108 W J MCGEE — SHEETFLOOD EROSION. 



and structure, that the sheetflood is an efficient agency (1) in transporta- 

 tion, (2) in corrasion, and incidentally (3) in deposition. Its efficiency 

 is enhanced in the Sonoran district by various conditions, comprising 

 all those essential to the existence of this form of water body, and in- 

 cluding notably the local tendency toward the production of disinte- 

 grated and comminuted detritus rather than residua over the surface of 

 mountain and plain — vegetation is too meager to produce appreciable 

 quantities of humus acid, water is too scanty to aid materially in chemic 

 action, frost is too rare and superficial and the included water too minute 

 to rend the rocks effectual^. Its efficiency is limited by the rarity of 

 rainfall in sufficient quantity to produce no wage in any form — over the 

 greater part of Papagueria, where the mean annual precipitation is prob- 

 ably no more than three to five inches, many tracts of 1,000 square miles 

 or over are missed by the midwinter and midsummer storms of one or 

 more years, the freshet-making storm may be three or five years in 

 coming, and the great mantle-moving torrents are apparently separated 

 by decades or centuries. 



The efficiency of the sheetflood in transportation was amply shown by 

 the relatively trivial torrent at Rancho de Bosque, which was literally a 

 thin mortar of mud rather than water, and the well-preserved traces of 

 sheetflood work in other localities gave unmistakable indications that 

 great quantities of detritus were collected and transported. Inferences 

 as to the behavior of greater sheetfloods rendered necessary by analogy 

 and by the distinctive topographic configuration indicate with strong* 

 probability that the volume of material transported by the debacle in- 

 creases in a higher ratio than the volume of water, so that the entire 

 detrital mantle may be saturated to the point of mobility and carried 

 down the slope in a sort of mud-flow peculiar only in its magnitude and 

 in the dimensional heterogeneity of the moving particles. 



The efficiency of the sheetflood in corrasion is made manifest by con- 

 sideration of its mechanism : The velocity of flow must be considerable, 

 else the flood is absorbed and evaporated ; the water must be laden to 

 its full capacity with abrasive material, else it runs clear and gathers into 

 streams. This material, whether fine or coarse, is exceptionally hard 

 and sharp rather than softened and rounded by decomposition, and the 

 internal currents in the shallow sheet are such as constantly to batter and 

 scour the subjacent surface as the mass half rolls and half slides across 

 it. The inference from the character of the sheetflood is consonant with 

 the necessary inference from the character of the baselevel surface. Over 

 dozens or scores of square miles in carefully examined localities, hard 

 rocks like those of the mountains, and with no sign of decomposition, 

 are planed almost as smooth as the subsoil by the plowshare, with noth- 



