128 



PHYSICAL GEOLOGY 



much water, and so favor the wedge work of ice, and, where 

 some of the constituents of the rock are soluble, solution. 

 Figures 118 and 119 illus- 

 trate the influence of joints 

 upon the weathering of 

 stratified rocks, and Fig- 

 ure 120 shows the effect of 

 joints upon the weather- 

 ing of granite. Dark ob- 

 jects heat and cool more 

 rapidly than light ones, 

 and dark rocks accord- 

 ingly favor splitting 

 through changes in tem- 

 perature. (2) No single 

 type of climate favors all 

 the processes of weather- 

 ing. It was seen on page 

 100 that the wedge work 

 of ice is most important 

 in moist regions where 

 there are frequent changes in temperature which involve the 

 freezing point. The chemical work of the atmosphere 

 and of ground water is, on the other hand, most important 

 in hot, moist climates. An arid climate with great daily 

 range in temperature favors rock splitting, but opposes the 

 work of plants, animals, and ground water. If rocks are 

 not buried too deeply with soil and subsoil, they probably 

 weather fastest, everything considered, in a hot and moist 

 climate; but flat lands in regions having such climates 

 usually have thick accumulations of mantle rock. It is 

 said to reach a thickness of 300 feet or more in parts of 

 Brazil. (3) If the products of weathering remain where 

 formed, they finally cover the bed rock so deeply that it 

 is more or less completely protected from further attack. 

 If, on the other hand, they are removed as fast as 



FIG. 119. Limestone columns weath- 

 ering away. The openings between the 

 columns are enlarged joints. The sur- 

 rounding rock has been removed by ero- 

 sion. Eastern Iowa. 



What are the various ways in which 

 these rocks are being weathered ? How 

 may the preservation of these rocks 

 after the removal of the surrounding 

 rocks be explained ? 



