IOWA ACADEMY OF SCIENCE 103 



even of fluviatile origin, have every appearance of being mainly eolic in 

 character. Although thickest near the Rocky Mountain front these 

 deposits in gradually lessening volume doubtless extend eastward be- 

 yond the Mississippi river. In the Caspian district, in the northern China 

 region, along the Nile, on the west coast of northern Africa, and on the 

 Pacific coast of the northern half of South America similar conditions 

 appear to prevail. 



A third class of deposits are those representing wind-blown dusts which 

 are laid down in the seas bordering desert lands. The origin of the vast 

 boraciferous formations of Tertiary and Quaternary ages occurring in 

 the Santa Clara valley, the Mojave basin and Deatli valley of southern 

 California has been thus explained. These deposits are from 5,000 to 

 8,000 feet in thickness. They now occupy an unbroken belt 100 miles 

 wide and more than 300 miles long ; representing the position of a great 

 shallow arm of the Pacific ocean. Playa deposits are another and more 

 ephemeral phase of wind-blown desert dusts accumulating in very shal- 

 low bodies of water. 



A notable example of the work of the process as it is going on at the 

 present time is in the region about the Gulf of California. In this great 

 valley, once occupied by the sea northward to the latitude of the Death 

 valley, deposition has been continuous ever since Early Tertiary times. 

 The great thickness of the so-called "river wash," from 1,200 to 2,000 

 feet deep, reported from many of the intermont plains of southern Ari- 

 zona and southeastern California is, in all probability, mainly the un- 

 consolidated deposits of the character mentioned, mingled at various 

 horizons with the coarse materials of arroyo out-wash. With this view 

 in mind these "plains deposits" need full investigation anew. Undis- 

 turbed by local water-work typical desert plains appear generally to 

 have rock-floors at no very great depths beneath the surface of the 

 ground. 



As shown more fully in another place such deposits as the Mojave des- 

 ert boraciferous clays appear to afford quantitative data for the deter- 

 mination of the duration of eolic erosion and the corref^ponding extent 

 of proposition. 



