CLIMATE AND TERRESTRIAL DEPOSITS i8l 



tions of Chamberlin in Greenland and of others in Alaska cited by 

 Merrill.' Observation and theory combine, however, in pointing 

 to the greater dominance of the forces of rock decay in warmer pluvial 

 climates and especially in the rainy portion of the torrid zone, the 

 natural activity of the warmer waters being further increased by the 

 organic acids supplied by the large amount of decaying vegetable 

 matter, giving rise to a mantle of rock waste of maximum thickness, 

 thoroughly hydrated and leached by the heavy periodic rains, and 

 thoroughly oxidized by the intervening seasons of dryness. The result 

 is the formation of the red or pink later ite soils of the tropics, and 

 •the characteristic red alluvium of the rivers,' alluvium poor in soluble 

 constituents, 3 



The effect of moderate cold, such as characterizes the winters of 

 the middle temperate zone, appears to have slight effect upon the 

 erosion of regions in topographic maturity, save that the melting of 

 the winter's snow gives a temporarily higher flood and greater erosive 

 power than would otherwise occur. The summer's heat being less 

 prolonged and intense, gives rise also to less intense oxidation of the 

 soil and less dehydration of the iron oxide, yellows and browns prevail- 

 ing as soil colors and yellows or grays characterizing the river silts in 

 place of the browns or reds of tropical rivers. 



In conclusion, therefore, it may be stated that an increase of tem- 

 perature away from a temperate mean in regions of heavy rainfall 

 will result in increased rock decay and decreased frost action, and in the 

 opposite characteristics in the case of a temperature decrease. Either 

 variation away from a climatic mean would therefore result in an 

 increase of rock destruction, but of opposite kinds. It is not known, 

 however, but that in the case of an increase of temperature with an 

 abundant rainfall the hold of the vegetation upon the soil may be 

 increased to such an extent as to neutralize the tendency toward 

 more rapid production of rock waste by decay. A close comparative 

 study of valley forms of similar age in the middle and southern 

 Appalachian states in similar rocks would tend to throw light on 

 this problem and show if erosion as distinguished from rock decay 



1 Rocks, Rock-weathering, and Soils, 1897, pp. 278, 279. 



2 Walther, Einleitung in die Geologie, p. 815. 



3 Hilgard, Soils, 1906, chap. xxi. 



