78 Cairnes — Some Suggested New Physiographic Terms. 



1. Plains of accumulation. 



2. Plains of erosion. 

 Other writers divide plains into : 



1. Peneplains. 



2. Constructional plains. 



These are typical of the various modern classifications of 

 plain-like surfaces where such are based upon genesis. Many 

 writers thus appear to consider that plain-like surfaces are pro- 

 duced entirely either by erosion or deposition, i. e. by either 

 subtraction of material from or addition of material to the 

 areas affected, that is by deplanation or applanation. No 

 credit is given to equiplanating processes, although these, 

 especially in desert countries, have frequently been described. 

 In such regions, the eroding or abrasive forces have been quite 

 as effective as those of deposition. 



Eq u iplanation. 



In the writer's original paper dealing with equiplanation, 

 this term is defined and the process is considered in a general 

 way. Also a special phase of equiplanation that was found to 

 be active in portions of Yukon and Alaska is also described. 



In portions of the Yukon plateau which is generally con- 

 sidered to represent an uplifted and subsequently dissected 

 peneplaned surface,* extensive portions of but gently undulat- 

 ing upland from 3,000 to 3,500 feet above sea-level are still 

 preserved that are now dissected by deep, steep-walled, depres- 

 sions representing the main drainage-ways of the areas. On 

 the upland, forces are engaged in dissolving and disintegrating 

 the rock material constituting the residuals or monadnocks, 

 and are transporting this to the intervening depressions, where 

 it is deposited. On account of the arctic climate there prevail- 

 ing, the material, wherever deposited, is soon frozen or becomes 

 associated with other frozen debris, and is consequently held, 

 and added to the flat or nearly flat surface accumulations of 

 superficial deposits which extend over considerable portions of 

 the plateau that are but very imperfectly drained on account of 

 the perpetual frost in the ground. In this way the bedrock 

 depressions of the upland are becoming filled with material 

 derived from the adjoining higher elevations. Thus, the 



* Dawson, G. M., Trans. Roy. Soc. of Can., vol. viii, sec. 4, 1890, p. 13. 

 Brooks, A. H., "Geography and geology of Alaska," IT. S. Geol. Surv., 

 1906, Prof. Paper, No. 45, pp. 36-41, 286-290. Spurr, J. E., " Geology of 

 Yukon gold district, Alaska," Eighteenth Ann. Rep. U. S. Geol. Surv., 

 Pt. Ill, 1898, p. 260. Spencer, Arthur C, "Pacific mountain system in 

 British Columbia and Alaska," Bull. Geol. Soc. of Am., vol. xiv, pp. 117- 

 132, 1903. Hayes, C. E., "Expedition through the Yukon district," Nat. 

 Geog. Mag., vol. iv, p. 129, 1893. 



