mountain, town or city. Index maps showing the location and names of quadrangles mapped in any state or region are published.

In addition to regular topographic atlas sheets many special topographic maps have been published of areas having particular interest, such as large cities and their environs, or regions embracing natural wonders. The Survey has also been engaged for some time in participating in the preparation of the United States portion of the millionth scale map of the world—that is, a map having a scale of one to one million, or a scale of about sixteen miles to the inch. This map is to be part of an international map of the world upon a uniform scale. A few sheets of this map have already been published by the Survey. A number of base maps of the several states and of the United States are also published by the Survey.

Historically considered, the topographic maps produced by the Survey are primarily of use to the Survey itself as necessary bases for geologic maps, but they are now to be regarded as the basic and standard topographic maps of the country for all purposes. They are used by virtually all departments of the national, state, and local governments in the location of field offices, the determination of the boundaries of administrative districts and the consideration of projects for public works, a use in which they are of special value. Before these maps were made, for example, every city was obliged to spend large sums in surveys for water supply. These topographic maps now furnish all necessary data for a far larger area than would be embraced in a local survey. In engineering works, whether municipal or constructed by private capital, and in industrial, mining, and agricultural operations of every sort, the Survey's topographic maps are also of the highest value. The maps are widely used, moreover, in traveling and in countless other activities of every-day life. The extent of their use is indicated by the annual sales, which now amount to over half a million copies.

Attention should be called to the especial value which at

taches to the Survey's topographic maps of Alaska, because of the undeveloped state of the country. Demands for them come from prospectors, engineers, capitalists, and school teachers, and they form one of the most essential preliminaries to any form of development of the country. They indicate routes of travel to the prospector and explorer, railway and wagon routes to the locating engineer, and possible sources of water power to the mine operator.

Cost and Progress. The cost of the surveys first made, on the scale of 4 miles to the inch, averaged $1.75 a square mile. Those made later, on the scale of about 2 miles to the inch, averaged in cost $4 a square mile, and those made on the scale of approximately 1 mile to the inch, averaged in cost $10 a square mile. During the field season of 1884 a single party mapped over 11,000 square miles on the scale of 4 miles to the inch. A few years later the output of a single party, on the scale of 2 miles to the inch, was about 3,000 square miles in a season. Today the more refined and detailed maps, on the scale of about 2 miles to the inch, with a contour interval of 100 feet, cost from $7 to $11 a square mile, according to the country, and a single party can rarely map over 600 to 1,000 square miles in a season. On the scale of 1 mile to the inch, a party rarely maps more than 500 square miles in a season, and the cost of this work varies between $12 and $30 a square mile, according to the nature of the country.

It would be difficult to compute with accuracy the total cost of the topographic surveys and of the topographic maps to date. As already stated, the Survey inherited from the several western surveys which preceded it a large body of material which it was able to utilize in the preparation of its early topographic maps. Again, it was not until eight years after the Survey began topographic surveys that specific appropriation was made for them by Congress. The state contributions to topographic work, amounting to nearly two and one-half million dollars, must also be taken into account. The total specific appropriations for topographic surveys to date have been

over eight million dollars. Thus, it seems safe to say that the total cost of these surveys and of the preparation of the topographic maps has been not less than twelve million dollars.

Of the 3,026,789 square miles in the United States proper, 1,257,623 square miles have now been mapped, covering about 2,225 sheets. The accompanying map, taken from the annual report of the Director of the Survey for 1917, shows the progress that had been made up to June 30 of that year in the execution of the topographic survey of the country.

In addition, there have been mapped 202,361 square miles in Alaska, comprising over 34 per cent. of the total area; and 1,393 square miles in Hawaii, comprising 22 per cent. of the total area.

Erection of Bench Marks. In connection with its topographic work, the Survey erects bench marks in areas surveyed, a bench mark being a monument upon which is noted its elevation. This work has been carried on since 1896, in pursuance of an act of Congress passed in that year which required that thereafter in topographic surveys "west of the ninety-fifth meridian, elevations above a base-level located in each area under survey shall be determined and marked on the ground by iron or stone posts or permanent bench marks, at least two such posts or bench marks to be established in each township or equivalent area, except in the forest-clad and mountain areas, where at least one shall be established, and these shall be placed, whenever practicable, near the township corners of the public land surveys; and in the areas east of the ninety-fifth meridian at least one such post or bench mark shall be similarly established in each area equivalent to the area of a township of the public land survey."

A bench mark serves two purposes-that ordinarily served by a monument of establishing an exact point of reference in the use of the topographic maps (the location of the bench mark being indicated on the map), and that of furnishing for all purposes a permanent record on the land itself of its eleva

tion. Bench marks also are of value in the progress of the Survey's topographic work as fixed points of reference for the extension of its level lines.

It will be noted that the act of 1896 does not require that the elevations of the bench marks above sea level be determined, but only "the elevations above a base level located in each area." Any attempt to determine elevations above sea level in the initial survey of an area might have necessitated the running of thousands of miles of precise levels in order to connect with sea level the initial points within each of the areas under topographic survey. This would have cost immense sums and would have occupied several years, during which the topographic surveys would have had to be continued without spirit leveling. The Survey, therefore, designated some point within each area under topographic survey as a central datum point for that area, and determined the elevation of that point as nearly as practicable from existing elevations adjusted by railway levels or other levels brought from the sea. In consequence, though all the elevations referred to the same central datum point agree one with the other, they may not be reduced to mean sea level because of the differences between the primary elevations on which the leveling is based. However, prior to and since 1896 preciselevel lines have been extended by the Survey and other organizations to more accurately determine inland elevations above sea level, and the elevations originally determined have been corrected from time to time, so that at present nearly all the central points have been reduced to mean sea-level datum and carry with them all the levels resting thereon. As the area referred to a single base is gradually widened, it becomes possible to correct the elevations with reference to a new base, the corrections being, if practicable, entered upon the bench mark.

The location and elevations of the bench marks are published in two ways-first, by a symbol on the atlas sheets, accompanied by the letters "B.M." and the elevation to the near

est foot; second, by lists of bench marks in the annual reports or bulletins of the Survey, with a full description of each bench mark and its exact elevation above sea level to the thousandth of a foot, as adjusted and referred to the various central datum points, these lists being corrected in publications from time to time as better connections are made with sea level. These publications are issued separately for each state.

Geological Survey. As perhaps its basic activity, the Survey is carrying on a survey of the areal geology and geological structure of the United States. By means of this survey it seeks to ascertain the distribution, structural relations, mineralogic character, economic value, and geologic history of the formations of each area surveyed. The results of this survey it publishes in the form of maps, illustrations, and textual descriptions.

The selection of areas for geologic survey is determined by a variety of considerations, corresponding closely to those mentioned in connection with the distribution of the topographic surveys. Still another element is the desirability of giving preference to the public land areas because of the value of geologic mapping in furnishing information needed by the General Land Office in the administration of the land laws.

The degree of refinement of the geologic survey varies with the character of the area and the results desired. The scale of the map on which the results are to be published is determined by the same considerations as in the case of topographic mapping. The three scales used in the topographic mapping— 1:250,000, 1:125,000 and 1:62,500-are used also in the geologic mapping and according as one or the other map scale is contemplated the survey is said to be exploratory, reconnaissance, or detailed. Larger scales also are used.

The geologic survey of an area is usually begun after the topographic map has been completed, and that map is used as a base. Occasionally, however, the geologist works with the

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