280 TEANSPOETATION OF BOCK DEBRIS 



face, proving ineontestably that, with the exception of the 

 small amount of organic matter that had since been added, 

 not an ounce of the soil was truly native, but all of foreign 

 birth, and a mongrel creature of compulsory migration. We 

 shall dwell more fully upon the character and distribution of 

 these soils later. The single illustration above given will 

 answer present purposes. 



In a less degree the ice along the shores of lakes and rivers 

 may exert a transporting influence. Thus the ice first formed 

 along the shores encloses sundry pebbles, boulders, and sand. 

 Through the expansive force of the freezing water as the entire 

 surface becomes frozen over, this shore ice, together with its 

 enclosures, may be pushed up some distance beyond the water 

 line, where the included debris is deposited on melting. Or, 

 on the breaking up of the ice in the spring, the shore ice may 

 be drifted to other parts of the lake, or down the stream, per- 

 haps for miles before melting sufficiently to cause it to deposit 

 its load. 



(3) Action of Wind.^ — "While abrasion by the wind is im- 

 possible without transportation, the converse is by no means 

 true ; indeed it is as an agent of transportation for rock detritus, 

 without appreciable abrasion, that the wind accomplishes its 

 greatest work, though in like manner this phase is most manifest 

 in arid regions. 



It is stated by Darwin that for several months of the year 

 large quantities of dust are blown from the northwestern shores 

 of Africa into the Atlantic over a space some 1600 miles in 

 width and for a distance of from 300 to 600 and even 1000 

 miles from the coast. During a stay of three weeks at St. Jago 

 in the Cape Verde Archipelago, this authority found the atmos- 

 phere almost always hazy from the extremely fine dust coming 

 from Africa and falling upon the land and water for miles 

 around. So abundant was this dust that a distance of between 

 300 and 400 miles from the coast the water was distinctly colored 



^ See article on Erosion performed by tlie Wind, by Professor J. A. Udden, 

 Journal of Geology, Vol. II, 1894, p. 318. Attention is here called to the 

 fact that the speed of the wind upon which its power of transportation 

 depends is lowest near the ground, and hence that materials to be trans- 

 ported any great distance, at any one time, must be lifted through this 

 zone of low velocity. Professor XJdden estimates that to he subject to trans- 

 portation hy ordinary strong winds mineral particles must be comminuted 

 to not above one millimeter in diameter. 



