270 Ice, and its Natural History. [May 8, 



place in regions where the atmospheric temperature" is high, chemical 

 decomposition accompanied by disintegration is the resnlt, and it is 

 followed by degradation under the all-pervading influence of gravity. 

 This is the primary process in the wasting of land substance, and we 

 have called it chemical and gravitational degradation. The conditions 

 which favour the exhibition of chemical and gravitational degradation 

 are high temperature, abundance of moisture, and scarcity of water. 

 These conditions are met with together in the tropical regions of both 

 continents. The word " tropical " is here used in its restricted sense 

 and excludes " equatorial." The equatorial climate is not only moist 

 but very wet. It also is productive of energetic chemical and gravi- 

 tational degradation, but its primary effect is profoundly modified by 

 the secondary effect of running water. This modification is equiva- 

 lent to intensification, in so far as the running water, by removing the 

 products of decomposition, and thus denuding the rock, exposes fresh 

 sm-faces to the decomposing agency. 



In its simplest and most perfect form the effect of chemical and 

 gravitational degradation, which has been active through all the ages 

 that a distinctly tropical climate has existed, is to be witnessed on the 

 prairie and the pampa. 



Here the wayfarer finds himself on a surface as featureless as the 

 ocean, but equally spherical ; and he may use his horizon for the 

 purpose of finding his position astronomically with almost as mucli 

 confidence as he would at sea. 



Water finds its level quickly ; sea ice, slowly ; land ice, more slowly ; 

 and land, in a time which, though very great, is still far from infinite. 



[J. Y. B.] 



