Page:A Treatise on Geology, volume 2.djvu/138

 124 of 90°, in which oblique motions will be at a maximum. One of these (tangential) planes, therefore, will dip in the same direction (but not necessarily at the same angle) as the beds, and the other in the opposite direction.

These things premised, we may by referring the pressures to the plane of stratification discover their effect on the outlines of organic remains, which, for the sake of comparison with our observations already recorded, we shall assume to be semicircular, and always laid with the hinge or diametral line parallel to the line of dip.

First, let it be supposed that pressure is applied perpendicular to the stratification and tension produced parallel to the strata. In this case there will be a symmetrical extension of the figure in the direction of the line of dip. If tension be applied perpendicular to the strata and pressure exerted parallel to them, the semicircle will be symmetrically contracted to a semi-ellipse, as in the example already represented in the diagram (p. 119.); and, finally, if either pressure or tension be applied in a direction meeting the plane of stratification at 45°, so that a plane of maximum tangential action shall coincide with the stratification, the semicircle will be unsymmetrically changed in form, so as to become elliptical with its diameter lying obliquely across the line of dip—in fact, to be angularly distorted.

When, therefore, angular distortion occurs in an equilateral shell placed symmetrically with respect to the line of dip, we may be sure the result is due to the tangential movements developed by pressure. If this happens chiefly or exclusively when the cleavage nearly coincides with the stratification, and happens rarely or not at all when the cleavage meets the strata at or about an angle of 45°, we may conclude that the cleavage plane is not perpendicular to any axis of antecedent pressure or tension, but is coincident with one of the two planes of maximum tangential movement.