannot be explained 

 by the replacement theory is then given. 



The final conclusion is that the chert, being originally a 



