102 IOWA ACADEMY OF SCIENCE 



ucts may be regarded as entirely mechanically fornled, and as mainly 

 accomplished by means of the sun's rays causing great daily fluctuations 

 of temperature. Insolation is the most appropriate title for this dis- 

 tinctive phenomenon. Through means of it rock materials are prepared 

 for transportation by the winds as effectually as chemical decomposition 

 prepares them for movement by streams. The vast extent to which this 

 particular process goes on under favorable conditions is often difficult to 

 fully appreciate. 



In the consideration of desert erosion the main interest concerning 

 wind-work centers about the transportation of rock-waste and hence the 

 deflative phases absorb principal attention. 



Judging from the literature on the subject the depositional effects of 

 wind-action are geologically of small import. Even the eolian origin of 

 such deposits as the loess is still often questioned. Only upon the ephe- 

 meral sand-dunes of the sea-shore and of the deserts is there general 

 agreement of opinion as to the adequacy of wind-action to produce them. 



The erosive phenomena of the dry regions of the globe are mainly 

 viewed only within the domains of the deserts themselves. The prin- 

 cipal depositional products are not to be sought within the boundaries 

 of the arid tracts but outside of them; in the same way that the prin- 

 cipal areas for the final resting place of sediments is not looked for 

 within the limits of stream-basins but beyond in the sea, where the 

 physical conditions are entirely different. 



Contrary to general opinion typical and prevailing desert deposits 

 are not gravels and coarse materials, but sands and porous clay-like ma- 

 terials, of which lass and adobe are the most familiar representatives. 

 In size the component grains are about half-way between that of sand 

 and that of clay. Size and character of grain should be made an im- 

 portant criterion for distinguishing water-laid from wind-formed de- 

 posits. 



Areas of proposition are determined by three chief circumstances. 

 Large rivers traversing desert tracts carry much of the coarse material 

 constantly blown into them, as in the cases of the Nile, the Rio Grande 

 and the Volga river. In its final resting place little of such material 

 can be properly called an eolie deposit. 



The important eolian deposits are on the leeward side of the deserts, 

 where the exported dusts come to rest under moister climatic conditions. 

 Such deposits are probably very much more extensive than there is at 

 present time any adequate notion of. Some of the so-called fresh-water 

 tertiaries of the Great Plains instead of being either sediments of vast 

 lakes existing during periods of moister climates than at present, or 



