410 Tarv — Origin of the Chert in the Burlington Limestone. 



Introduction. 



Many explanations have been advanced to acconnt for the 

 occurrence and distribntion of chert in Hmestone and dolo- 

 mite. Since chert is found in these rocks in all parts of the 

 world, ^'eologists have had many opportunities to study its 

 occurrence and to accumulate enoupjh facts to give an ade- 

 quate explanation of its origin. Tliis has not been done, how- 

 ever, for careful descriptions of chert, its mode of occurrence, 

 physical characteristics, and composition are lacking in most 

 references to it. There are a few good descriptions but these 

 are mainly old ; y^v^ few of the articles dealing with chert, 

 even in a general way, are later than 1900, and these, as a rule, 

 do not present new evidence but cite the earlier views as to 

 origin and conclude that they are adequate. In studying the 

 chert of the Burlington limestone in the vicinity of Columbia, 

 Mo., the writer observed many facts which do not accord with 

 the prevalent view that chert is due to the segregation of 

 silica in limestone into the characteristic chert nodules. The 

 following paper presents the evidence which has been collected 

 and its interpretation as the writer sees it. 



I. Theories as to Origin. 



There have been many different views advanced to explain 

 the origin of chert, but the majority can be classed under two 

 heads : (1) theories that seek to explain the origin through 

 segregation by organic means ; and (2) theories maintaining 

 that the material of the chert was a direct chemical precipitate. 

 There are, however, other theories, as the following complete 

 table shows : 



1. Chert is the result of direct chemical precipitation. 



2. Chert is the result of the secretion of silica by organisms. 



3. It is the result of the replacement of calcareous material. 



4. It is of organic origin, but not through replacement. 



5. It is a spring deposit. 



6. It is a mechanical sediment. 



7. It is regarded as being due to weathering. *= 



It would be entirely out of place to present the various 

 theories in full in this paper, but a brief statement of the essen- 

 tial point in each one may be made, and the bibliography at 

 the end of the paper furniahes many excellent summaries and 

 further citations to the literature. 



The theory that chert originated through the precipitation 

 of silica in the colloidal form upon the sea bottom was advanced 

 by Prestwich in 1888. The silica was thought to have accu- 

 mulated about the siliceous spicules of sponges, which acted as 

 nuclei, or in the absence of such material, decaying organic 



