p86 studies for STUDENTS 



common third term, while the influence of warm and cold climates upon 

 erosion may be studied by comparing the erosional rate of glacial 

 times with that of the same regions at present, here the common 

 third term being the topography. This has already been done under 

 the topic of the effects of increased cold. 



Turning to the ratio of erosion in arid and rainy climates, as 

 compared with marine denudation, it must be noted that a conclusion 

 should be founded on the average of many instances rather than 

 on a few, since in each the age of the cycle, the attitude and strength 

 of the rock masses, and the strength of the marine erosion will vary- 

 The present discussion must therefore be considered as merely 

 tentative. Very different views have been developed in Great 

 Britain and the United States as to the marine or subaerial production 

 of uplifted and dissected peneplains of Cretaceous and Tertiary 

 age which border the continents, but the growth of knowledge in 

 regard to the capacity of subaerial denudation first in America and 

 more recently in England has given rise to the belief that these are 

 mostly due to subaerial erosion, views confirmed by the application 

 of such criteria as can be applied.^ In general in rainy climates the 

 rivers are observed to sink rapidly toward base level upon an uplift 

 of the land, to open out interior plains in soft formations, and to 

 dissect deeply the hard ones, while the sea, compelled to work on the 

 outlying formations whether they be hard or soft, cuts inland over a 

 comparatively small area and with increasing difficulty. The problem 

 in such climates is to find good and unquestioned examples of elevated 

 plains of marine denudation comparable to the elevated plains of 

 subaerial origin. 



Along the arid coasts of the world very different conditions are, 

 however, found to prevail. To cite examples: 



While the Patagonian plains have an altitude of some three thousand feet at 

 the base of the Andes, they slope very gently to the eastv^^ard, and at a distance of 

 some fifty miles from the Atlantic coast, their elevation is more rapidly decreased 

 by a series of <iscarpments or terrace-like slopes, v^^hich face to the eastward and 

 terminate a succession of level plains, decreasing in altitude as one passes from 

 the interior to the coast and finally ending in the lowermost, which, with an average 

 altitude of some three hundred and fifty feet, extends almost uninterruptedly along 



I W. M. Davis, "Plains of Marine and Subaerial Denudation," Bulletin of the 

 Geological Society of America, Vol. VII, 1895, pp. 377-98. 



