278 Ice and its Natural History 



liberated by water freezing in its interstices. I know of no 

 detailed description of the process, nor have I met or read 

 of any one who has actually witnessed a rock being split in 

 this way. 



Discontinuities in the rock are postulated in order to 

 admit the water which is to be frozen, but no detailed speci- 

 fication is furnished of how the opening is to be closed and 

 the freezing is to be effected after the water has entered. 

 But it has been shown above that if the water gains admission, 

 it will in time disintegrate the rock by chemical action, and 

 gravity without assistance will complete the degradation. 

 Therefore freezing is unnecessary in order to account for 

 the facts. Moreover, the covering of a mountain top by 

 fragments of its own rock is a common occurrence in lati- 

 tudes where frost is rare or absent. 



In discussing the natural history of ice, it has been 

 necessary to some extent to include that of ice and steam. 

 In its physiographic relations we have found that ice is as 

 efficient a preserver of mineral matter as it is of vegetable 

 or animal matter in the everyday relations of life. We have 

 also found that the substance which chemists indicate by 

 the symbol H 2 O has the most destructive action on mineral 

 matter when it has passed from the gaseous state in the 

 atmosphere to the liquid state on the surface of the earth, 

 and when this takes place in regions where the atmospheric 

 temperature is high. Chemical decomposition accompanied 

 by disintegration is the result, and it is followed by degrada- 

 tion 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 gra- 

 vitational 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 ex- 

 cludes "equatorial." The equatorial climate is not only moist 

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

 gravitational degradation, but its primary effect is profoundly 



