DEFLATION 57 



Character and Amount of DcHatioiial Denudation. On the sea- 

 shore and in the deserts all the dust and fine sand grains are blown 

 away, leaving only the coarser sands and the rock fragments. The 

 latter are characteristic of the stony desert, the Hamada type, which 

 is found in many deserts the world over. The pebbles are generally 

 smooth from wind wear, and may resemble water-laid deposits. 

 That some pavements of this kind are due to water work is be- 

 lieved to be the case by some observers (Free-33 : ji', ^"^ works 

 cited there). Not infrequently the surface of the pebbles is covered 

 with the brown desert varnish, indicating that corrasion has come to 

 an end for the time being. The desert varnish is a brown to black, 

 often highly polished coating of iron oxide or hydrate, or of man- 

 ganese oxide, which covers the stones and ledges of arid regions. 

 The amount of the coating, which is seldom more than a small 

 fraction of a millimeter thick, increases with the increase in aridity. 

 Of|;en it is developed only on the upper, exposed side of the stones 

 of the Hamada, the under surfaces remaining uncoated. Chem- 

 ically, the material forming the rind is a mixture varying from pure 

 iron oxide to nearly pure manganese oxide, with all intermediate 

 combinations. The formation of the rind begins with the absorp- 

 tion of moisture by the porous rock and the formation of various 

 compounds, through the substances present in the rock or derived 

 from without. The solutions formed are drawn to the surface by 

 capillarity, and under the intense heat of the desert they evaporate, 

 leaving the coating of oxide upon the surface. What appears to be 

 a case of desert varnish in PaLneozoic rocks is found in a layer of 

 pebbles at the base of the Siluric iron ore deposit of Wisconsin. 

 These rest upon an eroded surface of Ordovicic shale, and are suc- 

 ceeded by the iron ore. 



A marked characteristic of many desert surfaces is the wealth of 

 fossil organisms which are left behind after the disintegration and 

 deflation of the rock mass originally containing them. Even if the 

 original rock is only sparingly fossiliferous, a long-continued defla- 

 tion will result in the concentration of a considerable number of 

 specimens on the desert surfaces. (Walther-104 : jc?.) 



Extensive eolian erosion, referable almost entirely to deflation 

 of materials loosened by other agencies, is shown in the plateau 

 country of western North America. The Jurassic sandstone of 

 Utah shows this especially well, numerous isolated remnants or 

 buttes testifying to the great extent of this work. The buttes them- 

 selves often show striking features, as in the case of Casa Colorado 

 and Looking Glass Rock in Dry Valley,. Utah. In the latter a large 

 alcove or cavern has been hollowed out by the wind, a feature not 



