served fluctuations of the water table, which represent the filling or emptying of the underground reservoir. In other areas it is practicable, by observation of surface conditions, to determine approximately the quantity of water that annually enters or is discharged from the underground reservoir. These observations comprise stream measurements to ascertain the quantities of surface waters that percolate into the ground, and measurements of the discharge of ground water through springs and by evaporation from soil and plants. These methods are laborious and difficult and none of them are likely to afford precise results, but they are nevertheless valuable, because they give a tangible basis for estimating the supplies of ground water.

In addition to determining the quantity and quality of the waters a survey of ground waters discloses important factors affecting the practicability of recovering them, such as hydrostatic pressure and the nature of the rock to be encountered in sinking wells. The investigation of these elements is involved in the study of the local geology, which is supplemented by examinations and tests of the existing wells in the area surveyed.

The data gathered as to sources of ground water are naturally of value chiefly in the arid and semiarid regions of the West, where such supplies are needed both for domestic use and for irrigation, but the importance of deep ground water supplies over the whole country is being emphasized. It is highly desirable to obtain supplies of domestic water from sources other than the shallow wells, some of them open, that are sunk near many houses. The water obtained from deep wells has percolated through sands and other material for so great a distance that its impurities have been removed by filtration, and it possesses a sanitary value that cannot well be overestimated, for it is free from the bacteria that cause typhoid fever and the protozoa that cause amoebic dysentery,. and its use obviates the necessity for shallow wells that may

serve as a breeding place for Anopheles, the mosquito to which malarial infection is due.

Altogether the Survey has accumulated a vast amount of accurate data of both scientific and economic value on the ground waters of the United States. A total of 474 papers has been published relating in some way to ground water, of which 271 papers relate primarily to this subject. Many of these papers include detailed maps showing the ground-water conditions in specific areas. In addition to the data already published there is a great quantity on file at the Geological Survey still unpublished.

About 75 per cent of the area of the United States (excluding Alaska and the Insular Territories) has been covered by ground-water surveys of some sort, and about 25 per cent has been covered by what may be called standard groundwater surveys. The total area covered by intensive surveys is very small, amounting to only a fraction of 1 per cent of the total area of the United States.

The Survey for some years has published in its water supply papers notes on desert watering places, but in 1917 a specific appropriation of $10,000 was granted by Congress "for discovering, developing, protecting, and rendering more accessible, springs, streams, and water holes on arid public lands of the United States; for erecting and maintaining suitable and durable monuments and sign-boards; and for providing convenient and ready means, apparatus, and appliances by which water may be brought to the earth's surface." The Survey has devoted the funds thus appropriated to prosecuting more vigorously than has hitherto been possible the survey of watering places, and to preparing guides and maps, and erecting sign-posts directing to them.

The total area sufficiently arid to require maps and guides to desert watering places is about 570,000 square miles, or nearly one-fifth of the area of the United States, exclusive of Alaska and the Insular Territories. Of this area about 75,000 square miles have been surveyed since July 1, 1917,

and additional areas of approximately the same aggregate extent are covered by older guides to desert watering places. The region to be covered lies east of the Cascade and Sierra Nevada Mountains and west of the following cities: Boise, Twin Falls, Salt Lake City, Grand Junction, Santa Fé, Roswell, Pecos, and Fort Stockton. The area has been tentatively divided into twenty-six districts, for each of which a guide is ultimately to be prepared. The preparation of maps and guides and the erection of a fairly adequate number of sign-posts for the entire area will cost about $100,000. It is hoped that this work can be done within the next few years.

Investigation in Problems of Water Utilization and Control. From time to time the Survey undertakes the investigation of large problems of water utilization and control. The early investigations made by the Survey into the problem of irrigating the arid lands of the West, and the more systematic work of the Powell Irrigation Survey, in 1888-90, have already been mentioned. It is worthy of note that in his report for 1888, in discussing the problem of irrigation, Major Powell adverted also to the possible reclamation of the flood plains of the lower Mississippi valley by the control of the headwaters of that stream and its chief tributaries. A problem closely akin to this was brought expressly within the province of the Survey by the act of 1908, to which more extended reference will be made in the next section, imposing upon the Survey the duty of examining forest lands proposed to be purchased by the government for the prevention of floods on navigable streams.

While one or another problem of water utilization or control has thus from time to time been brought within the purview of the Survey's activities, no express authorization to investigate generally problems of this type has ever been conferred by statute. Such investigations as are currently carried on are conducted under the general authorization to investigate "the water resources of the United States."

Illustrative of the type of investigation undertaken by the Survey in this direction are the continuing investigations of the problems of flood prevention and of reclamation of marsh and swamp lands, and the special investigation of the hydraulic mining débris problem. The investigation last mentioned, one of the most extensive and original of those made in this field by the Survey, was undertaken at the request of the California Miners' Association with a view to devising measures whereby the interests of agriculture and navigation in the streams carrying mining débris could be harmonized with those of the placer mining industry. The investigation by the Survey, which extended over more than ten years, involved research in the existing literature on the subject, laboratory study of the laws of transportation of detritus by running water, and a field examination of the entire course of the Sierra Nevada River. The report of this investigation, which was published in 1917, recommends a coöperative control by all the major users of the Sierra's water whether for mining, irrigation, or power development.

Examination Under Act of March 1, 1911, of Land Proposed to be Purchased by the Government for Protection of Navigable Streams. The act approved March 1, 1911, known as the Weeks Act, which provides for the acquisition of land for the purpose of conserving the navigability of navigable streams, imposed a new duty on the Geological Survey. The lands to be purchased must be selected by the National Forest Reservation Commission, but under section 6 of the act they may be purchased only after the Geological Survey has examined them and submitted a report on them to the Secretary of Agriculture "showing that the control of such lands will promote or protect the navigation of streams on whose watersheds they lie." The law thus places upon the Geological Survey the responsibility of determining the fundamental question whether the control of any specific tract of land will promote or protect navigability, and stipulates that

a favorable report to the Secretary of Agriculture must precede the purchase of the tract considered.

Following is a description of the methods employed in beginning this work as contained in the annual report of the Director of the Survey for 1911:

On the initiative of the Survey a conference was arranged between representatives of the Department of Agriculture and of the Department of the Interior, and an agreement was reached concerning procedure in the administration of this new law, so far as the Forest Service and the Geological Survey are concerned, to the end that the examinations of land by the two bureaus might be coördinated. In this agreement, which was approved by the two Secretaries on May 3, 1911, it is set forth that the examination by the Geological Survey will include the determination of the relation of the headwater streams to the navigable streams to which they are tributary, the local observation of the headwater stream or streams draining the tract or tracts in question with reference to runoff characteristics and to nature and amount of suspended material, the classification of the surface formations of the tract with reference to permeability and storage capacity and to resistance to erosion, and the securing of such additional topographic data, in coöperation with the Forest Service, as are needed by the two bureaus in their examination of the tract.

The greater part of the examinations necessitated by this act were completed within the first two years after the passage of the act, but other examinations have since been made from time to time.

Investigation of the Character and Value of Public Lands Made Necessary by the Public Land Laws. As has been seen in the preceding sections, the surveys and investigations made by the Survey, though they extend over the whole country, have in large measure related to what may be termed the public land region of the United States. Indirectly, therefore, the Survey's general work has had and continues to have a far-reaching influence upon the development of the public