Page:Popular Science Monthly Volume 46.djvu/502

486 together the scattered results of all these in a comprehensive whole, before a beginning could be made on the publication of the geologic map. This work was carried forward for ten years before the first folio of the final geologic map was published. Most of the larger questions affecting the classification and nomenclature of the sedimentary and volcanic rocks were brought to a satisfactory conclusion. Great progress was made in the study of the altered (metamorphic) rocks and of the complex of crystalline rocks grouped under the term "Archean." A satisfactory solution of the lower Mesozoic (Juratrias) series is yet to be reached; and there is a great field in the pre-Paleozoic sedimentary and crystalline formations, in which further study will bring out important data for classification and geologic mapping; but the areas affected are relatively small within the United States, and the areal geologic mapping can go on without serious injury on this account. Director Powell and members of the survey gave much time and thought as to the best method of representing the geology on maps. The result is a color scheme that, under the skillful application of the editor of geologic maps, has thus far met the demands made upon it.

The areal geologic map in its final form, as presented in the geologic folios, is intended to place before the geologist, mining engineer, student, and all persons interested the topography and geology of the area included within each atlas sheet. The topographic map has already been mentioned. In each folio a brief explanation is printed of the topographic and geologic maps and of the uses to which they can be applied. It is a simple, short lesson for the layman, to enable him to make use of the folios intelligently.

The areal geologic map represents all that the geologist preparing it knows of the areal distribution of the rocks occurring within its area, so far as he can delineate such knowledge within the scale of the map. Taken in connection with the topographic base, it presents the geologic distribution of the various rocks in a form for the use of geologists and students, but it does not appeal directly to persons interested in the mineral resources o.f the region. To meet this important demand a second map is prepared, upon which the rocks carrying minerals of economic value are clearly indicated by distinct colors, the import of which is shown by the colored legend on the margin of the map. Thus the distribution of the coal-and iron-bearing rocks of the Appalachians in Tennessee and other States, and of the gold-bearing rocks of California and elsewhere, is clearly presented. These maps refer only to the areal distribution of the rocks. What is known of the underground geology is graphically illustrated on a structure section sheet and a sheet of columnar sections. With