Tarr — Origin of the Chert in the Burlington Limestone. 433 



It is readily seen that the chert in the Cambrian-Ordoviciau 

 series is related to a time when the seas were extensive and the 

 lands low-ljing. The Silurian contains very little chert, due 

 probably to the submergence of the portions of the continent 

 which might furnish an abundance of silica and to the land 

 areas being eroded rapidly, as in the eastern United States. 



The shallow, interior lower Devonian sea received some 

 silica along with the calcareous salts. Some silica is found 

 with the formations of the southern Appalachians. 



In Mississippian times the Mississippi valley region was again 

 receiving much silica, for there is an abundance oi chert in the 

 lower formations, especially in the Burlington. 



With the change to elastics in the Pennsylvanian the depo- 

 sition of silica in this region came to a close. This change was 

 the result of renewed mechanical erosion, and the great quanti- 

 ties of shale of this period were doubtless due to long-continued 

 chemical weathering on the low-lying lands of the continent. 

 Some chert occurs in the Pennsylvanian of the Cordilleran 

 region where carbonate rocks continued to be deposited. 



On the whole, it appears that the abundance of chert in the 

 formations of the various periods is more or less dependent 

 upon the character of the adjacent lands. If these are low the 

 chemical removal of much silica is favored and chert may 

 result, but if they are high enough to permit mechanical ero- 

 sion to remove the products of weathering, the limestone off 

 shore will be relatively free from silica. Low-lying lands, 

 then, such as are developed when a given region approaches a 

 peneplain, become a probable source of silica and favor the 

 deposition of much chert. 



(d) Derived from shore worlt. — There is no reason to think 

 that the decomposition of the rocks along the sea shore does 

 not furnish silica to the sea water. Though the action of the 

 waves is largely mechanical, the rocks, after they have been 

 broken up, are attacked chemically by the sea water and further 

 decomposition occurs. This alteration furnishes a certain 

 amount of silica which is added to that already present in the 

 water. 



2. Dispersion of Silica in Sea Water. 



The colloidal silica carried to the sea by streams is distributed 

 by the currents in the sea water, for the rate of diffusion of 

 colloids is low, probably less than the movement of the currents. 

 Since the material is in solution it is readily transported as far 

 as the silica can be carried before becoming coagulated. Dis- 

 persion is aided by the slow rate of concentration of the silica. 

 Certain salts in solution aid in this dispersion by preventing 

 the formation of the gel. Hydrogen sulphide may do this, and 

 so also may some potassium salts and other compounds. 



