This mechanical action of water appears in many cases to have contributed in an eminent degree to the hollowing out of valleys ; but all valleys have not been formed in this manner ; for many and very extensive valleys are formed by mountain groups disposed in a circular form, as is the case io Bohemia, Hungary, Tran sylvania, &c. ;• others by the original inequalities of the crust of the earth ; some by the unequal deposition of formations, and others by the widening of great rents.
It is also observed, that numerous rents and fissures, and the fall of great masses of mountains, take place during floods or wet seasons. These falls are oc casioned either by the weight of the masses being in creased by the great quantity of absorbed water, or by the diminished cohesion of the parts of the rock effected by the same cause, or by the splitting of great masse.,, by freezing of water, or any other power that interrupts the continuity of the rock, and favours its separation into different masses. The fall of rocks is also occasioned by the softening and removal of subjacent strata or beds by means of water.
These masses sometimes interrupt the course of rivers, and thus form lakes. These lakes in their turn again force a passage through this inclosing barrier, and some times so suddenly as to deluge and desolate the lower country.
In the year 1618, the once considerable town of Plurs, in Graubunden, with the neighbouring village of Schi lano, were overwhelmed by an immense mountain mass, which separated with a flightful noise from the south side of the mountain of Corto. In 1673, the sinking of several great masses of rocks in the Pyrenees caused very vio lent ovcifluwings in Gascony. In 1714, the west side of the Diablerct, in the Vallais, fell down, and covered the neighbouring country with its ruins for two miles in length and breadth, and the immense blocks of stone in terrupted the course of the rivers ; and lakes were thus formed. Many similar instances are on record in France, Italy, England, Scotland, &c. When the barriers of lakes, in high and mountainous countries, burst during great thaws or uncommon floods of rain, tremendous °vet flow ings of the lower country that almost exceed conception are the consequence. Immense masses of rock are torn away and carried to an inconceivable dis tance, and whole countries are desolated. Many of the
deluges mentioned by ancient writers appear to have been caused by the bursting of lakes.
The Waller Lake, in the Ty, ol, rose so much, owing to the melting of the ice of the neighbouring glaciers, that it broke through its natural barrier ; and its water was precipitated with so much velocity and violence into the low et country, that whole valleys and plains NI/ere desolated. The bursting of a lake in the valley of Bagne occasioned great devastation. There are many other instances of takes having emptied themselves in other parts of Switzerland, as also in the Fichtelgebirge, &c.
The waters of the ocean also act very powerfully in breaking down the land. Its waves and currents are particularly active in these destroying opet ations. They either hollow out the locks on the coast into caves of gi eater or less magnitude, or, by washing away softer suljace.nt strata, cause sinkings and fallings of great masses of rock. The caves in the islands of Arran and Jura have been partly formed in this manner.
If many streams act in different directions on the saute coast, or in conjunction with land-floods, as is often the case, the destroying effect is very great. Frequently also the power of the flood is increased by ebb and flood tide. In this manner many maritime countries have been overwhelmed by the sea.
The Baltic Sea affords examples of these destroying effects ; thus the island of Rugen was formerly joined to the Continent, but, by the violent action of the sea, has been much diminished in magnitude, and separated from it. The effects it has produced on the coasts of Carniola, Dalmatia and Egypt, are well known. The Zuyder-zee, which is contained between the provinces of Holland, Utrecht, Gelders, Overyssel, and Friesland, was formerly a lake, through which an arm of the Rhine, named the Flevo, flowed towards the ocean. In the 13th century the sea broke in, covered the whole country, and left only detached portions of the land, which now form the islands denominated Texel, Vlieland, Schclling, Newland, and others, that now serve to defend this sea from the power of the waves of the ocean. This remarkable change is supposed to have been occasioned by a violent land-flood, in conjunction with high tides, and a high wind blowing in an opposite direction to the course of the river.