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page 20
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schists are obtained along joints. These joints are most common near the surface, diminishing in number and in definiteness as depth increases. VEGETATION AS A GUIDE. The flora of the usual thoroughly arid sections of the desert varies but little in character. Tracts for miles in extent are covered by an almost uniform vegetation consisting of but a few species that show only here and there slight variations, due to the local differences in the physical or chemical nature of the soil. The presence of water, however, produces marked changes, hence the character of the vegetation is an excellent indication of the possibility of obtaining water near the surface. At scattered points in the broader valleys or in the more restricted gulches are numbers of dead sycamores or cottonwoods. Their presence usually indicates the lowering of the water table in that region, and it is not likely that a supply will be found within 20 feet of the surface. The dry lakes are nearly all absolutely free from vegetation. This lack is probably due to the intermittent flooding of the surface, to the frequently renewed deposits of mud, and to the excess of alkali present. Although mesquite (Prosopis juliflora) grows luxuriantly where water is abundant, its presence is not a good guide to water, for its growth may depend on either the proximity of the ground water or the periodical flooding of a small area. Where its growth is due to ground water it furnishes no suggestion as to the depth to the water. It may lie within a few feet of the surface or it may be 50 feet below. Where its life is sustained, as in restricted gulches, by occasional floods, obviously water will not be found by digging during the drier periods. Tules, or bullrushes, always indicate water at or very near the surface, and generally water of good quality. Lowland purslane (Susevium portulacastrum) is a plant that grows on moist alkaline soils. It indicates water, but usually water of poor quality. Wild heliotrope (Heliotropium curassavicum), sometimes called "Chinese pusley," is one of the rarer plants. It grows only in moist soil, but since it has strong alkali-resisting powers the water near it may be brackish. Arrowweed (Pluchea sericea) is another moisture-loving plant found in the desert. Where it grows to a height of 6 or 8 feet in a dense tangle it is reasonably certain that the ground water lies within 20 feet of the surface and that its quality is at least fair.

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