But there are worse dangers in the mountains than falls of snow and ice, for sometimes masses of rock come hurtling down, or worse still, the whole side of a mountain gives way and spreads ruin far and wide. Perpendicular or overhanging rocks, which seem securely fastened, suddenly become detached and rush headlong down the mountain-side. In their rapid fall, they raise a cloud of dust like the ashes vomited forth by a volcano; a horrible darkness is spread over a once pleasant valley; and the unfortunate inhabitants, unable to see what is taking place, are only aware of the trembling of the ground, and the crashing din of the rocks as they strike together and shatter one another in pieces. When the cloud of dust is cleared away, nothing but heaps of stones and rubbish are to be seen where pastures once grew, or the peasant ploughed his acres in peace. The stream flowing down the valley is obstructed in its course, and changed into a muddy lake; the rampart of rocks from which some débris still comes crumbling down has lost its old form; the sharpened edges point out the denuded cliff from which a large part of the mountain has broken away. In the Pyrenees, Alps, and other important ranges there are but few valleys where one cannot see the confused heaps of fallen rocks. Many of these catastrophes, known as the "Bergfall," have been recorded; and the records tell of the fearful havoc and destruction to life and property due to this cause. In Italy the ancient Roman town of Velleja was buried, about the fourth century, by the downfall of the mountain of Rovinazzo; and the large quantity of bones and coins that have been found proves that the fall was so sudden that the inhabitants had no time to escape. Taurentum, another Roman town, situated, it is said, on the banks of Lake Geneva, at the base of one of the spurs of the Dent d'Oche, was completely crushed in A. D. 563 by a downfall of rocks. The sloping heap of débris thus formed may still be seen advancing like a headland into the waters of the lake. A terrible flood-wave, produced by the deluge of stones, reached the opposite shores of the lake and swept away all the inhabitants. Every town and village on the banks, from Morges to Vevay, was demolished, and they did not begin the work of rebuilding till the following century. Some say, however, that the disaster was caused by a landslip which fell from the Grammont or Derochiaz across the valley of the Rhone, just above the spot where it flows into the Lake of Geneva. Hundreds of such falls have taken place within the Alps and neighbouring mountains within historic times.