GEOLOGY. 



395 



more considerable the quantity of water, and tne 

 more mountainous the country. These floods are 

 still more destructive, when the mountain rocks are 

 of such a nature, as to afford little resistance to the 

 impetuosity of the water ; that is, when they are 

 decomposed, loose in their texture, or have such a 

 shape as to allow the water to act more easily on 

 them. If we compare together all these circum- 

 stances, we shall find that mountainous countries are 

 more liable to sutler from the effects of floods, than 

 low and flat countries. To this, indeed, there are 

 exceptions, as in the case in some granites, and other 

 rocks that long resist the effects of the most powerful 

 and violent floods. 



The water of these floods, in its progress towards 

 the lower parts of the earth, flows either into ravines, 

 and from these into valleys and beds of rivers ; or 

 when it meets with no ravine, scoops out a bed for 

 itself, wherever it meets with a soft yielding rock or 

 a slight hollow. The junction of these mountain- 

 streams with the river of the district not only increases 

 ita power by the addition of a considerable quantity 

 of water, but also causes it to overflow its banks, 

 and deluge the neighbouring country, and thus to 

 occasion great changes on its surface. The different 

 loose materials are carried towards the sea, and are 

 deposited at different distances from the mouth of 

 the river ; and these are proportioned to the magni- 

 tude of the masses. The finest or loamy parts reach 

 the sea ; the sand, gravel, and larger rolled masses 

 being left on the surface at greater or less distance 

 from the sea, according to the relative magnitude of 

 their parts. 



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 ex- 

 tensive valleys are formed by mountain groups dis- 

 posed in a circular form, as is the case in Bohemia, 

 Hungary, Transylvania, &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 fis- 

 sures, and the fall of great masses of mountains, take 

 place during floods or wet seasons. These falls are 

 occasioned either by the weight of the masses being 

 increased 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 

 masses 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 enclosing 

 barrier, and sometimes so suddenly as to deluge and 

 desolate the lower country. 



The waters of the ocean also act very powerfully 

 in breaking down the land. Its waves and currents 

 are particularly active in these destroying operations. 

 They either hollow out the rocks on the coast into 

 caves of greater or less magnitude, or, by washing 

 away softer subjacent strata, cause sinkings and fal- 

 lings of great masses of rock. The caves in the 

 islands of Arran and Jura have been partly formed 

 in this manner. In some of the Shetland islands, the 

 sea has formed a passage through large rocks. See 

 a representation in the Plate (41) of the destroying 

 effects of water in three several places in Shetland. 

 Also of stony fragments which have been drifted by 

 the sea. 



If manv streams act in different directions on the 



same coast, or in conjunction with land-floods, as w 

 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 destroy- 

 ing 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 thirteenth century the sea broke 

 in, and covered the whole country, and left only de- 

 tached portions of the land, which now form the 

 islands denominated Texel, Vlieland, Schelling, New- 

 land, and others. This remarkable change is sup- 

 posed 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. 



Water in the state of ice, also produces consider- 

 able changes on the surface of the earth. Thus we 

 often observe masses from a hundred-weight to many 

 tons floated by rivers during thaw-floods, and these 

 frequently break up the banks of the rivers, and even 

 tear away immense masses of solid rock. Sea-ice 

 also produces similar effects on coasts, but on a greater 

 scale. 



The freezing of water contained in the fissures of 

 rocks also occasions considerable alterations on the 

 surface of the earth. This is observed most parti- 

 cularly in those rocks that have perpendicular fis- 

 sures, because these allow the water to enter more 

 easily, and favour the separation of the masses when 

 the water expands during the process of freezing. 

 Hence we find no species of rock more changed by 

 the effects of frost than basalt and porphyry slate. 



The chemical effects of water, particularly the 

 destroying effects, depend on the kind of rock over 

 which it flows ; for some allow water to act on them 

 chemically, others do not. Limestone, gypsum, and 

 rock-salt, are more particularly acted on by water 

 than most other rocks. 



By this agency of water, the height of limestone 

 and gypsum mountains is gradually diminished, caves 

 are excavated in them, and the water of such coun- 

 tries is much impregnated with gypseous and calca* 

 reous matters. The rock-salt which occurs in hills of 

 gypsum, is often dissolved by the water, and thus 

 cavities of considerable magnitude are formed ; and 

 by the continued action of the water on the gypsum, 

 the cavities increase in size, until the superincumbent 

 pressure becomes too great, and then the roof falls 

 in and forms those remarkable funnel-shaped hollows 

 so often observed in gypsum countries. 



Sometimes, as in felspar rocks, the percolating water 

 washes away the alkaline ingredient ; in other cases, 

 the moisture combines with iron, and forms hydrate, 

 or by its decomposition oxidates the metallic sub- 

 stances in a greater or less degree. By its action on 

 sulphurous compounds, as on pyrites, it gives rise to 

 sulphates or vitriols. As iron is the most general 

 and abundant metal in the mineral kingdom, and is 

 easily acted on by air and moisture, it follows that it 

 must be one of the most active agents in the disin- 

 tegration of mineral substances. 



\Ve sliall now consider the forming effects of water, 

 which are either-mechanical or chemical. 



It is evident, that every mechanical destruction will 

 be followed by a mechanical formation; for the masst's 

 which are separated by the water will be again dcpo- 



