40 INITIAL CAUSES. 



owing to their number and relatively small size. Of topographic agencies, 

 volcanoes alone produce no erosion, unless the violent removal of portions of 

 volcanic cones be regarded as such. 



In erosion, agents usually act alone, though it is often the case that one 

 agent will erode an area deposited by another. It is true that water and 

 gravity are regularly associated in erosion by water, but gravity is hardly 

 to be regarded as controlling, except in the d^integration of peaks and cliffs, 

 and in the case of avalanches, whether of snow or of rock and soil. 



Eate and degree of erosion. — ^While the force and dmation of the chief 

 eroding agents, water and wind, differ greatly, they are critical in determin- 

 ingthe rate of erosion and the degree to which it will act. These are also affected 

 in the first degree by the hardness and compactness of the surface acted upon, 

 as is shown by the formation of boulders and ledges in rock strata. The ero- 

 sive force of rain-water depends upon the rate of precipitation and the angle 

 of slope, that of running water upon the fall or current and the load carried. 

 While these vary in all possible degrees, the essential fact is that they are 

 more or less constant for a particular area. In many areas they are suscep- 

 tible of approximate measurement and expression, at least. The erosive 

 force of wind is determined by the velocity and by the exposure of the slope 

 acted upon. Prairies and plains, deserts, ridges, moimtain peaks, and shores 

 are the chief areas characterized by forceful winds. Apart from velocity and 

 exposure, the erosive influence of wind is determined by the length of the 

 period for which it acts and the frequency of such periods. Certain areas, 

 sand-hills, dunes, strands, and mountain-tops, for example, may have winds 

 forceful enough to pick up sand or dust every day for all or most of the year. 

 In the case of compact soils or rock surfaces the action of the wind is confined 

 to removing weathered material, unless the wind carries a load of abrasive 

 particles. 



In the case of water erosion, intensity often compensates for lack of duration 

 or frequency, especially where the slope is great and vegetation scanty. This 

 is especially true of regions with torrential rains, such as the deserts of the 

 southwest and the Black Hills and Rocky Mountains, where the characteristic 

 "bad lands" occur. The density or hardness of the eroded smiace, its rough- 

 ness, and the amount and kind of dead or living cover, together with slope and 

 exposure, are all factors of moment in determining the final effect of erosion. 

 These are factors which permit of quantitative study with a minuteness and 

 thoroughness not yet attempted. Such study seems inevitable if we are to 

 make an accurate analysis of the forces which influence migration and occu- 

 pation and direct the water-content basis of successional development. 



Fragmentary and superficial erosion. — Erosion may act over the whole sur- 

 face of an area with greater or less imiformity ; it may be restricted to particular 

 portions or localized in the most minute way. Striking illustration of this is 

 found in the comparison of ridge and slope with valley. Moreover, while 

 the contrast between slope and valley is of the greatest, similar slopes exhibit 

 similar or identical behavior. Marked examples of local erosion by wind are 

 found in the blow-outs of sand-hills and dimes, while sand-draws and washes 

 furnish similar cases of water erosion. Fragmentary erosion is a feature, 

 however, of lateral erosion by running water, and of cliff and ridge erosion 

 due to gravity. It furnishes a bewildering array of areas of all sizes and 



