Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/326

Rh fact is so, but that the river marks the direction of the great continuous chains, and of the direction also, in which the valley formations are least broken and discontinuous, namely, parallel to the alignment of the flanking chains.

The same principles apply, in explanation of the fourth condition stated. The wave travels best "end on" to the strata, and not transverse, more or less, to them.

When the wave-path is transverse, or obliquely transverse to mountain chains, on the contrary, everything tends to oppose its progress, and hasten its extinction. The surfaces of discontinuity are as numerous, as the multiplied beds that we should cross, in boring through the base of the range, to say nothing of fissures, or other casual breaches of connection. The planes of the bedding, are continually altering in direction, with reference to the wave-path, by which the wave sufifers continual loss of vis vivâ, and is dissipated by dispersion. As the wave is transmitted from valley to range, and from range to valley, in perhaps many successions, it is continually passing from one formation into another, with a change of velocity in each, and therefore loss of vis vivâ at the junction. As we ascend, in the order, from the deeper to the more superficial formations, reposing upon the flanks of the ranges, we in general find