DEPARTMENT OF BOTANICAL RESEARCH. 93 



to occur in the intermediate zones. The outer zones will not have 

 sufficient clay or caliche seals to confine successfully the water of the 

 channels. The inner zones will be so far from the mountains that most 

 of the sand-streaks of the buried channels will have pinched out or 

 been choked with clay so that they no longer serve as water conduits. 



It must not be imagined that the requirement of bowl-shaped 

 structure in the alluvial beds necessitates that the valley be an inclosed 

 basin without outward drainage. It is true that the necessary struc- 

 ture and the typical artesian conditions are best exhibited in such 

 undrained basins, but the same structure and sufficiently favorable 

 conditions may occur also in open valleys, provided there is enough 

 breadth and depth of alluvial fill to produce the proper contour of the 

 underground beds. The important criterion of artesian occurrence 

 is not inclosure of the drainage, but the slope of the valley and the 

 breadth and flatness of the alluvial fill. In valleys which are steep or 

 narrow the materials of the alluvial deposits are poorly sorted. All of 

 the beds contain much coarse material and are penneable to water 

 both horizontally and vertically. Broad valleys and slopes gentle 

 enough to permit standing or slow-moving water are essential to the 

 collection and deposition of clay, and the deposition of clay layers 

 is essential to the seaUng of the buried channels and the production of 

 artesian conditions. 



From the considerations outhned in the discussion one can formulate 

 the first necessary condition for the occurrence of artesian water in the 

 desert valleys, namely, the existence of a deep alluvial fill the super- 

 posed layers of which have the structure of a shallow bowl, flat or 

 nearly so toward the center. Breaks in the rock wall of this bowl, 

 allowing the escape of surface drainage, are not important, provided 

 the stated condition is satisfied. Since the water, as in all artesian 

 areas, must be pro\dded by rainfall on higher land, this forms another 

 necessary condition. In the desert valleys this means the existence 

 around the valley of fairly high mountains, on which the fall of rain 

 and snow will be considerable and from which the water can drain into 

 the talus slopes and enter the buried channels. These two are the only 

 conditions necessary for artesian occurrence. 



If these criteria be apphed to the valleys of the Great Basin it is 

 obvious at once that many valleys are too narrow or too steep to have 

 the necessary bowl-shaped stinicture of the underground beds. Many 

 others have insufficient drainage areas or have watersheds of such low 

 altitude that precipitation upon them is inconsiderable. There are, 

 however, many desert valleys in which the necessary conditions 

 appear to be satisfied. In several of these artesian waters have 

 already been discovered; for instance, in the Salton Basin, California, 

 and in the Carson Sink, the Las Vegas Valley, and Railroad Valley, 

 Nevada. Doubtless water will be discovered in others of the valleys 



