414 



SCIENCE. 



[N. S. Vol. XV. No. 376. 



Dragoon Mountains and the Whetstones 

 and liuachucas. The phenomena bear tes- 

 timony also to the great epeirogenic uplift 

 since the Miocene. A depression of four 

 thousand feet would submerge the greater 

 part of southern and southeastern Arizona, 

 including the great valleys of the Gila 

 river, Salt river and of the Santa Cru?, 

 leaving only a few widely separated islands 

 above the Pliocene Sea. 



The Dehris Fans of the Arid Region in 

 their Relation to the Water Supply: B. 

 W. HiLGAED, Berkeley, Cal. 

 The debris fans or cones of the torrential 

 periodic streams that enter the broad and 

 deep valleys of the Cordilleran region are 

 wholly different in their genesis and struc- 

 ture from the alluvial fans of the streams 

 that enter lakes, as described by Gilbert 

 (Monograph No. 1, U. S. Geol. Survey). 

 Immediately in front of the caiion mouth 

 we always find an accumulation of cobbles 

 and boulders, the latter sometimes of enor- 

 mous size; these grade off into smaller 

 cobbles and gravel as the distance increases, 

 but there is always an irregularly semi- 

 elliptical area, of an extent proportioned to 

 the size of the stream, on which the water 

 is partly or wholly absorbed unless the dis- 

 charge exceeds a certain amount, when a 

 portion of it passes over the gravel area, 

 carrying with it the finer materials, which 

 are deposited beyond. As the valley is 

 filled up and the slope decreases, it takes 

 exceptional floods to carry the coarse ma- 

 terials to any great distance from the caiion 

 mouth. Yet while the slope \Vas steep and 

 the valley channel relatively narrow, the 

 cobbles were often carried to considerable 

 distances. The water so absorbed in the 

 coarse materials forms a pressure column 

 behind the main body of the fan, which 

 when large becomes a prolific source of 

 artesian water, as is prominently exempli- 

 fied in the iipper San Bernardino valley 



and elsewhere in Cixlifornia. The extreme 

 irregulai'ity of structure within the fan, 

 and the variations in the quantity and 

 course of the main discharge of the stream, 

 cause corresponding irregularity in the 

 flows and static pressures of wells; high 

 pressure being frequently coincident with 

 small flows, and vice versa. Spontaneous 

 outflows also frequently occur in times of 

 high floods, or as the result of erosion on 

 the fan slope. ' Artesian ' springs and 

 streaans thus formed are important sources 

 of irrigation water in southern California; 

 they respond to the variations of the sea- 

 sonal rainfall in from three to six months. 

 Hence these debris fans form natural stor- 

 age and regulating reservoirs of great im- 

 portance. A striking example of the effect 

 of these conditions upon the topography 

 and hydrography of the valleys is afforded 

 by the debris fan of San Antonio creek, a 

 snow- fed torrent descending from the Sierra 

 Madre northward of Pomona, Los Angeles 

 County. Its typically regular fan has ex- 

 tended clear across the valley (nine miles) 

 to the foot of the hills opposite, thus form- 

 ing a water-divide between the Santa Ana 

 and San Gabriel rivers. The creek itself 

 has in the past evidently discharged alter- 

 nately into the two drainage basins, which 

 originally were probably a single one drain- 

 ing through the Los Angeles plain into the 

 sea. This is an easily verifiable illustra- 

 tion of the manner in which the broad Cor- 

 dilleran valleys have been filled in, as 

 lately discussed by Shaler. Similar though 

 less obvious examples exist in the Great 

 Valley of California, as well as in the Santa 

 Clara valley on San Francisco Bay. 



A Post-Tertiary Elevation of the Sierra 

 Nevada shotvn iy a Comparison of the 

 Grades of the Neocene and Present Tuo- 

 lumne Rivers: H. W. Turner, San 

 Francisco. 

 The Neocene Tuolumne occupied the 



