GEOLOGY.
Geology, as it finds application in the petroleum industry, con cerns itself chiefly with the study of sedimentary rocks and their structure, or that branch known as stratigraphy. Igneous rocks, which are of volcanic origin, and metamorphic rocks, formed by the action of pressure and heat on either igneous or sedimentary rocks, are never the primary source of oil, and such oil as has in rare instances been found in them has escaped thereto from the sedimentary formations.
In the study of the earth's form we find many agencies at work on it, constantly altering its configuration. Rains, winds, and frost are changing the surface by tearing down material at one point and transporting it to another, doing this slowly but with a great cumulative effect throughout the centuries in which geological time is measured. Rivers bring down immense quantities of sand and silt, depositing these in lakes, lagoons, and the sea. Waves are breaking into the shore line and washing material back under the water, to be deposited there again. Through these and the many other influences at work new bodies of slightly consolidated sediments are constantly being deposited under water, and in this way are formed the stratified rocks, as differentiated from the igneous, which are of volcanic origin and have been fused. In the latter class are the granites, porphyries, and other crystallines more generally associated with metallic ore deposits. Heat, and often great pressure, have been important factors in the process of their formation and they are most readily recognized by their compact ness and crystalline structure.
The stratified rocks, which include the sandstones, limestones, shales, and clays are more apt to be loose and friable and are characterized by their division into parallel sheet-like masses known as strata. About nine-tenths of the surface, as well as the entire sea-bottom of course, consist of stratified rocks, the former having been brought to their present position through the elevation of what at one time lay under water. Much of the history of the surface of the earth in past ages has been learned from the study of the stratified rocks. Fossils, which are the remains of either
animal or vegetable matter existing at the time the sedimentary strata were deposited, throw light on the life of the time and are valuable aids in correlating and identifying measures in the field.
These measures are found to have an historical sequence in the order of their deposition, and in some districts their chronologi cal relations have been worked out in great detail. The greater periods of geologic time are known as Eras; these are divided into a number of Periods, the Periods into Epochs and the latter further subdivided into stages represented in the rocks by Formations. It should be noted that the kind of rock and its appearance, whether sandstone,' shale or limestone, has no direct connection with the age, inasmuch as different combinations of these are repeated in all Epochs ; and oil has been found in the rocks of nearly every Period. In the United States, the eastern oils are obtained from the geologically older measures and those of the southern and western fields from the more recent. Gas shows an equally wide geological distribution.
Classes of Sedimentary Rocks.
The stratified rocks are classed according to the material of which they are chiefly composed such as sand, lime, etc. These classes are then further divided and identified by other character istics such as color, compactness, size of the individual grains com prising them, and the cementing material occupying the interstices between the grains. The latter is an especially important feature in its effect on the stone as a whole. Sandstone, colored red by a cement of iron oxide which is not soluble in water, is often valuable for building stone, while sandstone with a lime cement would have no value for this purpose because of its eventual disintegration due to the ease with which the limestone washes out. If lime is the cementing material the rock is known as calcareous; it is ferrugi nous if the cement is one of the iron oxides ; siliceous if it is silica ; and argillaceous if it is clayey.