ARRANGEMENT OF PEBBLES BY WATER. 53 



like drawn wire. If the Aoav were very great (literally infinite) the lines 

 along which relative movement took place at the inception of strain 

 would become horizontal. The schistose partings would then in each set 

 range through the %ngle «>. 



Relative motion, in a mass subjected to direct uniformly distributed 

 pressure, can only take place perpendicularly to the line of pressure when 

 the strain ellipsoids are infinitely thin discs or when the rigidity is zero. 

 In other words, only liquids, viscous or otherwise, can act in this manner. 

 The behavior of semi-fluid material, like wet clay, approximates closely 

 to that of a viscous fluid. 



Rigid Disc in rcxixliiHj Medium. — The behavior of an elastic mass under 

 simple pressure leads to an extremely simple method of proving a 

 dynamical proposition of much importance to geologists. A simple 

 pressure acting against a resistance converts any sphere of unstrained 

 matter into an oblate ellipsoid of revolution, the minor axis of which is 

 in the direction of the pressure. If the constant pressure were to exceed 

 the constant resistance, the mass would move in the direction of the 

 pressure and of the minor axis of the oblate ellipsoid. Now, it is a well- 

 known fact that the whole or any portion of an elastic mass which is in 

 equilibrium, whether at rest or in motion, may be supposed to become 

 infinitely rigid without disturbing the equilibrium. This is an almost 

 self-evident proposition, for a mass is in equilibrium only when there is 

 no influence tending to change its form, and it therefore makes no differ- 

 ence whether this form is capable of change or not. Hence in the present 

 case the strain ellipsoid may be supposed replaced by a rigid mass. 

 Consequently a rigid ellipsoid of revolution moving under the influence 

 of a pressure against a resistance will be in equilibrium when it opposes 

 its greatest surface to the resistance. 



Similarly an elastic sphere under tension becomes a prolate ellipsoid, 

 and consequently a rigid prolate ellipsoid moving under the influence 

 of tension against resistance will be in equilibrium when its longest axis 

 coincides in direction with the tension. 



If a cube were circumscribed about either of these spheres, with four 

 of its edges in the direction of the force, it would become a rectangular 

 parallelopiped with sides parallel to the axes of the ellipsoid. Any plate 

 or rod may be made up of a single layer or row of such flattened or 

 elongated cubes. Hence any rigid disc or rod moving against a resist- 

 ance under the influence of pressure will be in equilibrium when its 

 smallest dimension is in the direction of pressure. If it moves under the 

 influence of traction, its longest axis will fall into the line of traction. 



If a flattened pebble is dropped into a running stream, the water will 

 exert a pressure upon the stone until its inertia is overcome, and during 



