INFLUENCE AFFECTING FORMATION OF LOESS 249 



to the pebbles now exposed — a feature which shows that the surface has 

 not been perceptibly lowered by eolian erosion. 



Influences affecting Formation of Loess 



vegetation 



A comparison of the rock surfaces of the vegetated ui)lands, where no 

 wind erosion occurs, Avith those of the near-by arid valleys, where this 

 action is at present gouig on, shows a perceptible though inconsiderable 

 difference in the condition of tlie exposed rocks in the respective fields. 

 Within the high areas, where the snows hold until late in the spring and 

 gather early in the autumn, the well watered soil is completely bound by 

 the growth of trees and lesser plants, so that it is quite undisturbed by 

 the movements of the air. To this upland realm no dust is borne, ex- 

 cept a little of an extremely fine nature. Here we find the rock faces 

 as clearly the work of water or of ice as they are in any part of the Ap- 

 palachian district. There are, moreover, no traces of loess deposits dis- 

 closed along the banks of the streams or in other positions where they 

 would naturally be found. It should also be noticed that, as regards 

 tlie eolian wearing, the i)assage from the arid valleys to the verdant high 

 country is, so far as observable, rather gradual, indicating it would ap- 

 pear a progressive advance, in the process of desiccation, up to its present 

 limit. There are no reasons for supposing that there is at present any 

 rapid diminution of the rainfall, if indeed there is any change of this 

 kind in progress. 



FIRES 



The forests, except where devastated by fires, appear to be holding 

 their lower margins in a fixed position, renewing the growth as tlie 

 trees die of old age. It is unhappily otherwise where these woods 

 are swept by fire. Where conflagrations destroy the young plants, and 

 even where they do no more than burn the scanty covering of vegetable 

 mould, the edge of the wood is likely to be pushed upward for the height 

 of 500 or 1,000 feet, the area from which the trees are driven being occu- 

 pied by a growth of lesser plants so scanty that they afford an insuffi- 

 cient protection to the soil. In this way the field open to eolian erosion 

 is being rapidly extended. 



IRRIGATION 



Some compensation for the increase in the area which is subjected to 

 wind erosion through the action of fire is brought about by the exten - 



