530 A TREATISE ON METAMORPHiSM. 



are favorable and the forces and agents of weathering have been long at 

 work, greater or less thicknesses of material in various stages of weathering 

 have accumulated. 



Some of the regions will be mentioned in which the weathering pro 

 cesses are far advanced. Perhaps one of the most striking illustrations of 

 advanced weathering is furnished by various limestone regions. The 

 destruction of limestones located in the belt of weathering occurs both at 

 the surface and below the surface. Below the surface the process is mainly 

 that of solution, and in many regions the same statement applies to the 

 surface reactions. Solution below the surface is the chief cause of the 

 porous, cellular, and cavernous character of limestone formations above the 

 level of ground water. As a result of solution at the surface, residual clays 

 are formed. Residual clays in limestone regions vary in thickness from a 

 few centimeters to 7 meters or more. Chamberlin and Salisbury give the 

 average thickness of the residual clay of the driftless area of the upper 

 Mississippi Valley as more than 2 meters." 1 Whitney estimates the thick- 

 ness of the clay in this same area as 3 meters b This he believed to corre- 

 spond to an original thickness of from 105 to 120 meters of limestone and 

 shale. 



In regions of noncalcareous rocks advanced decomposition may 

 extend to the depth of a score or more of meters. The comparatively late 

 coast ranges of California at many places are much weathered. The 

 weathering affects all the rocks, but to a different depth. The Montara 

 granite in many places is disintegrated to a depth of 6 to 10 meters. The 

 complete disintegration and advanced decomposition of the rocks of the 

 Piedmont Plateau, from Pennsylvania to Georgia, is well known to have 

 extended from a depth of 2 or 3 meters to nearly 30 meters. In Georgia, 

 according to Spencer, decomposition has gone on to some extent at a 

 depth of 60 meters. Merrill says that in many places the Sierra Nevada 

 granodiorites are disintegrated and partly decomposed to a depth of 60 

 meters. 1 * Belt states that the igneous rocks of Nicaragua are decomposed 



"Chamberliu, T. C, and Salisbury, R. D., Driftless area of the upper Mississippi Valley: Sixth 

 Ann. Kept. U. S. Geol. Survey, 1885, p. 254. 



''Hall, James, and Whitney, J. D., Geology of Wisconsin, vol. 1, 1862, pp. 121-125. 



(■Spencer, J. W., The Paleozoic group of northwestern Georgia: Geol. Survey Georgia, 1893, pp. 

 22-24. 



'l Merrill, George P., Rocks, rock- weathering, and soils, Macmillan Co., New York, 1897, p. 274. 

 See also Merrill on extent of weathering, pp. 276-27S. 



