WEATHEKING IN COLD AND WARM CLIMATES 269 



passes through this snperficial blanket of mould that can be 

 instrumental in promoting rock decomposition. Hence the 

 presence of such a blanket may exert a protective, or at least 

 conservative, rather than destructive action. Further than this, 

 we have to remember that plant growth tends to reduce the 

 extremes of temperature and, even more, to diminish evapora- 

 tion from the immediate surface. The constant action of gravity 

 and capillarity in pumping the water down and up through 

 the soil is therefore largely diminished. Since it is by tempera- 

 ture changes and water action that decomposition is so largely 

 brought about, it is apparent that one must not be too hasty in 

 assuming that forest action is actually destructive; it may be 

 largely conservative. It is probable that the apparent amount 

 of decomposition in wooded areas is due to protection from ero- 

 sion, and the consequent accumulation of the residuary material. 

 (8) Difference in Kind of Weathering in Cold and Warm 

 Climates. — That there may be a difference in kind in the de- 

 generation in warm and cold climates, or at least in moist and 

 dry climates, is possible and even probable.^ In cold and in 

 dry climates subject to extremes of temperature, as in the arctic 

 regions or in the arid regions of lower latitudes, the weathering 

 is at first almost wholly in the nature of disintegration, a process 

 of disaggregation whereby the rock is resolved into, first, a gravel 

 and ultimately a sand composed of the isolated mineral particles 

 which have suffeied scarcely at all from decomposition. The 

 writer has elsewhere referred to this form of degeneration as 

 manifested in the desert regions of the Lower Californian penin- 

 sula.^ In a warm, moist climate chemical decomposition may 

 or may not keep pace with the disintegration, according to local 

 conditions, so that the resultant material may be in the form of 

 an arkose sand, as in the District of Columbia, or a residual 

 clay, as in the more superficial portions of the residual deposits 

 to the southward. In certain cases, or among certain classes of 

 rocks, the decomposition proceeds at so rapid a rate that there is 

 scarcely any apparent preliminary disintegration. Local cir- 

 cumstances and character of rock masses being the same, we are, 



^The majority of writers have failed to discriminate between decomposi- 

 tion and disintegration. That there may be a very marked difference, due 

 mainly to climatic conditions, is the point I wish to emphasize here. See 

 also Walthers, Denudation in der Wiiste, p. 22. 



2 BuU. Geol. Soc. of America, Vol. Y, 1894, p. 499. 



