258 STUDIES FOR STUDENTS 



opposing fans dam up the line of confluent drainage. The final 

 conclusion, therefore, is that over such piedmont slopes there is a great 

 freedom from areas of swamp and the greatest opportunity for drying 

 and aeration of the soil between the times of flood. Such deposits 

 should normally show complete oxidation of the iron and a complete 

 absence of carbon. In hot climates evaporation from the soil is very 

 rapid and oxidation of the humus is also rapid, resulting in the red 

 clays characteristic of the moist tropical climates. Even the short 

 intervals between rains which occur in the most pluvial of tropical 

 regions are sufficient, therefore, to bring about the oxidation of the iron 

 and elimination of the carbon from all but permanently swampy 

 areas, giving rise to the red muds of the Amazon and Congo rivers. 

 In cool climates, on the contrary, evaporation and oxidation are 

 both diminished in intensity with the result that carbon may accumu- 

 late upon slopes to an extent impossible in warmer climates, giving 

 rise upon consolidation to carbonaceous sandstones or even conglom- 

 erates. The existence of the latter chemical conditions, leading to the 

 accumulation of carbon even upon sloping surfaces, is found only in 

 climates which approach a continuously rainy character and possess 

 in addition cool summers. As an example, such climatic conditions 

 are found at present upon the western slopes of Ireland, though the 

 accumulating piedmont slopes are there wanting. Peat swamps, 

 however, are observed to exist upon hilltops, slopes, and valley- 

 bottoms, covering one-seventh of the entire island. Many bogs pos- 

 sess a grade sufficient so that in times of excessive rain they may swell 

 and burst and disastrously flood the lower valleys. Less familiar 

 but still better examples are oft'ered by the cool and humid maritime 

 mountain slopes of Alaska. Of these Russell states: 



About the shores of Unalaska and for fully 2,000 feet up its rugged mountain 

 slopes the vegetation is essentially the same as at St. Michaels upon the Yukon 

 delta. In climbing the steep slopes about Iliuliuk I often had great assistance 

 from the dense mat of vegetation two or three feet thick, which, clinging to the 

 rocks, converts their angular crags and shattered crests into smooth domes of 

 soft, yielding moss. On the steep slopes, as in the swamps, the vegetation is 

 always water soaked, owing to the extreme humidity of the climate in which it 

 thrives. Lakelets are common on slopes and hillsides that would be well drained 

 were it not for the spongy nature of their mossy banks. ^ 



I "Notes on the Surface Geology of Alaska," Bulletin of the Geological Society 

 0} America, Vol. I, 1890, pp. 125, 126. 



