Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/373

308 the same medium, should be greater, considerably, than the velocity of actual wave transit in the same. The preceding velocity of 3640 feet per second, being that due to the $$\sqrt[2]{\epsilon}$$, is, as we shall see hereafter, nearly four times as great, as was the actual transit of the wave, in the limestone; and some retardation, more or less, must undoubtedly have arisen, in passing through several miles of rock, more or less shattered, heterogeneous, and discontinuous beforehand, transversely to the line of fracture.

If, therefore, we assume the inferior limit of $$\mathrm V$$, to be equal, only to the transit rate of the shock wave, in the same formation, the duration of the precedent, tremors, would be about $$4 \times 8 = 32$$. The longest observed periods that I have recorded, vary between $$30$$ and $$40$$.

It is with considerable interest also, that, on referring to our Maps, A and B, and still better, on comparing with the localities, in Zannoni's great map, it will be found, that the curve of contrary flexure, assigned to the form of the fissure, conforms in a remarkable way, to the existing natural lines of surface contour, of the country through which it extends; and to such lines of pre-existent dislocation, as we may predicate, from those, of the valleys, hills, and water-courses. Thus, at its northern extremity, it follows, in part, the lines of the deep valleys, of the rivers Turno and Landro; crosses the Campostrina ridge, at one of its several transverse depressions, or minor valleys, follows diagonally, the general line of the valley, between Caggiano, and Pertosa; and then to the extreme southern end, strikes right into the deep valley, between the ranges, of Monte Tomaso, and that of the Costa del Castagneto, to the west of Polla.