Page:Great Neapolitan Earthquake of 1857.djvu/402

318 These give a declination varying between 13° 30' and 15° 30' west. I also took an observation of the sun, but on computing I find that some decisive error must have occurred in my note of the sun's azimuth, which renders the observation valueless, as there was no ground for supposing any material disturbance of the ordinary amount of declination at this spot, more than at La Duchessa, where the same error appears to have been made.

As I left Polla in the afternoon. the grand plain of the Vallone di Diano opened before me, level almost as a sea, of deep rich alluvial clays, which, as they approach the roots of the mountains that rise almost abruptly from the plain, assumes the form of a sort of sloping and almost continuous terrace all round the plain, elevated everywhere a few feet above its average surface, seldom rising more than 40 feet above it, frequently not half that.

The outline section of this terrace, as it sweeps towards the centre of the plain, is that of a flat, hollow, parabola-like curve, with a slope varying from 15 to 1 to 60 to 1, or even still more gradual, and suggests at once to the eye, the geologic conditions that produced the piano; once a large lake, or arm of the sea, which found egress for its waters at a level probably not lower than the summits of the Campestrina Pass. In that condition of things, the rich bed of mud was deposited, that now forms the basis of the valley and of its agricultural wealth.

The great fracture, through which the Calore now finds its course sub dio, must have been subsequently formed, and through it the lake was drained down to the level of its marginal terrace. Its subsequent progress of desiccation must have been gradual, and dependent upon the rate