Page:A Treatise on Geology, volume 2.djvu/137

 CHAP. VII. of this investigation, a short statement of the bearing of it may suffice to put geologists on the right track for further inquiry, and perhaps to show them how much of beautiful illustration of geology is lost by those who permit themselves to be deterred by mathematical expressions from a close survey of physical truths. Pressure and tension being taken as of opposite meanings, and coexistent in a mass of rock, we may admit as representing their directions three coordinate axes passing through a central point at right angles to each other. Along these axes the effects of pressure and of tension will be direct and total, so that a small plane situated at right angles to one of these lines will be subject to the pressures or tensions of that line only, and will be moved, if at all, along that line; but a small plane placed in some other position will be moved in a line not having the same direction. As these pressures and tensions are assumed to be general, and so to affect all the particles, it is obvious that we shall have three co-ordinate planes, parallel to which direct forward or backward motion is possible, and between them other (tangential) planes in which the possible motions are oblique.

Now in the case before us one of the axes of direct pressure or tension may be regarded as of little or no effect, viz., that which coincides with the strike of the cleavage and the strike of the strata. And from this it follows, that direct motion from pressure should take place along lines lying in one plane only, viz., that which is perpendicular to the anticlinal, and only in two directions crossing each other at right angles in this plane, one of these directions being that of pressure, the other that of tension. Planes perpendicular to these two lines will be planes of direct tension or pressure, and their strike will be that of the beds. There will be also crossing these planes (but having the same strike with them) two other (tangential) planes, making with them angles of 45°, but with each other angles