496 A TREATISE ON METAMORPHISM. 



exceedingly minute, the surface exposed to action is enormous. As illus- 

 trating this, Whitney has calculated that there are 22 billion grains of sand 

 and clay in a gram of residual subsoil from a limestone, and that the surface 

 of these particles has an area of 5,000 square centimeters. This is an 

 extreme case, but Whitney estimates the number of grains in ordinary soils 

 to vary from 2 billions to 15 billions per gram." 



In an early stage of the process of subdivision disintegration usually 

 plays the dominant part. As the material becomes finer chemical action 

 becomes more important. When the particles become minute, chemical 

 action is dominant in producing subdivision. As shown on pages 432-434, 

 it may well be doubted whether the exceedingly minute subdivision of the 

 residual clays from limestones could be accomplished by the mechanical 

 agents of weathering alone. 



It has been seen that the mechanical work of weathering is accom- 

 plished by wind, water, ice, change from water to ice, change in tempera- 

 ature, plants, and animals. The chemical work is accomplished by plants, 

 animals, and solutions. The various factors in this mechanical and chemical 

 work interlock in a most complex fashion. An increase in the amount of 

 work of one of the factors may increase or decrease the amount of work 

 of another factor. Moreover, the kind of work accomplished by one factor 

 is so different from that of another factor that it is difficult or impossible 

 to make quantitative comparisons between them. Thus it is impossible to 

 compare the mechanical work of disintegration with the chemical work 

 of decomposition. 



The particular combination of agents and forces most actively at work 

 at a given place is largely dependent upon humidity, latitude, elevation, 

 and life. There are very numerous permutations and combinations of these, 

 so that the number of variations in the rate of weathering are indefinitely 

 great. 



REGIONS FAVORABLE TO PROMINENCE OF DISINTEGRATION. 



The regions favorable to prominence of disintegration are those of (a) 

 aridity, (b) high latitude, (c) marked topographic relief, (d) sparseness of 

 plants and animals, and (e) nearness to the sea. 



Arid regions. — It has already been seen (pp. 438-439) that in arid regions 

 the mechanical stresses due to rapid change in temperature and to the wind 



a Whitney, Milton, The soils of Maryland: Bull. Maryland Agr. Exp. Sta. > T o. 21, 1893, pp. 8, 33-57. 



