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



cliances of finding such remains are considered to be best in 

 the chert, because the formation of the nodules is supposed to 

 take place around siliceous organic remains. Why such depo- 

 sition would be the case it is difficult to see in view of the fact 

 that the silica in the fossils is supposed to be readilj^ soluble. 



The limestone also was carefully examined for traces of sili- 

 ceous organisms. A large quantity of the i-ock was carefully 

 dissolved in weak acetic acid and the insoluble material exam- 

 ined with a microscope. No traces of siliceous fossils were 

 found. There was a small amount of silica in the residue in 



Fig. 12 



Fig. 13 



Fig. 12 (red"uced one-lialf). Chert containing a Syringopora coral sur- 

 rounded by chert. The tubes are nearly vertical in the chert. From Bur- 

 lington limestone, Columbia, Mo. 



Fig. 13 (slightly reduced). Impression of a crinoid on the exterior of a 

 chert nodule. From Burlington limestone, Columbia, Mo. 



the form of euhedral quartz crystals, the largest of which 

 rarely exceeded O*!"""' in diameter, mostof tliem being much less. 

 That all traces of any siliceous organisms which may have once 

 been in the limestone have been removed is too improbable to 

 believe. 



It is concluded, therefore, that the silica in the chert was 

 not derived from siliceous organisms in the adjacent limestone. 

 The inclusion of the fossils in the chert is believed to have 

 taken place when the nodule was forming on the sea bottom, 

 the remains of the organism falling into the soft colloidal silica 

 and being buried and preserved. 



7. Conclusions as to Origin under the Theory. 



It is believed that silica which formed the chert nodules in 

 the Burlington limestone was deposited upon the sea bottom 

 in the form of colloidal silica simultaneously with the deposi- 



