12 GENESIS AND DEVELOPMENT 



2.7 times heavier than water. A strong breeze is therefore required 

 to raise the dust of a road for transportation by the wind, and a still 

 stronger breeze to raise quartz sand; while large pebbles are seldom 

 lifted from the ground. The winds are also extremely irregular in their 

 movements and action. The trades over the ocean have a higher degree 

 of uniformity than other winds, but the velocity is generally only 10 to 

 20 km. an hour. The winds that do the chief part of eolian geological 

 work are those of storms, whose velocity per hour is from 50 to more 

 than 100 km. Such winds are very unsteady in their action, blowing 

 in gusts, in which there is a sudden increase to a maximum and a slower 

 decline to a minimum. There is no constancy in force even for an 

 hour, and no uniformity over large areas. 



The transporting power of water, on the contrary, is very great; 

 strong waves or torrents being able to move rocks weighing hundreds 

 of tons. By experiments it has been found that a current moving at 

 the rate of 25 cm. per second is able to carry fine sand, while a velocity 

 of 50 em. is sufficient to transport coarse gravel. The action of water 

 is, moreover, very constant as a rule, and the waves on a long coast, 

 for instance, exert their uniform influence over a considerable area. 



We must not, however, confound the transporting power of these 

 agencies, wind and water, with their erosive power. In one ease it is 

 the weight, in the other the cohesion, that offers the resistance. Neither 

 wind nor water has any greater erosive power by itself. It is where 

 mud or sand is carried by the wind or water, that a friction arises which 

 removes the particles, loosened by decaying and other processes, from 

 their original place. 



Water is efficient in denudation by 1) dissolving of rocks, 2) trans- 

 portation of the material which assists in the eroding work, and 3) car- 

 rying away the debris. The analogous functions of wind are: 1) trans- 

 portation of the material which triturates and erodes all substances in 

 its way, and 2) distribution. 



A water current when overloaded with solids will deposit ; when under- 

 loaded it will erode. A sandladen wind always both cuts and deposits. 

 Dry sand, wind borne, is an unobtrusive agent, working silently but 

 diligently on the task of paring away the surface. It leaves no monu- 

 ments to show the magnitude of its results, as does denudation by 

 water. Eiver beds and sandbanks are examples of the excavating and 

 building up through sedimentation by the water. 



Water is a base>ieveler in the sense that it transfers material from 

 higher places to lower ; but where it erodes, it always works more rapidly 



