fissure, at , (Fig. 345,) in the direction , is expended in compressing and altering the relative positions, of the clays,
Fig. 345.
gravels, rock fragments, detritus, or whatever other loose and incoherent materials the fissure contains. Upon the width of the fissure, in proportion to the amplitude of the wave, at its entering the loose material at , and upon the degree of looseness, incoherence, and capability of being crushed and moved, possessed by the latter, will depend tbh residual unextinguished portion, that shall be transmitted on to the opposite wall of the fissure , and so into and on through, the solid rocks beyond. Where at the same time the rock at both sides, has been so tilted, that its bedding is vertical, and nearly parallel with the walls of the fissure, transverse to the wave-path, then the obstacles to the wave are the greatest possible; and where the plane, or various planes, of the fissure, are met obliquely, by the emergent wave-path, to , instead of perpendicularly, then we have, in addition to the foregoing, further loss by refraction and reflection, as already described. This effect of fissures, was presented upon a grand scale by the earthquake of December, 1857.
Lastly, deep and continuous valleys with precipitous sides, cut off and extinguish the wave at their free lying
