Page:Great Neapolitan Earthquake of 1857.djvu/219

Rh been all swept off from them and levelled under water in the valley bottoms, to form these rich plains of agriculture. The soil is commonly a dense tenacious brown or red brown loam, often free from stone for large areas, and composed of the detritus of the calcareous and argillaceous rocks. That combining the sand of the cretaceous rocks, and the clay of the marl beds, is of high fertility.

Where the fall is small, the rivers (as the Tanagro in the Val di Diano) run slowly, upon beds of these clays, and form for themselves a permanent bottom, pared by the angular fragments and boulders of limestone washed from the soil; but when the fall is rapid, as in the Rio Agri—a view in the valley of which, is given in Photog. No. 110 (Coll. Roy. Soc.), then erosion takes place upon a scale of great grandeur. The alluvial soil is cut through to the bottom; the marl beds follow it; and the river runs upon a bed of loose blocks of limestone, resting on the bare rock, at the bottom of a ravine or "nullah", with steep sloping banks, which are continually washing away and slipping in, and at a level often of 300 to 700 feet below the flat of the piano, or valley bottom, that spreads out above, for miles at either side. The rocks in the foreground of this photograph, are of the calcareous breccia described. The rivers themselves, fed by the heavy rains of the wet season, which fall with something of the regularity and violence of the tropical rainy seasons, by the wet of winter, and by the sudden melting off, of the vast masses of snow that fall on the higher ridges, (and at elevations exceeding 2,000 feet above the sea, lie accumulating during winter,) are of a torrential character, and almost all flow thick, and turbid with brown alluvial sediment, to the sea.