Page:Great Neapolitan Earthquake of 1857.djvu/220

170 North of the great transverse ridge about Atella, we begin to encounter the great tufa deposit, of the ancient volcanic region around Monte Vulture, and extending northward to beyond Melfi, where it is evident that denudation, has levelled and spread out, over the limestone, prodigious quantities of the tufas and decomposed lavas, at a period probably long anterior to the ejection, of the earliest of the tufas of the Vesuvian tract, and where torrent and rain erosion, present features of the largest, and most instructive character.

Details, either of that, or of the Vesuvian volcanic tract, are beside our present purpose, however, except to remark, that in both, the limestone laps in under the superficial volcanic products to an immense distance; indeed, in the Terra di Lavoro—as we travel, for example, from Naples to Caserta and Capua—it is obvious that the level plain of tufa over which we pass, out of which the limestone mountains rise sheer and abrupt on all sides, has been run in and levelled between them, and has traced out by its surface, the contour, of every sinuosity of their narrow winding valleys, when all were under water, and that the limestone of the hills, in reality underlies almost the whole of the great tufa bed. Earthquake vibrations, therefore, penetrate both these volcanic regions, through the intervention of the harder and more elastic limestone beneath, the tufas being thus shaken like plastic clay in a saucer, just as the great alluvial beds in the more southern valleys, are shaken by the vibrations, primarily propagated through the limestone surrounding them. Of the two, probably the tufa is the worse material, for the easy propagation of earth wave.