54 



PHYSICAL GEOLOGY 



of such dust, in some regions to a depth of iooo to 2000 feet. The fertility of the 

 soil of these regions is remarkable. Although cultivated for many thousands of years 

 without artificial fertilizer, it still retains its fertility. This is due largely to the con- 

 stant supply of new dust from the desert. It is stated that the limit of the loess 

 practically marks the extreme limit of the extension of Chinese agriculture and com- 

 merce. (Richthofen.) Large areas in the United States (p. 657) and in Argentina 

 are also covered with loess, and in all such regions grass and grains flourish, although 

 trees are usually few. The principal deposits of loess in the United States were de- 

 rived from the fine material of glacial deposits which were caught up by the winds 

 during the dry phases of the interglacial periods (p. 657). 



Loess has the property of maintaining a vertical face when cut through artificially 

 or by streams. In China the roads of the loess region are often in nearly vertical, 

 walled canyons (Fig. 31), many feet below the surface, having been deepened by the 

 blowing out of the dust of the traveled road. On either side of these roads cave houses 

 have been excavated and furnish homes for many thousands of people. 



Dust is obtained by the winds from sources other than desert sand, 

 such as fine volcanic ash, solid particles of smoke, pollen of flowers, 

 and spores of plants. The amount of material thrown into the air 

 during volcanic eruptions is enormous. The volcano Krakatao in the 

 East Indies, for example, in 1883 threw volcanic dust to a height of 

 several miles, which in fifteen days had encircled the globe. So 

 abundant was the dust in the air that for many months after the 

 eruption the sunsets were remarkably brilliant. In Kansas and 

 Nebraska there are deposits of volcanic dust, locally 30 feet thick, 

 which had their source in ancient volcanoes hundreds of miles 

 away. 



REFERENCES FOR THE WORK OF THE WIND 



Beadnell, H. J. L., — Sand Dunes of the Libyan Desert: Geog. Jour., Vol. 35, 1910, 



PP- 379-395- 

 Cobb, C, — Where the Wind Does the Work : Nat. Geog. Mag., Vol. 17, 1906, pp. 310- 



317. 

 Davis, W. M., — The Geographical Cycle in an Arid Climate: Jour. Geol., Vol. 13, 



1905, PP- 381-407- 

 De Martonne, E., — Geographie Physique, 1909, pp. 649-672. 

 Free, E. E., — The Movement of Soil Material by Wind: Bull. U. S. Bureau of Soils, 



No. 68, 191 1. 

 Geikie, J., — Earth Sculpture, 1898, pp. 250-265. 

 Haug, E., — Traite de Geologie, 191 1, pp. 387-403. 



Hobbs, W. H., — Earth Features and their Meaning, 1912, pp. 197-222. 

 Huntington, Ellsworth, — The Pulse of Asia, 1907. 

 Keyes, C. R., — Relation of Present Profiles and Geologic Structures in Desert Ranges: 



Bull. Geol. Soc. America, Vol. 21, 1910, pp. 543-563. (Dr. Keyes holds extreme 



views on wind erosion.) 



