98 



PHYSICAL GEOLOGY 



FIG. 90. Dunes in Colorado Desert, Cal. 

 (Fairbanks.) 



unfilled portions become depressions without outlets. Such 

 depressions may be occupied by marshes and temporary ponds 

 and lakes. 



Taking the world as a whole, the amount of sand deposited 



by the wind, but not 

 forming distinct ele- 

 vations, probably far 

 exceeds that in dunes. 

 -'-"' Eolian sandstone. 



Wind-laid sand 

 may be cemented 

 into sandstone, 

 though this is less 

 likely to occur than 

 in the case of water- 

 laid sand (Why?). 

 Sand subjected to long-continued action by the wind consists 

 chiefly of quartz, for the softer minerals have usually been 

 reduced to dust and blown 

 away. The sand grains 

 have often been worn to 

 small size. Water-laid 

 sand is carried in suspen- 

 sion more or less, and so 

 subjected to less wear; its 

 particles are therefore 

 likely to be larger. Its 

 composition, too, is more often mixed. The origin of a sand- 

 stone may be revealed also by its bedding, by the fossils it 

 contains, and in other ways. It has been possible to determine 

 that the sand of even very ancient sandstones was deposited 

 by the wind. 



Loess. The wind has been concerned in the deposition 

 in certain regions of loess. This is a silt, often buff-colored, 

 that is intermediate in coarseness between sand and clay, and 

 whose particles are remarkably uniform in size. Extensive 



FIG. 91. Dunes on the coast of north- 

 ern Denmark. (Engsig.) 



