LEVELLING ACTION OF WATER. 5 
important influence, in this respect, is even yet exercised 
by water at every moment. As it falls down as rain, 
trickling through the upper strata of the earth’s crust, 
and flowing down from heights into hollows, it chemically 
dissolves different mineral parts of the ground, and mechani- 
cally washes away the loose particles. In flowing down 
from mountains water carries their débris into the plains, 
or deposits it as mud in stagnant lakes. Thus it con- 
tinually works at lowering mountains and filling up 
valleys. In like manner the breakers of the sea work 
uninterruptedly at the destruction of the coasts and at 
filling up the bottom of the sea with the débris they 
wash down. The action of water alone, if it were not 
counteracted by other circumstances, would in time level the 
whole earth. There can be no doubt that the mountain 
masses—which are annually carried down as mud into the 
sea, and deposited on its floor—are so great that in the 
course of a longer or shorter period, say a few millions 
of years, the surface of the earth would be completely 
levelled and become enclosed by a continuous sheet of water. 
That this does not happen is owing to the perpetual volcanic 
action of the fiery-fluid centre of the earth. The surging of 
the melted nucleus against the firm crust necessitates con- 
tinual alternations of elevation and depression on the 
different parts of the earth’s surface. These elevations and 
depressions for the most part take place very slowly; but, 
as they continue for théusands of years, by the combined 
effect of small, interrupted movements, they produce results 
no less grand than does the counteracting and levelling 
action of water. 
Since the elevations and depressions of the different parts 
