432 Tarr — Orngiii of the Chert in the Burliyigton Limestone, 



callv all of this silica is removed as colloidal silicic acid. If 

 mechanical erosion does not interfere, any quartz present in 

 the rock will finally be removed in solution and thus augment 

 the amount derived from the decay of silicates. 



These figures are sufiicient to indicate that the chemical 

 decay of rocks furnishes a large amount of silica to the streams 

 and ultimately to the oceans. 



(c) Relationship to ])eriodsof penejplanation. — A peneplain 

 favors chemical denudation, and the deposits made in those 

 periods during which the lands were low-lying should contain 

 chert^ the amount depending upon the petrol ogic character of 

 the lands drained. During periods of low-lying land areas 

 deposits of limestone and dolomite usuall}^ predominate over 

 those of the other sediments, a fact that should be correlated 

 with chemical denudation. It is in these same limestones and 

 dolomites also that the greatest abundance of chert is found, 

 unless there should be considerable silt deposited along the 

 shores, in which case much of the silica will be carried down 

 W'ith it. 



Campbell"^ has shown that when an area is flat and yet well 

 drained, and the solutions contain acids which can attack 

 quartz, the ground-water becomes a very good solvent and much 

 quartz is removed. These acids are thought to be humic acid, 

 but their efficiency as solvents for silica is seriously questioned. 

 C. W. Hayes t suggests that the azo-humic acids are the chief 

 solvents. This also is questioned. The same thing is true of 

 the development of lateritic soils under conditions of chemical 

 weathering, only the very insoluble substances remaining. 



Table showing Geologic and Areal Distribution of Chert in the 

 Pcdeozoic System. 



Pennsylvanian. — Common in Missouri, Iowa, Kansas, Oklahoma 

 and some parts of the Rocky Mountains. 



Mississippian. — Abundant in lower part of the Mississippian 

 formation in the Mississippi valley. 



Devonian. — Found in lower part of some Appalachian states and 

 central Mississippi valley. 



Silurian. — A little reported in upper Mississippi valley and in the 

 Appalachian region. 



Ordovician.-— Abundant in Knox dolomite and Shenandoah lime- 

 stone in lower part of Ordovician and top of 

 Cambrian. In Mississippian valley and to the 

 southwest in Texas, New Mexico and Arizona. 



Cambrian. — Upper Cambrian in Appalachian region, Mississippi 

 valley, and in the western part of the United 

 States. 



* Campbell, M. E., "Chemical erosion at baselevel," Bull. Geol. Soc. Am., 

 vol. viii, pp. 221-226, 1897. 



t Hayes, C. W., "The solution of silica," Bull. Geol. Soc. Am., vol. viii, 

 p. 218. 1897. 



