58 PRINCIPLES OF STRATIGRAPHY 



uncommon in these sandstones where some parts are very friable. 

 (Cross-2i : pi. 4, fig. 2.) Hollows of this type are very common in 

 sandstones, especially where concretionary segregation of this iron 

 oxide has left some of the sands in an unconsolidated condition. 

 (See Chapter XVII.) Deflational erosion of rocks in moist tem- 

 perate regions is shown by the classical example of the Heidelberg 

 Schloss. The western tower of this castle, built in 1533 and partly 

 blown up in 1689, shows a narrow passageway, through which the 

 wind blows with considerable force. The wall of this passage is 

 built of heavy sandstone blocks, some of which have been hollowed 

 out for a depth of 10 cm., entirely by the blowing away of the 

 disintegrated sand grains, the situation being such as to exclude 

 corrasion. 



Eolian erosion over wide areas is well shown in the Kalahari 

 Desert of South Africa, where, according to Passarge, extensive 

 plains have been produced by this agent, the amount of deflational 

 work being often indicated by the butte-like 'Tnselberge" still ris- 

 ing above the plain as unconsumed remnants. (Bornhardt-8 rj/.) 

 A similar origin has been advocated for a part at least of the desert 

 plains or "bolsons" of southwestern United States. (Hill-50; 

 Keyes-56.) That some of the extensive depressions of the earth's 

 crust occupied by deserts owe their character to deflational work of 

 the wind seems quite certain. In the Inselberg-landscape of South 

 Africa the country rock is a crystalline rock, mostly biotite gneiss, 

 biotite muscovite gneiss, garnet gneiss, and amphibolite. The resid- 

 ual peaks are harder rocks, chiefly pegmatite dikes and other rocks 

 high in quartz. The individual Inselberge are separated by dis- 

 tances varying up to 25 km. and over ; the differences in height of 

 various parts of this plain are not great, 80 meters having been 

 found in a distance of over 150 km. Sometimes the floor of the 

 plain seems as smooth as a table top, the minor depressions being 

 filled in by eolian deposits. Near the Inselberge, sometimes at their 

 very bases, are heavy masses of talus, consisting of the products of 

 recent rock decay, inaugurated probably by a change in climate to 

 a more pluvial one. (Hecker-46.) 



Distance of Eolian Transportation. Only the finest dust par- 

 ticles remain more or less permanently suspended in the air, or imtil 

 washed out by rains. Most of the wind-borne particles travel by 

 means of leaps and bounds of varying magnitude. The length 

 of the leap depends upon the size and shape of the particles, the wind 

 velocity, and to some extent on the topography of the country. 

 From a number of analyses of wind-transported material Udden has 

 constructed the following table of approximate maximum distances 



