44 



GEOLOGY. 



I. Temperature Effects. 



When the sun shines on bare rock its surface is heated and expanded, 

 and the expanded particles crowd one another with great force. Since 

 rock is a poor conductor of heat its surface is heated and expanded 

 notably more than parts beneath the surface. It follows that strains 

 are set up between the expanded outer portion and the cooler and less 

 expanded parts Avithin. In the cooling of the same rock mass it is the 

 outermost portion which cools first and fastest, and, contracting as 

 it cools, strains are again set up between the outer part, which is cooled 

 more, and the inner part, which is cooled less. The result may be illus- 

 trated by the effect of cold water on hot glass, or of hot water on cold 

 glass. In either case the fracture is the result of the sudden and con- 

 siderable differential expansion or contraction. Since the heating and 

 coohng of rock are much slower than the heating and cooling of glass 

 under the conditions mentioned, the rupturing effects are less conspicu- 

 ous, but none the less real. The actual effects of temperature changes 

 are illustrated by famihar phenomena. The surface portions of bowlders 

 exposed to the sun are frequently seen to be shelling off (Fig. 26). The 



Fkl 26.- 



-Exfoliation. A bowlder of weathering, the rock being granite. 

 Mountains, Oklahoma. 



Wichita 



loosened concentric shells may be a fraction of an inch, or sometimes 

 even several inches in thickness. This process of exfoliation affects 

 not only bowlders, but bare rock surfaces wherever exposed to the sun 

 (Figs. 27, 28). It is often conspicuous on the faces of chffs. 



