284 TEANSPORTATION' OP EOCK BEBEIS 



SO covered by drifted sands that at the time of the Greek 

 Xenophon. (about 400 b. c.) the very site of the once famous 

 city was unknown. Marsh^ gives the rate of movement of dunes 

 along the western coast of Jutland and Schleswig-Holstein as 

 averaging 13J feet a year, while Anderson estimates the aver- 

 age depth of the sand over the entire area as about 30 feet, 

 equalling therefore about 1| cubic miles for the total quantity. 



It is not in all cases possible to trace the drifted sands to 

 their various sources. Dunes along the sea-coasts are in nearly 

 all cases composed of materials thrown up by the waves on 

 the beaches in the immediate vicinity. This is the case with 

 those of Hatteras, Cape Cod, Gascony, Algeria, and Schleswig- 

 Holstein. But the origin of the large inland dunes, like those 

 of Nevada, is not always so clear. It has been suggested that 

 these last are formed of beach sand driven in by the prevail- 

 ing westerly winds from the Pacific coast. This is, however, 

 a matter of very grave doubt, and it seems more probable, as 

 stated by geologist Russell,^ that they were derived from the 

 disintegrating granites of the Sierras. They certainly have 

 traveled far, and are not a product of disintegration of rocks 

 in the immediate vicinity.^ 



By wind action, accompanied by the carrying power of spas- 

 modic or perennial streams, were formed the wide stretches 

 of adobe in the western United States, and according to many 

 authorities the deposits of loess which cover, as in Europe and 

 Asia, areas aggregating many square miles and have a depth, 

 in extreme eases, of 2000 feet.^ 



* The Earth as Modified by Human Action, p. 562. 



^Quaternary History of Lake Lalionton, Nevada, Monograpli, IT, S. 



Geol. Survey, 1885. 



'The sands covering the Egyptian Sphinx and Pyramids are stated to 

 have come mainly from the sea on the north, and not from the desert, as 

 is popularly supposed. Sand showers having their origin in the desert of 

 Sahara extend across the Mediterranean, and as far as northern Italy 

 (Nature, July 18, 1889, p. 286). 



*The wind plays an important part in the transportation of soils in 

 Wyoming, owing to their incoherent state, which is due to a lack of clay. 

 The arid regions of this state, which are chiefly Tertiary and Cretaceous 

 plains and tablelands, receive very little rain. Consequently the soils be- 

 come loosened, and during the dry and windy winter weather are trans- 

 ported to the broken country and distant hills and mountains in dense 

 clouds, which almost suffocate travellers. In a single season it is not an 

 uncommon sight to see banks of earth, like huge banks of snow, behind a 

 reef of rock, or in the lee of large bunches of sage brushes (IT. S. Dept. of 

 Agriculture, Ofice of Experiment Stations, Yol. V, No. 6, 1894, p. 567). 



