RAPIDITY OF ROCK-WEATHERING. 361 



ground than though it was poured down during the comparatively brief 

 period of a shower. How far these agencies may go toward counter- 

 balancing the effects of the continued higher temperatures of the tropics 

 we have no means of judging.* 



It is even questionable if decomposition has actually gone on to greater 

 depths in regions covered by forests, as contended by Hartt,t than else- 

 where. The accumulation of a large amount of organic matter is un- 

 doubtedly favorable to decomposition, but the growing vegetation con- 

 stantly robs the atmosphere of carbonic acid and the soil beneath of 

 moisture and other elements necessary for its groAvth, storing them away 

 in the form of woody fiber or sending them off into the atmosphere once 

 more. The amount of moisture that a full-grown tree evaporates daily 

 through its leaves is simply enormous, and is often made conspicuously 

 apparent b}'' the dry knolls which may be seen surrounding isolated 

 trees or groups of trees in swampy areas. J 



The apparent amount of decomposition in wooded areas may be greater, 

 simply for the reason that the ground is protected thereby from the erosive 

 action of running water. 



It is my present belief that the opinion regarding the more rapid rate 

 of degeneration in warm climates and forested areas is founded on no 

 other basis than the visible accumulation of rock debris, and that this 

 accumulation is rendered possible through the protective action of plant 

 life, which is naturally more profuse in warm than in cold climates. 



That, however, there may be a difference in Jcind, in the degeneration, 

 in warm and cold climates, or at least in moist and dry climates, is possi- 

 ble 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 degeneration is first almost wholly in the nature of disin- 

 tegration, a process of disaggregation whereby the rock is resolved intot< *" 

 first, a gravel, and ultimately a sand composed of the isolated mineral 

 particles which have suffered 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 peninsula.§ In warm, 

 moist climates 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 



* There is an old saying among eastern farmers to the effect that a late spring snowstorm is as 

 good as a dressing of manure. It undoubtedly arose from an appreciation by the farmers of the 

 fact that the snow was more beneficial than rain, for the reasons above mentioned. 



+ Geology and Physical Geography of Brazil. 



J It is stated that a grove of 500 full-grown healthy trees emit during every 12 hours of daylight 

 4,000 tons of moisture. (H. de Varigny : The Air and Life. Eeport Smithsonian Institution, 1893.) 



§ Bull. Geol. Soc. Am., vol. 5, 1894, p. 499, 



,*' 





