Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/435

370 the subterranean conduit at Polla is in its usual state turbid; and it issues clear or much clearer at the other end. Great enlargements of the tube by caverns, &c., exist therefore, along its length, in which there is but a very feeble velocity; and in this slack water the fine mud of turbidity is deposited, and so the waters pass on clear. The dislodgment of a few masses of stone from the roof or sides of one of these cavernous "settling basins," or the under-water slippage, of one of the soft incoherent beds, of chalky mud in the bottom, would be sufficient to produce turbidity of the issuing water for a long time afterwards, the length of time being greater, as the cubic capacity of the "settling basins," was larger, in proportion to that of the tubular duct leading the water from it. This was also the entire mystery of the discolouration of the springs issuing at La Sala, at Atena, and at Marsico Nuovo. The muddyings alleged to have been observed in some earthquakes, (nothing of the sort was stated to me as respects the present one,) of the water of common draw-wells (dug in the ground) precedent to and forerunning the shock, are, I believe, mere superstitions, but, at any rate, require further investigation as to the actual fact.

In the case of the great earthslip, into the bed of the Agri, in the Valley of Viggiano, its circumstances, and the mode and rapid progress, of its removal in mud, have been stated in Part II. Its toe, and the mass behind, were not so circumstanced, as to produce upon the occurrence of its descent, an effective dam across the whole river bed to any considerable depth; but had the mere accidents, of the height of the slipped mass at the point, the total volume