16 DYNAMICAL GEOLOGY. 



integration. Water penetrating the joints, and freezing, 

 expands with such force that the rocks are riven asunder ; 

 and then, penetrating again into the capillary fissures 

 and freezing, these blocks are in their turn broken into 

 smaller fragments, until the whole crumbles to dust. 



Wind. Again, loose earth, sand, and dust, especially in 

 dry climates, are carried by winds, and sometimes accu- 

 mulate in large quantity and form a peculiar soil. Thus, 

 the sands of Sahara are in some places encroaching on 

 the fertile lands of Egypt. Thus, also, sea-sands are 

 often carried inland from shore, and cover up and destroy 

 fertile lands. The sand-hills to the west of San Fran- 

 cisco are made in this way. The phenomena of sand- 

 dunes may be observed in many places along the coasts 

 of nearly all countries. Some geologists think that in the 

 interior of dry countries, like Asia or the western part of 

 our own country, soil of great thickness has been formed 

 by accumulation of dust. 



