Page:Popular Science Monthly Volume 46.djvu/499

Rh feet. Special maps, for the detailed survey of areas of unusual mining or scientific interest, are made on still larger scales, up to 1:10,000. With, the increase in scale there has been an increase in cost; but the latter has been in a considerably smaller ratio than the increase in the value of the maps produced. Since the beginning of topographic surveys by the Geological Survey there has been a steady improvement of methods, and the survey has been making better maps during the past two years than ever before.

Primarily the topographic maps are for the use of the geologist, and their scale has been determined largely by this fact. "With the progress of the survey from year to year, the public became more and more acquainted with the maps, and a strong demand arose for the topographic maps as such. Massachusetts, Rhode Island, Connecticut, and New York asked to have the work pushed more rapidly within their respective boundaries; this was done on the condition that the State pay one half of the cost of the work. Of the States mentioned, all have a completed topographic map with the exception of New York. In the great semi-arid region of the interior the maps were requested as an aid in the development of their resources in artesian water and in the application of drainage waters to irrigation. Severe criticism of the survey has been made on account of the extent of the topographic surveys in the semi-arid region; but when it is considered that the Geological Survey is a national institution, it is evident that the great interior has as just a claim for consideration as the mining regions of the mountain areas of the eastern and western sides of the continent. If water is the principal mineral resource, it should receive due attention in making the topographic map. The uses of the topographic maps are many, and it is the policy of the survey to give them as high a standard of accuracy as the limit of scale will permit. A map may cost one dollar or one thousand dollars, or more, a square mile, according to its scale and its contents. For general purposes an excellent map can be made for ten dollars a square mile, one that will subserve the uses of the geologist and the people. This will answer for nine tenths or more of the area of the country; and when more detailed, expensive maps are required for the remaining tenth, they can and will be made. In the meantime the development of the country will be assisted in many ways by the maps constructed on the scales now adopted.

The following table exhibits the area of topographic work completed up to December 1, 1894. Of this, a considerable portion of the 1: 250,000 scale (four miles to the inch) will be revised as detailed geologic work is carried forward. A thorough revision will also need to be made of certain areas in the Appalachian Mountains and west of the Mississippi River, to the Pacific.