The stratified rocks as found exposed on the surface of the earth are rarely horizontal and uniformly continuous, but instead may be tilted, folded, or have portions thrown off and their continuity broken to such an extent that their exact interrelation may be established only by a careful survey over an extended area. Such work becomes more complex through the fact that as soon as strata are elevated above sea level their degradation begins and, as they stand now, only small portions of some remain, the rest having been eroded and carried away.
The dip of a stratum is the angle between its inclination and a horizontal plane. This is expressed in degrees and in direction— thus 15° N42W. For measuring the dip, several forms of clinometers are used, the simplest of which is similar in appearance to an ordinary pocket folding rule with two legs working on a hinge. One leg is placed on the stratum in the direction of its greatest inclination, and other is swung upwards until it is horizontal as indicated by the bubble in a level which it holds. The angle is then read on a circular scale attached to it. The direction it takes when placed at the maximum inclination is the direction of the dip. Other forms of clinometers, with which compasses are combined, give direct readings of the dip and direction at the same time. As strata often contain minor small waves it is better when taking the dip and a sufficiently wide exposure can be found, to place a board or stick on it, conform ing to the general direction and to place the clinometer on the board.
The strike is the line of direction taken by strata, or the line that would be formed by the intersection of the strata and a horizontal plane. This is represented by the line ad in Fig. 15. ObviOngy 'this is at right angles with the direction of the dip, and when the strata are not bent, it will be a straight line. Should the strata not only dip but bend also, then the strike will be a curve and when the measures have been upturned into a dome-like structure; so that each stratum occupies the position of an inverted bowl, the strike takes the form of the circumference of a circle.
Anticline is the name given to the arch-like position taken by strata when they have been folded. The corresponding position of strata when they are bent down and then up is known as a syncline. and frequently the crinkling in the earth's crust that has brought about the folding structure has resulted in a series of wave-like alternating anticlines and synclines (Fig. 16).
Where a series of strata is in an inclined position without the development of folding apparent or nearby, the structural form is known as a monocline. A monocline is really only one portion of a broad general fold. The line along the top of an anticline is the anticlinal axis; that along the bottom of the syncline is the synclinal axis.
The anticlinal theory, of I. C. White, relating to oil formation was first brought out in, connection with the development of the Appalachian fields and has had a wide application since then in many districts. It holds that, where strata are horizontal the oil and gas are irregularly scattered through the measure containing them, while in folded districts the oil and gas collect at the sum mits of the anticlines, and the synclines between are apt to be bar ren or to hold water. Another theory, that of Lesley and Ash burner, assumes porous areas of rock in which the oil has gath ered, and is also applicable in some fields.
Aside from theories, however, it is now a well-established fact that practically all petroleum is obtained from sedimentaries and that the major portion is derived from the sands and sandstones, and that these productive measures are usually overlain with a so-called cap rock. The cap rock is an impervious layer, of clay, shale, or some other compact material, which prevents ascension on the part of the gas and oil into higher strata and is especially important in connection with the .anticlinal theory.
In connection with the latter, the evidence developed in many fields shows that the fluids confined in a sedimentary measure tend in the course of time to separate according to their respective spe cific gravities. The gas rises to the topmost point available while the water, if such be present (and salt water is almost invariably associated with petroleum) displaces the oil by reason of its greater weight. Thus there are three fairly well-marked zones, first the gas, then the oil, and finally at the bottom the water. (Fig. 17.) The transition from gas to oil is not as definite and may not be so clearly shown as that from oil to water. In the latter it is not uncommon to trace out within a short distance, along a line of wells which penetrates the oil at greater and greater depths, a gradual change from oil with no water content to that containing a slight and then increasing percentage till finally a well far enough out on the trough of the syncline will be drilled which yields water only and no oil.