AGENCIES IN THE ALTERATION OF ROCKS. 145 
ness of the crust of the globe has been formed of the débris of 
pre-existing formations, that have been ground down and held in solu- 
tion till deposited in the layers afterwards hardened into rock. Whence, 
then, this immense accumulation of matter, and what the disintegrating 
agents ? 
1. Atmospheric Agency.—The atmosphere, by its chemical action, and by 
the combined effects of alternate heat and cold, wetness and dryness, is 
continually crumbling down all exposed surfaces, forming new soil, and 
thus increasing the earthy covering of the globe. The wind, also, has an 
incredible power of drifting and heaping up sandhills along the shore— 
as in the county of Elgin, where an ancient barony has been entirely 
reduced to a desert through this means—and in raising the waves of the 
sea, and wearing the rocks through the mighty force of its swooping 
~billows. Frost, too, is one of the quietest but most powerful disinte- 
grating agents ; for when water has percolated a mass of rock, the act of 
freezing exerts a great expansive force which cracks the rock. But 
frost can work on a grander scale, for to its agency is due the existence 
of avalanches, glaciers, and icebergs; which, whether sweeping with 
overwhelming convulsion, or crawling down the mountain side, or 
floating and grating on the ocean floor, continually and with terrible 
effect, wear down or dash to pieces every rock that obstructs their 
irresistible course. 
2. Aqueous Agency.—The most extensive aqueous agent is rain, which 
wears, softens, percolates, and gradually wastes away every rock on which 
it falls. The rain-water also gathers under the ground in large cavities, 
where springs are formed, which dissolve the interior rocks, and, bursting 
out, deposit their solutions of lime, iron, sulphur, soda, flint, and bitumen. 
One of the most powerful degrading agents is, of course, the sea; which, as 
it beats on its rocky shores, wears, rolls, and grinds to powdery sand the 
flintiest rocks, and presents as monuments of its mighty power of waste 
those lofty cliffs that guard its shores. But more powerful, but less 
obvious agents of destruction than the sea, are the many streams that 
everywhere traverse the land on their way to this boundless reservoir. 
The power of rivers in excavating and wearing away the surface of the 
globe is much greater than at first thought might be supposed. Every 
valley, however deep, has been mainly worn down by river-action, extend- 
ing over immense periods of time. When we contemplate the mighty 
valleys enclosed by towering peaks capped with eternal snows, that lie 
hid amidst the mountain solitudes of the Alps, the Andes, or the 
Himalayas, we may well be astonished at such a statement. But that 
these huge excavations have been mainly produced by the combined 
action of air, frost, rain, and river, has been demonstrated beyond a doubt 
by a vast accumulation of facts and reasonings on phenomena in all parts 
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