4J:8 Tarr — Origin of the Chert in the Burlington Limestone. 



evidence to disprove liis own and Ulricli's statements. The 

 chert in the conglomerates at the base of the various formations 

 in Missouri is sufficient proof that it is not due to weathering, 

 especially in view of its fresh character in such conglomerates. 



Isaturally, chert could not accumulate on the face of a bluff, 

 though such a very insoluble, resistant substance as chert could 

 well be expected as a residual material on all slopes w^iere the 

 rate of erosion was not sufficiently fast to remove it. This is 

 the condition over the entire Ozarks, and cannot be interpreted 

 as evidence that the chert is growing at the surface at the pres- 

 ent time. 



It is always the resistant materials which are left behind in 

 the mantle rock, and men have long recognized that the chert 

 (usually called flint in Missouri by laymen and many geologists) 

 is insokible and accumulates abundantly at the surface. Cer- 

 tainly we cannot believe that the chert is growing in the 

 residual clays. If the chei't masses are increasing in size the}^ 

 should show signs of cementation at the surface, forming chert 

 breccias, and in many parts of Missouri, at least, a solid sheet 

 of chert over the entire surface, for there certainly is enough 

 residual chert at the surface to supply all the silica necessarj^ 



A careful study of the residual chert failed to show any frag- 

 ments more or less dissolved or any to which material was 

 being added. 



Deep drill holes in all parts of the state, which have pene- 

 trated tlie formations below the Pennsylvanian, show much 

 chert as a part of the sections. This is sufficient evidence that 

 it is original and not formed at the surface by the process of 

 weathering. 



{f) Conclusions as to rejylacement. — That the chert in the 

 Burlington limestone is not due to replacement is shown by the 

 following lines of evidence : 



1. The uniform distribution of the chert along planes in 

 the limestone and the absence of structural control of these 

 planes of chert. 



2. The impermeability of the limestone. 



3. Absence of a source of silica in the limestone or adjacent 

 rocks. 



4. No siliceous organisms in either chert or limestone. 



5. Deposition of the silica does not occur in the main solu- 

 tion channels in limestone. 



6. Elliptical or circular forms more readily explanied by col- 

 loidal precipitation theory. 



7. No structural features of tlie limestone preserved in the 

 chert. 



8. Occurrence of angular, brecciated fragments of chert in 

 larger masses of chert. 



