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After the scheme for the publication of the map had assumed final form the necessity for these broader studies was less urgent and the work of the Survey returned more and more to economic lines. The early economic work was confined to the investigation of the ores of the precious metals, but it is now distributed over the whole field of mineral production, embracing the nonmetalliferous minerals as well as the metalliferous ores. Of the 53 parties engaged in field work during the year 1903–4, 24 were engaged in work which was primarily economic, while 14 others were employed on work which was more or less directly economic in character.
The division of geology and paleontology is now under the general direction of the geologist in charge. Since, however, the work is somewhat varied, scientific control along various lines is vested in chiefs of the following sections:
The first-named section has immediate charge of the work of making a geologic map of the United States. The sections of Pleistocene and pre-Cambrian geology represent specializations of that map work. The petrologic section is concerned with the study of the rocks themselves, the paleontologic with the fossils in the rocks, and the two remaining sections are concerned with the metalliferous ores and the nonmetalliferous minerals.
The section chiefs are in effect consulting geologists who help the individual workers in the formulation and solution of their problems. Each field party works under the supervision of one or more section chiefs, selected according to the nature of the problems of the area. Before publication all manuscripts are submitted for criticism and approval to the chiefs concerned.
The main purpose of the Survey is defined by law to be the making of a geologic map of the United States. In discriminating the different formations and determining the complicated structure of the various mining districts, however, many facts of interest and importance in themselves are brought out. With very little additional work it is usually possible to give a fairly complete account of the ore bodies, and such work absorbs a very large share of the time and energy of the corps. This is as it should be, since one of the main purposes of the map is to aid in the development of our natural resources.
Geologic maps are valuable in many ways. They show the areas within which coal, iron, and other useful minerals occur, the limits of the artesian basins, the course of metalliferous veins, and many other things needful to the development of a region. In connection with structure sections, which usually accompany such a map, it is often possible to indicate very precisely not only the localities but the extent of beds of commercial importance and many of the conditions which influence their availability in mining.
The geologic map of the United States which is being made by the Survey is in large part on scales of 1 and 2 miles to the inch. Such a map of the whole country on the scale of 1 mile to the inch would require paper dimensions of about 240 by 180 feet-about the area of half a city block. This would be too large for general use, so the map is being made piece by piece, each sheet on the scales named representing one sixteenth or one-quarter of a square degree of the earth's surface (see pp. 60–61). The individual sheets as they are completed are bound up with a brief descriptive text in folios (see pp. 99–100). Thus the information relating to each area is available at once, and it is not necessary to wait for the completion of the whole.
Of the 3,025,000 square miles of area of the United States, excluding Alaska and island possessions, the geologic map of 100,000 square miles has now been surveyed and published. A large additional area has been covered by preliminary or reconnaissance surveys, and the field parties are at work in widely scattered areas, as shown by Pl. I.
The base map used by the geologist is prepared by the topographic branch, though occasionally the geologist works with the topographer, or even at times does preliminary work in advance of topographic mapping. In such cases he must measure his own distances, usually by pacing, and determine altitudes by means of the barometer.
In order to make his map, the geologist needs to determine the character and distribution of the individual rock masses and their relations to one another. To do this it is necessary to travel over the whole area and carefully plot all outcrops and ledges of rock. The dips of the rocks must be measured in order to determine the amount of deformation they have undergone, and since in places they have been repeatedly folded and broken it requires many careful observations to determine the position of the individual beds beneath the surface.
In some districts, as in much of the Lake Superior region, magnetic surveys are made in connection with the geologic work. In that case observations are made with a dipping needle at many points, and the results are plotted to show the underground distribution of the ores. Unfortunately, very few ores are magnetic, so that this method can be used but rarely.
Many rocks do not show their true character in the outcrop, where they have been changed by the process of weathering. It is also difficult, if not impossible, to determine by the eye the nature of many igneous rocks. Samples are accordingly taken and, when necessary, chemical analyses of these are made. More commonly the nature of a rock may be determined by studying it with the microscope, and this