Tarr — Origin of the Chert in the Burlington Limest07ie. 411 



matter might also have acted in the same capacity. Once 

 started, the aggregate would, according to the theory, continue 

 to attract silica to itself from the surrounding muds as long as 

 material was available. Hull and Hardman, M. A. Renard, C. 

 A. White, E. O. Hovey, C. R. Van Hise and R. D. Irving, 

 and N. H. and H. W. Winchell have all advocated the same 

 view, although the first two men thought that the silica so pre- 

 cipitated had later replaced calcareous material. Hovey based 

 his theory upon a study of chert from the Burlington and the 

 Cambro-Ordovician formations of Missouri, which makes his 

 conclusions especially interesting by way of comparison, since 

 this paper also is based upon a study of the chert of the Bur- 

 lington and the Cambro-Ordovician formations. He states : 



" Regarding the Lower Magnesian and Lower Carboniferous 

 cherts from southern and southwestern Missouri, the present 

 writer's conclusion is, that they are due to chemical precipitation, 

 probably at the time of the deposition of the strata in which 

 they occur or before their consolidation. " 



The advocates of the theory that the silica was first segre- 

 gated by some organism have presented much evidence in its 

 support. Their main evidence is the finding of the remains of 

 sponges and diatoms scattered throughout the chert. This 

 theory was early advanced in England by Bowerman and was 

 ably supported by the researches of Sollas, Hinde, Jukes- 

 Brown and Hill, and ISorby. It is largely accepted by Ameri- 

 can geologists as is shown by the statements made in all our 

 latest text-books. Had these early investigations been made on 

 material collected in Missouri, very likely a different theory 

 would have resulted, because Missouri chert shows no remains 

 of siliceous organisms. 



The advocates of the theory that the chert and flint -nodules 

 and beds are the result of the replacement of calcareous mate- 

 rial of the limestone early called attention to the fact that 

 fossils known to be originally calcareous were often found 

 silicified undoubtedly through replacement. A. H. Church 

 was able to replace with silica the calcium carbonate of a frag- 

 ment of coral by allowing a weak solution of colloidal silica to 

 percolate over it.'^ On account of the finding of fossils on the 

 interior of the chert and flint nodules which were originally 

 calcareous, the conclusion was drawn that the silica had 

 replaced the fossil and that the nodule had grown by further 

 additions on the outside, each particle of silica replacing an 

 equivalent amount of calcareous material. The presence of 

 siliceous spicules of sponges in chert and flint suggested a pos- 

 sible source for the silica in the nodules, and since the silica in 

 *F. W. Clarke, Bull., U. S. G. S., 616, pp. 515, 543. 



