330 STUDIES FOR STUDENTS 



age, as Passarge has shown, a thin layer of sandy or gravelly waste^ is 

 almost universal. 



Besides these features, chiefly made or modified by the work cf 

 water, it is well known that wind transportation plays an important 

 part in desert erosion and deposition. Immense stony w^astes, as the 

 belt of the Sierran Hamada, may in this way have all soil removed, 

 the finer dust being carried to great distances and ultimately cut of 

 the desert region, the sand being swept in the form of dunes over 

 great areas of country; the dunes themselves, frequently hundreds of 

 feet in height, being but the upper, wind-tcssed portion of a deep 

 deposit cf sand. From the study of the surface of arid regions it 

 would seem that a conservative estimate would arrive at the conclu- 

 sion that at least one-half of the desert areas, and consequently one- 

 tenth of the land surface of the world, is covered with more or less 

 important deposits of recent desert accumulations, only a small por- 

 tion of which are characterized by salt and gypsum. In interpreting 

 desert conditions from the sedimentary record of previous ages, red- 

 ness of formations, indicating subaerial oxidation, and roundness of 

 sand grains, as indicating aeolian action, and other features, are some- 

 times used; the presence of salt and gypsum is, however, the only 

 characteristic which is determined at a glance and considered as a 

 positive indication of an arid climate, and the only feature which is 

 commonly used. But from the small proportion of present desert 

 areas which are characterized by these deposits, and the great amount 

 of land surface which is now desert, it would seem that the problem of 

 ancient desert deposits should enter much more largely into geological 

 history than is usually appreciated. 



PIEDMONT RIVER DEPOSITS 



Piedmont river deposits are built up in front of young and lofty 

 mountain regions removed from the sea, by torrential rivers, which 

 on escaping from the mountains are leaded with waste which they are 

 unable to carry acrcss the gentler slopes of the plains. As a region 

 where such work is actively in progress at the present time m.ay be 

 cited the Pampas of northern Argentina. An early and excellent 

 account cf this region is given by John Miers, writing in 1825, who, 



I Review by W. M. Davis in Science, N. S., Vol. XXI (1905), pp. 825-28. 



