52 PHYSICAL GEOLOGY 



great series of white dunes, 60 to 100 feet in height, covering an area 

 15 miles by 40 miles in extent. Dunes are formed also of fine clay, 

 as well as of disintegrated granite sand. 



Height of Dunes. — The height of dunes in regions where the direc- 

 tion of the wind is fairly constant is seldom more than 300 feet, but 

 in such deserts as the Sahara, where the wind varies from season to 

 season, the height may reach 1500 feet. In the latter case, the dunes 

 do not migrate, and their greater height is due to the piling up of the 

 sand from different directions. 



Eolian Sandstone. — Extensive strata of sandstone of very ancient 

 date are known to have been formed of wind-blown sand. Rocks of 

 this origin can often be distinguished from the sandstones laid down 

 on the ocean bottom. The following differences assist in recog- 

 nizing the source of the original deposits. (1) The former consist 

 chiefly of quartz, the softer minerals having been worn to dust and 

 carried away, while in the latter the softer and harder minerals are 

 more likely to occur together. (2) Since water-laid sands are carried 

 in suspension, they are subjected to less wear than eolian sands, as 

 the water between the particles acts as a cushion, and the grains are 

 consequently less worn and more angular than the sand grains which 

 have been buffeted by the winds. (3) The stratification of eolian 

 sand (Fig. 26, p. 48) usually exhibits cross or false bedding (p. 47), 

 i.e., it is not horizontal but varies greatly in inclination and direction 

 within short distances. (4) Wind-blown sands may also be dis- 

 tinguished from marine sandstones by the character of the fossils, if 

 such exist. 



Dust. — As sand grains are borne to and fro by the wind, striking 

 against each other or against rock surfaces, the softer grains are 

 reduced to dust, and even the harder ones may finally reach a similar 

 state. The dust thus formed is carried by air currents, often to great 

 distances. In a single storm in 1901 it is estimated that 1,960,420 tons 

 of dust were carried from the Sahara desert to Europe, reaching Italy 

 on the second day of the storm, and Germany and Denmark on the 

 fifth day. It is probable that every square mile of the earth's surface 

 has dust upon it from every other square mile. Even the snows of 

 mountain glaciers and those of the Arctic and Antarctic regions con- 

 tain dust, carried to them from lands hundreds of miles away. 



Loess. — One striking result of the transportation of dust by winds is that regions 

 \o the leeward of deserts are constantly receiving dust which settles gradually upon 

 them. Such a deposit of fine dust is called loess. The fine dust is carried by the 



