86 Cairnes — Some Suggested New Physiographic Terms. 



the various desert areas of the world by Walther,* Passarge,* 

 Penck,* Davis,* and many others have shown that it is by the 

 slow process of exportation of fine wind-blown material that 

 the mean altitudes of arid regions are continually decreased. 

 These vast quantities of transported fines thus assist in appla- 

 nating the surfaces of the adjoining regions upon which they 

 fall; and in certain countries the accumulations of these 

 deposits acquire very considerable thicknesses, as in the case of 

 the loess of China or of the central plains of the United States. 

 This wind-borne dust has frequently been shown to have been 

 transported almost incredible distances from the arid or semi- 

 arid districts from which it was derived. It is well known 

 that the sails of vessels of the Atlantic to the leeward of the 

 Sahara commonly become tinged with red, owing to the wind- 

 blown dust from the desert ; and sand grains are dredged with 

 the true pelagic material from the bottom of this portion of 

 the Atlantic. 



Grlaciation by inland-ice has also contributed vast amounts of 

 material in places, to form drift or till plains. This also con- 

 stitutes a true applanating process. Extensive plain-like areas 

 occur on this continent just south of the 49th parallel, the sur- 

 faces of which are composed of materials that owe their origin 

 to the continental glaciers that advanced from the north. 

 These materials are not always so deposited as to have even, 

 horizontal surfaces ; in fact, their deposition, in places, has had 

 a decided roughening effect on the topography. Nevertheless, 

 extensive accumulations of drift tend to produce, in a general 

 way, a plain-like topography, much more plain-like, in places, 

 than the underlying bedrock surface, f 



Yulcanism is also an important applanating process, and 

 throughout extensive tracts in different countries, lavas, scoria, 

 ashes, etc., have almost, or quite, obscured the pre-existing 

 land surface, and have produced typical volcanic plains or 

 plateaus. Probably the greatest lava fields in the world occur 

 in the Columbia Plateaus of the western United States.:): 

 There, a region previously possessing a topography of consid- 

 erable relief was subjected to successive lava flows until only 

 the higher summits were left projecting through the volcanic 



* See references to writings by these authors on a preceding page. 



f Bowman, Isaiah, " Physiography of the United States," in Forest Phys- 

 iography, 1911, pp. 460-477. Martin, Lawrence, "Physical geography of 

 the Lake Superior region," U. S. Geol. Surv., Mon. lii, 1911, pp. 85-112. 

 Carman, J. E., " The Mississippi valley between Savanna and Davenport," 

 111. State Geol. Surv., Bull. No. 13, 1909, pp. 31-55. 



^Russell, I. C, " A reconnaissance of Southern Washington," W. S. & 

 Irr. paper, No. 4, U. S. Geol. Surv., 1897. Smith, G. 0., " Ellen sburg 

 folio," No. 86, 1903, U. S. Geol. Surv.; "Geology and physiography of 

 Central "Washington, "U. S. Geol. Surv., Prof. Paper, No. 19* 1903, pp. 9-38. 



