page 18


page 18
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These accumulated waters may usually be developed by the mining community or by the prospector who wishes to establish a permanent camp. The favorable place for undertaking such development, however, is not in the mud flat—-the playa itself-—but around its borders. All flowing and percolating waters dissolve alkalies from the rocks, sands, and gravels over and through which they flow. At the point where the waters are evaporated these salts are left behind as saline deposits. As the bottoms of these sinks are merely great evaporating pans, accumulated surface waters and rising subterranean waters both passing into the air there, they are almost invariably alkaline, and usually the alkalies extend to considerable depths. The waters in the center of the sink are impregnated with these salts and are generally not usable. But as the ground waters always percolate toward the lowest point, the waters found about the borders and at short distances back from the playa are relatively pure, since they have not yet entered the highly alkaline area. To reach the water plane at these points about the borders of the sink it may be necessary to penetrate to a greater depth than would be required nearer its center, but usually this is the only way in which potable waters can be developed at such places. GEOLOGIC HINTS. As already stated, pure water may be found in springs, pools, and tanks in the areas of granite, lava, or other compact rock. The favorable time to find such water is immediately after the rains or snows of winter. Many of these springs and pools are short-lived, but they furnish water of the best quality, and even immediately after rains no water can be found in the desert valleys, for their sands and gravels absorb the rain as fast as it falls. Many inquiries are made as to the value of tunneling along a crevice from which a slight seepage escapes, in order to develop a greater supply. Of course it is well to clean out the opening from which the water comes, so that it may flow freely, and to go deep enough to see that none escapes by side channels, but more extensive development work is usually disappointing. If the water of the spring is derived from a saturated body of gravel, tunneling will result in a temporary increase of flow, as the drainage of the gravel body is improved. But in rock even a temporary increase is often not procured, so that there is no return for the expense. A bedrock tunnel to develop water in arid regions is in the majority of cases a failure. The experience of miners in developing their prospects shows that most of the tunnels that are run into the desert mountains are dry, at least until they have been driven a great distance, and that they develop water only under exceptional conditions.

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