198 HISTORY OF GEOLOGY AND PALEONTOLOGY 



According to Tietze (1886), the debris in the sterile rainless 

 mountain-territories of Persia is chiefly produced by the dis- 

 integrating influence of insolation. Again, in the region above 

 the snow-line in mountain-chains, the accumulations of debris 

 are attributable to the daily alternation of frost and warmth; 

 but in most areas the strain is occasioned not so much by the 

 rapid change of temperature as by the presence of water in 

 the fine rock-fissures, and the pressure exerted during alternate 

 freezing and evaporation of the water. 



The geological effects of the wind are of importance. 

 Neglecting here the disturbances caused by hurricanes, many 

 striking phenomena have been traced to the influence of 

 wind-borne sand or dust. As early as 1847, Naumann 

 described polished and furrowed rocks near Hohburg, in 

 Saxony, and erroneously ascribed the appearance to the 

 action of ice. Heim in two papers, in 1870 and 1874, 

 showed that the markings on the rocks had been pro- 

 duced by wind-swept grains of dust and sand. Similar 

 wind scratches had been mentioned by Blake in 1855, and 

 by Gilbert in 1874, from the western states of America. 

 Zittel, Rolland, Walther, and others have reported how fre- 

 quently one may observe wind-worn rocks in the Sahara 

 with a polished glassy surface, dotted with cavities, or deeply 

 scored and fluted. 



Other phenomena of a more imposing nature in the great 

 desert wastes and steppes owe their origin to the wind. In 

 the Monument Park of Colorado, the numerous picturesque- 

 looking rocky pillars with a narrow basis have been explained 

 by Gilbert as remnants left by wind-weathering. The clouds 

 of dust borne along by the wind attack chiefly the lower levels 

 of the pillars, and reduce these so that the top-heavy upper 

 portions are gradually undermined. Similar appearances in 

 the Arabian desert have been described by Fraas, who called 

 them " Fur-cap Rocks," on account of their characteristic form; 

 Walther called them "Mushroom" rocks. More recently, 

 " three-cornered" rocks in the dunes and steppes of Northern 

 Europe, in the Rhone Valley, in the neighbourhood of Vienna, 

 and other European localities, have been attributed to the work 

 of the wind. In the Sahara, pillars or table-like eminences 

 have been undercut by wind-borne dust and sand, and remain 

 as "island rocks" the so-called "gurs of the desert." Long- 

 continued action of the wind may hollow out basin-shaped 



