GEOLOGIC PROCESSES OF THE ARID REGION 575 



of pebbles ou the surface is due principally to tbe fact that the wind blows 

 away the finer materials. Most of the gravel-surfaced mesas when upturned 

 by the plow give excellent loamy fields. The most notable examples on an 

 extensive scale are shown in the Bstancia valley. 



TENDENCY OF MANTLE TO MAKE THE PLAINS EVEN 



While the rock-floor of the plains is a plain itself, there are many minor 

 irregularities in the rock-surface. Between sheetflood transportation and 

 wind-drifted loams and sands these depressions are quickly filled up. The 

 soft mantle only makes the plains smoother. The nearest point where this 

 phenomenon is clearly shown is in the vicinity of Los Cerrillos. 



AB.SENCB OF DISTINCT WATERWAYS ON THE PLAINS 



Soon after leaving the foot of the mountains the drainage-lines entirely dis- 

 appear. Although the gradients are high, no drainage-systems are developed. 

 When channel-ways are corraded by unusual freshets in the mountains they 

 are quickly filled up by drifting sands. In the San Pedro arroyo this vanish- 

 ing of the drainage-ways is finely displayed. 



RQLE of SHEETFLOOD ACTION 



Sheetflood action is one of the most important of the plains-forming agen- 

 cies. Instead of water even tending to concentrate along certain lines, as in 

 the humid regions, heavy local rainfall in the mountains spreads out on reach- 

 ing the margins of the plains. Slight inequalities tend to become obliterated 

 by the effects of the sheetflood. 



LACK OF DIRECT EVIDENCE OF FORMER HUMID CLIJIATE 



In the New Mexican region there are discernible no facts suggesting a cli- 

 mate at no distant period of much greater humidity than at present. In the 

 Nevada region the main moulding of mountain and valley has recently been 

 ascribed to former greater rainfall and consequent more adequate water erosion. 

 Independent of any such change of climate the known erosional effects of the 

 winds under conditions of an arid climate may be considered amply sufficient 

 to account for all features of the present landscape. 



The next paper read was 



SHORELINE STUDIES ON LAKES ONTARIO AND ERIE 

 BY ALFRED W. G. WILSON 



The paper has l)een })ublished as pages 471-500 of tliis xolume. 

 The following papers were then read by title : 



FAULTS AND FOLDS OF THE OR AND CANYON DISTRICT 

 BY DOUGLAS WILSON JOHNSON 



COON BUTTE, ARIZONA 

 BY JOHN B. HASTINGS 



