Economic Geology (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1914 Excerpt: ...investigation of underground waters has assumed such importance in the last few years, that it is hardly possible to do it justice in the limited space which can be devoted to it here. However, some of the more salient points can perhaps be touched upon, and those who desire more detailed information are referred to the selected bibliography at the end of the topic. While much of the water used for supplying towns and cities, for irrigation purposes, etc., is obtained from below the surface, all of it originates in rainfall. The rain water falling on the surface is disposed of in part by evaporation and surface run-off, but a variable and sometimes large percentage seeps into the ground. Ground Water (5, 6)--A small part of the water soaking into the ground is retained by capillarity in the surface soil, to be returned again to the atmosphere, either by direct evaporation or through plants; but most of it finds its way into deeper layers of the soil, which it completely saturates. The water in this saturated zone, which is termed the ground water (Fig. 105), forms a great reservoir of supply for lakes, springs, and wells; and its upper surface, known as the water table, agrees somewhat closely with that of the land surface, but is farther from it under hills (Fig. 105), and nearer to it under the valleys. Under some depressions it may even reach the surface and form springs or swampy conditions (see Fig. 105). The depth of the water table is quite variable, being but a few feet below the surface in moist climates, while in arid regions it may be 100 feet or more. Fia. 105.--Ideal section across a river valley, showing the position of ground water and the undulations of the water table with reference to the surface of the ground and bed rock. (After ...