scopical 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 wo