534 A TREATISE ON METAMORPHISM. 



the openings are fine and close, as in the case of fissility, the process 

 extends downward uniformly and much more rapidly than where this struc- 

 ture is absent. 



As examples of very fine- and close-grained rocks which weather less 

 rapidly than other rocks of similar chemical and mineral composition may 

 be mentioned the Berlin rhyolite-gneiss and the Niagara limestone of Wis- 

 consin. The former has a maximum crashing strength of 3,304 kg. per 

 sq. cm.' and the latter a crushing strength of 2,812 kg. per sq. cm." These 

 are probably the highest crushing strengths yet recorded for such rocks. 

 While altogether unlike in composition and origin, they are alike in their 

 remarkable uniformity, fineness, and closeness of grain. Many of the 

 marbles are not more porous than the limestone mentioned, but the mineral 

 particles are much larger, and in this difference probably lies the explanation 

 of the very unusual strength and slow weathering of this limestone. 



From the above there follows the conclusion that in proportion as the 

 aggregation is coarse and open the rate of weathering is rapid; in propor- 

 tion as it is fine and close the rate of weathering is slow. These general 

 statements need modification with reference to disintegration through 

 freezing and thawing (see pp. 441-442). 



THICKNESS OF THE BELT OF WEATHERING. 



Since the entire belt of weathering is affected by the reactions charac- 

 teristic of that belt, the greater the thickness of the belt the more effective 

 the process of weathering. It has already been explained that, other things 

 being equal, the level of ground water is likely to be a considerable depth 

 below the surface in proportion as the elevation is great. It therefore 

 follows that the rate of weathering is greater in elevated than in low regions. 

 This is partly a direct consequence of the fact that the weathering agencies 

 have a greater volume of material upon which to work. But probably 

 even more important than this is the rapid removal of weathered material 

 in elevated regions. Material disintegrated and partly decomposed by the 

 weather is removed by erosion as rapidly as formed, and therefore contin- 

 uously exposes new surfaces to the agencies of weathering. 



"Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat. 

 Hist. Survey No. 4, 1898, pp. 360-363. 



