688 C. R. KEYES MID-CONTINENTAL EOLATION 



Prominent among such tracts is the country lying between the Eocky 

 Mountains and the Mississippi Eiver. Both for the origin of the vast 

 plains surface and of the so-called fresh-water Tertiaries underlying it a 

 more satisfactory explanation than any yei offered is now demanded. 



The Great Plains appear to display the effects of a general leveling 

 process to which but scanty attention has been given. On a grand scale 

 they seem to introduce to ns a mode of terranal genesis hitherto almost 

 unrecognized. Continental deposits thus begin to assume in this country 

 an importance which has been never before accorded them. 



The vast areal extent of the Great Plains terranes, their remarkable 

 uniformity in lithologic character, their unusual evenness or vagueness 

 of stratification, and their fineness and homogeneity of texture long 

 marked them as lake-formed deposits. Only recently has this generally 

 accepted interpretation of their genesis been seriously called into ques- 

 tion. As Davis^ well observes, there has been no critical discussion of the 

 proofs upon which the bare assertion of their lacustrine origin rests. 



When the lacustrine hypothesis is closely examined there appear many 

 incongruities in its unqualified application to the entire Great Plains 

 region. In order to overcome some of the most glaring obstacles aggra- 

 ding aid from present rivers is invoked. To this also there is serious 

 objection. There is, however, a third geologic agency which is now 

 known to l)e involved, of which little reference has yet been made. This 

 is the potent activity of the wind, both in degradation and in aggrada- 

 tion, in and bordering arid and semi-arid regions.^ To certain aspects 

 of the last mentioned agency attention is here mainly directed. 



Since, according to Murray, one-fifth of the entire land surface of the 

 globe is occupied by desert,* and another one-fifth is susceptible to a 

 greater or less accumulation of continental deposits of one sort or an- 

 other, subaerial formations come in for larger interest than it has been 

 customary to accord them. Of the United States fully one- third may be 

 regarded as arid, and a like large part may be considered as more or less 

 appreciably affected by subaerial deposition. In the past deposits now 

 regarded as continental in character have been usually explained on the 

 theory of water-action alone. Even in the Great Plains region processes 

 of the sea, the lake, and the river have been all drawn upon to account for 

 the phenomena presented, often where the wind appears to be almost the 

 sole erosive agency involved. 



2 Bull. Geol. Soc. America, vol. 11, 1000, p. 598. 



3 McGee's appropriate term eolation in its original general sense is adopted ; it covers 

 all phases of wind-action, the destructive phase heing known as deflation and the con- 

 structive phase as aeroposition. 



* Science, vol. xvi, 1890, p. 106. 



