EFFECTS OF WATER AND VOLCANOES ON THE SURFACE OF THE EARTH.
29. Having now described all the varieties of ine -luality observed on the surface of the dry land, it will be interesting to endeavour to discover how these have been formed.
Many of the elevations and hollows appear to be ori ginal formations, while others seem to be more particu larly the effects of water and of volcanic fire. We shall consider each of these in particular.
30. Water acts either chemically or mechanically on the surface of the earth : and it may further be considered, according to its destroying and forming effects.
Destroying Effects of Water We shall first consider the mechanical destroying effects of water.
Every long continued rain convinces us of the pow erful mechanical effects of water on the surface of the earth. The precipitated water penetrates the surface, then flows along mixed with the matter of the soil, and in its course forms small water-courses, and occasions considerable changes in the flat country and the decli vities of mountains. As it rains a very considerable portion of the year, and as every rain carries along with it a quantity of the soil, very obvious changes must in this manlier be induced on the surface of the globe. Thunder storms and water-spouts, although more un common phenomena, produce more considerable changes, and this either alone, or when their waters join or how into rivers. These deprive whole districts of their soil to the bare rock ; they sometimes even form small ra vines, and break down and carry away great masses of rock, that were either formerly much rent, or of such a form as to be easily overpowered by water. If such changes take place in the low land, they must be vastly more considerable in the high land.
The thaw-floods that take place in low countries to wards the end of winter and beginning of spring, and in mountainous districts during summr.., occasion still greater changes on the sui face of the earth. Their ef fects are truly frightful, particularly whe• accompanied with rain. The declivities in low countries, over which water flows, are less considerable than in high countries ; and besides, the water can exten:l itself far ther in low and flat countries ; hence its destroying effects are diminished in intensity. In mountainous
countries, on the contrary, the fall is much greater than in flat countries, and the water is compressed into narrow rocky valleys ; hence it follows, that rain floods must be more destructive, the more considerable the quantity of water, and the 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. II we compare together all these cir cumstances, we shall find that mountainous countries are more liable to suffer from the effects of floods, than low and flat countries. To this, indeed, there are excep tions, 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 or 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 its power by the addition of a considerable quantity of water, but also causes it to overflow its banks, and deluge the neigh bouring 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 magnitude 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 dis tance from the sea, according to the relative magnitude of their parts.