There is no phase of geology which at some time or place does not have its economic application. Many references to these applications are made in other chapters. It is proposed here to indicate briefly some of the phases of geologic science which are most necessary to the practice of economic geology. The student in his preparation cannot afford to eliminate any of them on the ground that they are merely "scientific" or "academic" or "theoretical."
Mineralogy and Petrology
Mineralogy, the study of minerals, and petrology, the study of rocks (aggregations of minerals), are of course elementary requisites in preparation. There must be familiarity with the principal minerals and rocks, and especially with the methods and processes of their identification, with their nature, and with their origin. This involves a study of their crystallography, chemical composition, physical qualities, and optical properties as studied with the microscope. In recent years the microscopical study of polished and etched surfaces of ores has proved a valuable tool.
Stratigraphy and Paleontology
Stratigraphy and paleontology are concerned with the sedimentary and life history of the earth. The determination of the ages of the earth's strata and of the conditions of their deposition is required in the practice of economic geology. For example, a detailed knowledge of the succession of rocks and their ages, as determined by fossils and other stratigraphic evidence, is vital to the interpretation of conditions in an oil or coal field, and to the successful exploration and development of its deposits. The success of certain paleontologists and stratigraphic specialists in oil exploration is an evidence of this situation. Certain iron ores, phosphates, salts, potash, and other minerals, as well as many of the common rocks used for economic purposes, are found in sedimentary deposits, and require for their successful exploration and development the application of stratigraphic and paleontologic knowledge.
Closely related to stratigraphy (as well as to physiography, see pp. 6-10) is the study of sedimentation,—i. e., the study of the physical, chemical, climatic, and topographic conditions of the deposition of sediments. This is coming to play an increasingly large part in geologic work, and is essential to the interpretation of many mineral deposits, particularly those in which stratigraphic and physiographic questions are involved.
Still another aspect of the problem of stratigraphy and sedimentation is covered by the study of paleogeography, or the areal distribution of the faunas and sediments of geologic periods caused by the alternating submergence and emergence of land areas. In the search for the treasures of sedimentary deposits, a knowledge of ancient geographies and of ancient faunas makes it possible to eliminate certain regions from consideration. From a study of the faunas of eastern Kansas and Missouri, and of those along the eastern part of the Rocky Mountains, it has been inferred that a ridge must have extended across eastern Kansas during early Pennsylvanian time,—a conclusion which is of considerable economic importance in relation to oil exploration.
Structural Geology
Structural geology is the study of the physical forms and relations of rocks which result mainly from deformation by earth forces. If rocks remained in their original forms the structural problem would be a comparatively easy one, but usually they do not. Often they are faulted and folded and mashed to such an extent that it is difficult to go behind the superposed structural features to the original conditions in order to work out the geologic history. Not only is structural study necessary for the interpretation of geologic history, but it is often more directly applicable to economic problems,—as when, for instance, ore deposits have been formed in the cracks and joints of rocks, and the ore deposits themselves have been faulted and folded. Water resources are often located in the cracks and other openings of rocks, and are limited in their distribution and flow because of the complex attitude of deformed rocks. Oil and gas deposits often bear a well-defined relation to structural features, the working out of which is almost essential to their discovery.
It is not desirable to stop with the merely descriptive aspects of structural geology, as is so often done; for much light can be thrown on the economic applications of this subject by consideration of the underlying principles of mechanics,—involving the relations of earth stresses to rock structures. The mere field mapping and description of faults and joints is useful, but in some cases it is necessary to go a step further and to ascertain the mechanical conditions of their origin in order to interpret them clearly. If, for illustration, there are successive groups of mineralized veins in a mining camp, the later ones cutting the earlier ones, these might be treated as separate structural units. But if it can be shown that the several sets of veins have formed from a single movement, that there is no sharp genetic separation between the different sets and that they are a part of a single system, this interpretation throws new light on exploration and development, and even on questions of ownership and extralateral rights (Chapter XVI).