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



the spicules is in a form regarded as being soluble in ground 

 water, it was tliouglit to have been taken into solution and 

 deposited about other sponge spicules which acted as nuclei. 

 Other silica-secreting organisms, such as radiolaria, have been 

 suggested as possible sources of the silica. In this and the 

 preceding theory the organisms were thought to have obtained 

 their silica either from the sea water in which it was held in 

 solution or by decomposing silicates occurring in the muds 

 upon the bottom. There seems to be room for doubting that 

 the latter method is an important mode for obtaining the silica, 

 for the silicates which accumulate in the muds have already 

 withstood the attacks of solutions upon the earth's surface and 

 are very stable. Most of the men who held the view given 

 above believe that the segregation of silica by organisms was 

 followed by its solution, removal, and deposition elsewhere, so 

 that these two theories are really no more than one. 



That the chert layers and nodules may have been due to the 

 accumulation of abundant organic remains upon the sea bottom 

 was an idea advanced by Wallich and has been favored by 

 others. The possibility that a part of the silica was a chemi- 

 cal precipitate derived from soluble sponge spicules was also 

 included in this theory, but the method of precipitation was 

 not explained. 



Lawson has suggested the theory that the chert of the Fran- 

 ciscan series in California is due to the precipitation of colloidal 

 silica brought to the sea water by thermal springs. The chert 

 contains sponge spicules, and Lawson states that these proba- 

 bly fell into the soft colloidal silica on the sea bottom and were 

 preserved. 



An unusual mode of origin was suggested by Penrose and 

 Buckley, who thought that the silica might be transported to 

 its present position as a fine siliceous mud. Penrose applies 

 his theory to the Boone chert in northern Arkansas and Buck- 

 ley, to the dark-colored, secondary chert in the Joplin district 

 of southwestern Missouri. It is difficult to explain the source 

 of such a siliceous mud. 



Another theory that has been suggested to explain the origin 

 of chert is that the aggregation of the silica is due to the ordi- 

 nary processes of weathering. It is thought by Ulrich, who 

 suggested this view, that the silica, being insoluble, is concen- 

 trated near the surface as the associated calcareous material is 

 removed by subaerial erosion. This view is held by some 

 others, but the finding of chert in the lower formations in some 

 of the deepest wells that have been drilled in Missouri is strong 

 evidence against it. Further, a careful study of the occurrence 

 of the chert in the area described by Ulrich shows that it does 

 not have unequal distribution, though Ulrich reported that it. 



