RELATIVE EAPIBITY OF WEATHERIKG 265 



writer, he further states that the diabases of the region are 

 fully as much decomposed as are those in the Adirondacks of 

 Kew York, and that the blocks of eruptive rocks occurring in 

 the moraines of Muir Glacier are far gone in decomposition. 



Mr. C. W. Purrington has made similar observations, and 

 states^ that on the south side of Silver Bow Basin, some three 

 miles west of Juneau, at an elevation of 2000 feet above sea- 

 level, he found schistose diorites disintegrated over a consider- 

 able area to a depth of 20 feet. The particular locality cited 

 was on a mountain slope, where landslides were frequent, and 

 other conditions prevailed such as would prevent the accumula- 

 tion of the debris throughout a prolonged geological period or 

 to a very great depth. There could be, however, no doubt as 

 to the residuary character of the material observed, and the 

 inference drawn was to the effect that the disintegration had 

 taken place within a comparatively brief space of time. G. E. 

 Culver has also described^ a diabase dike in Minnehaha County, 

 South Dakota, an arid region lying within the glaciated area, as 

 decomposed throughout the whole exposures from its upper 

 surface down to a depth of 20 or 25 feet, the limit of disinte- 

 gration being the drainage level of the region as marked by 

 the bed of a stream cutting through it. 



On the other hand. Professor I. C. Eussell, who has devoted 

 much attention to the subject of rock-weathering in both high 

 and low latitudes, is of the opinion that rock decay is a direct 

 result of existing climatic conditions. He states that decay goes 

 on most rapidly in warm regions where there is an abundant 

 rainfall, and is scarcely at all manifest in arid and frigid 

 regions.^ Professor Russell's observations are of more than ordi- 

 nary value, since he has discriminated between decay and dis- 

 integration, which most writers have failed to do. 



Since climate is dependent upon altitude as well as latitude 

 the relative rapidity of weathering in mountain regions and those 

 near sea-level is worthy of consideration. The sharp contrasts 

 of temperatures on mountain peaks bring about excessive ex- 

 foliation and disintegration, as has been noted by every traveler 

 in high altitudes. Indeed it has been suggested, I believe by 

 Penck, that the actual average height of mountains is limited 



1 Surface Geology of Alaska, Bull. Geol. See. of America, Yol. I, 1890. 



2 Wisconsin Academy of Sciences, Art, and Literature, 1886-91, p. 206. 



3 Personal Memoranda to the writer. 



