48 GEOLOGY. 



tion and slow conduction favor disruption. A given range of tempera- 

 ture is unequally effective on rocks of different mineral composition. 

 In general crystalline rocks (igneous and metamorphic) are more 

 subject to disruption by this means than sedimentary rocks, partly 

 because they are more compact, but especially because they are made up 

 of aggregates of crystals of different minerals which, under changes 

 of temperature, expand and contract at different rates, while the 

 common sedimentary rocks are made up largely of numerous par- 

 ticles of one mineral. 



Fig. 31. — Peak north of Kearsarge Pass, the Sierras. Shows the way in which ser- 

 rate peaks break up into angular blocks. 



The freezing of water in the pores of rock is effective in disrupting 

 them only when the pores are essentially full at the time of freezing. 

 Otherwise there is room for the expansion attending the freezing. If 

 the pores of the rock are large, the expansion on freezing may force 

 out sufficient water to balance the increase of volume, even though the 

 rock was completely saturated. If the pores be very small the water 

 passes out less readily, and if the rock is saturated, freezing is more 

 likely to be attended with disruption.^ 



^ Buckley. Wisconsin Survey, Bull. IV, 1899, pp. 81-3. 



