32 GEOLOGY 



ing off" is frequently seen in the bowlders of the field (Fig. 13) and 

 on the slopes of the hills and mountains (Fig. 14). Quick and 

 great changes of temperature are more important than slow and 

 slight ones in rock breaking. Thus annual changes are of little 

 consequence, as compared with daily changes. If the changes of 

 temperature involve a range above and below the freezing point 

 of water, they may be still more effective, for if the pores and 

 cracks of the rock are full of water, its expansion on freezing may 



Fig. 14. Exfoliation on the slope of a granite mountain near Royal Arch 

 Lake, Yosemite Quadrangle. (Turner, U. S. Geol. Surv.) 



break the rock. Great changes of daily temperature are found 

 especially in high and dry regions, and it is in such places that rock 

 breaking due to changes of temperature is most effective. Swift- 

 streams and waves, especially when they carry rock fragments, 

 may also break up igneous rock which is so situated as to b< 

 attacked by these agents of destruction, and the growth of roots 

 in cracks may contribute to the same end. All mechanical dis- 

 ruption of igneous rock leaves the fragments essentially like iho 

 original rock in composition. 



Chemical disintegration. The silicate minerals which make 

 up the larger part of all igneous rocks are usually complex, choini- 



