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was organized in independent parties, each chief of party reporting to the Director; and (4) from 1900 to the present, during which the geologic and paleontologic work has been in a single administrative division, but separated for scientific supervision into several sections.

(1) The law of 1879, under which the Survey was organized, defined the duties of the organization as the "classification of the public lands and examination of the geological structure, mineral resources, and products of the national domain." Under this law, as interpreted by the first Director, the activities of the Survey were confined entirely to the public lands, and consequently to the Western States. The work was organized under five divisions, each of which was practically complete in itself, carrying on geologic, topographic, and chemical investigations. This form of organization continued for two years, when topographic work independent of the geologic parties was begun. Shortly afterwards the same action was taken with regard to paleontology, and at the same time a separate division was formed for the collection of statistics of mineral production. At the end of this first period, therefore, the work of the Geological Survey had become well differentiated and most of the forms of activity in which it is at present engaged were represented. In 1882 an amendment to the sundry civil bill was passed providing for the preparation of a geologic map of the United States, and this was regarded as sanction for extending the work of the Survey over the whole country. New divisions were therefore established for the investigation of the eastern portion of the United States as well as the western.

(2) During the second period the work of the Survey was divided into three groups of divisions-geologic, geographic, and accessory. Under geologic were grouped the following divisions:

1. Division of Archean geology.

2. Division of Atlantic Coastal Plain geology.

3. Appalachian division.

4. Lake Superior division.

5. Division of glacial geology.

6. Montana division.

7. Yellowstone National Park division.

8. Colorado division.

9. California division.

10. Division of volcanic geology.

11. Mississippi division.

12. Potomac division.

Paleontology was also divided into the following divisions:

1. Division of vertebrate paleontology.

2. Division of Paleozoic invertebrate paleontology.

3. Division of Mesozoic invertebrate paleontology.

4. Division of Cenozoic invertebrate paleontology.

5. Division of paleobotany.

6. Division of fossil insects.

The next year a division of correlation was established, and a little later the Florida and New Jersey divisions, making in all 15 divisions in geology. The heads of these divisions reported to the Director until 1889, when the office of chief geologist was established.

(3) In 1893 all the geologic divisions were abolished and the work was organized under separate independent parties, each party chief reporting to the Director. This form of organization continued from 1893 to 1899, during which time the number of parties increased from 23 to 58. During this period details both of administration and of scientific supervision were attended to by the Director.

(4) With the increase of his duties the Director, in 1900, found it necessary to change the form of organization. Various sections were established under section chiefs, who had scientific supervision but no administrative control over the work. The latter remained with the Director until 1902, when the office of geologist in charge of geology was created and the administrative control of the division was transferred to that office. At the same time paleontology was included with the division of geology.

As already stated, the character of the early work of the Survey was determined in part by the problems inherited from previous organizations and in part by the urgent demand, particularly from mine operators, for the economic investigation of various mining districts. Thus, of the five divisions originally composing the Survey organization, three were devoted to the completion of studies already begun under previous organizations, namely, the investigation of the geology of the Colorado Plateaus, which had been begun under the Powell survey; the study of the Quaternary lakes in the Great Basin region, which had been started by the geologists of the Wheeler survey; and the geology of Montana, which was left incomplete by the Hayden survey. Three new investigations were begun, one in each of the three mining districts that were then most important-the Washoe district, containing the famous Comstock lode; the Eureka district, and the Leadville district. With the extension of the geologic work to the Eastern States, gradually less and less attention was given to the economic geology, and of the 15 divisions which existed in 1890 fewer than a third were engaged in work which had a direct economic bearing. The work was directed more largely to the investigation of the broad problems of geology as a scientific basis for the preparation of the geologic map of the United States. This work, which was the largest of its kind ever undertaken by a Government organization, required careful consideration and thorough investigation of many fundamental problems in geology which have only an indirect bearing upon the development of economic resources. The first folio of this atlas was issued in 1894, fifteen years after the organization of the Survey and eleven years after the necessary legal authority for the preparation of the map had been secured. (See pp. 98–103.)

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:

Areal geology.

Pleistocene geology.

Pre-Cambrian and metamorphic geology.

Petrology.

Paleontology.

Economic geology of metalliferous ores.

Economic geology of nonmetalliferous minerals.

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

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