RAPIDITY OF ROCK-WEATHERING. 359 



and also to the character of the alteration which had gone on prior to and 

 contemporaneous with degeneration. 



I have often noted, both in eastern Massachusetts and elsewhere, that 

 in cases where the mineral alteration had given rise to epidote and free 

 quartz, as is the case in many of the diabasic dikes hereabouts, they resist 

 decomposition even more successfully than do the granitic and other 

 rocks by which they may be inclosed. Where, on the other hand, the 

 alteration yielded mica, chloritic and zeolitic compounds, degeneration 

 almost invariably ensues. 



Relative Rapidity of Rock-weathering in high and low Latitudes. 



An impression is still to some extent prevalent to the effect that rocks 

 decompose more rapidly in warm and moist than in cold climates. This 

 is shown in the writings of Kerr, Stubbs, Storer, Branner and others. 

 While, owing to abundance of vegetation and other supposed favorable 

 conditions, a more rapid decomposition may possibly be expected, such 

 has not as yet been proven to actually take place, and indeed many 

 facts tend to prove the impression quite erroneous. Lack of decompo- 

 sition products in high latitudes is not infrequently due to glaciation or 

 erosion by other means, as has been suggested by Pumpelly.* 



Whitney ,t Chamberlain and Salisbury J have shown the presence of 

 residual clays of all thickness up to 25 feet in the driftless area of Wis- 

 consin, and Chamberlain § has described limited areas of strongly de- 

 composed gneiss in the nonglacial areas of Greenland. 



Moreover, we have no actual proof that the action of frost is on the 

 whole protective, as is stated by Branner. || It must be remembered that 

 frost penetrates to but a slight depth, and, while it undoubtedly puts a 

 temporary stop to chemical action on the immediate surface, it remains 

 yet to be shown that the mechanical disruption which there ensues is not 

 as efficacious as would have been the chemical agencies alone had they 

 been permitted to continue their work. Through bringing about a finely 

 fissile or pulverulent structure, whereby a vastly greater amount of mate- 

 rial becomes exposed, frost on and near the surface prepares the way for 

 chemical action at a thousandfold more rapid rate than could otherwise 

 have been possible.^ 



* Am. Jour. Sci., vol. xvii, 1879, p. 133. 



t Rep. Geol. Survey of Wisconsin, 1861. 



t Sixth Ann. Rep, U. S. Geol. Survey, 1884-85. 



gBull. Geol. Soc. Am., vol. 6, 1895, p. 218. 



I! Ibid., vol. 7, 1896, p. 282. 



^ That a frigid climate is much more trying on stone exposed in the walls of a building is a fact 

 apparently well established. Indeed, the action of frost and the constant expansion and eon- 

 traction from natural temperature variations are among the most potent of agencies in promoting 

 the disintegration of stones so used. See Stones for Building and Decoration, Wiley & Sons, New 

 York. 



