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ALLUVIUM. 
oa a 
128 
other low situations of the lower valley, a stratum 
of alluvial land. The force of running water, 
when directed against a natural embankment, or 
any other earthy or rocky obstacle in its course, 
is very considerable; and this force is greatly 
increased by the momentum of current down a 
steeply inclined plain, and by the presence in the 
current of a large mechanical mixture of sand 
and gravel. Large stones and pieces of rock are 
lighter in water than in air, to the amount of 
the weight of a mass of water equal to their own 
bulk ; and they, in consequence, roll along the 
current of a river with remarkable ease and velo- 
city, and are transported by floods to great dis- 
tances, and occasionally deposited in extraor- 
dinary situations. But with how much greater 
ease, or in how much feebler conditions of fluvia- 
tile current, may detritus, soils, and earth-banks 
be worn into fragments, and carried headlong 
for many miles to be deposited athwart the sur- 
face of a valley, or the delta of a river! The 
most ordinary observer needs but to look at the 
effects of one day’s common heavy rain, in order 
to see that hill-pastures are swept and washed 
like the pavements of a town, that newly tilled 
arable lands are deprived of their soil to the 
amount of several hundred weights per acre, that 
the banks and bed of streams are abraded and 
deprived of considerable portions of their sub- 
stance, that loose and decayed vegetation in the 
wood and on the field is extensively carried off, 
and that the streams, in the middle and lower 
parts of their course, are red and turbid with the 
load of the accumulated spoils. When a stream 
flows at the rate of three inches in the second, it 
tears up fine clay; six inches in the second, fine 
sand; twelve inches in the second, fine gravel; 
and thirty-six inches in the second, beds of such 
loose stones as have each the size of a hen’s egg. 
The power which results from these rates of velo- 
city is quite common in the streams of hilly and 
undulated countries, and is far exceeded by many 
of the streams of mountainous regions; and it 
obviously could not have been constantly exerted 
during thousands of years, in lines at the mean 
distance from one another of only a few furlongs 
over the great part of the terrestrial world, with- 
out effecting great changes in the configuration 
of the earth’s surface, and producing an enormous 
ageregate amount of alluvial deposit. Even in 
level countries, too, where the loss of power in a 
river is great from the diminution of its velocity, 
that loss is often very considerably compensated 
by the tortuosity of the river’s course, the weight 
of its accumulated volume of water, and the fre- 
quent and heavy abrasion of its current upon 
protrusions and salient angles of the banks. The 
stream, on entering a plain, may run obliquely 
so as speedily to form a steep bank or vertical 
cliff; it deflects, or is flung off from this, and 
runs obliquely to a point on the opposite side, 
there also to form a steep bank or vertical cliff ; 
it repeats this process at every one of its mazy 
windings, or rather at every curvature of its sinu- 
ous course ; and it in consequence rolls the whole 
weight of its large and heavy volume against a 
rapid series of salient angles, and carries on, from 
side to side, and from point to point of its banks, 
a constant and extensive process of undermining 
and erosion. When a tortuous river-course winds 
among solid rock, and constitutes a prolonged 
and stupendous ravine or natural canal, as in the 
case of the river Moselle, whose banks in some 
places are 600 feet in height, it affords the most 
forcible evidence of the great power of running 
water, and thoroughly exposes the absurdity of 
the theories which ascribe a chief part, or almost 
any part, of the configuration of valleys to the 
action of fire, or to mere disruption of the earth’s 
crust. 
In 1827, after a heavy fall of rain, the little 
Northumberland rivulet called the College, car- 
ried blocks of stone upwards of half a ton in 
weight two miles down its channel, and one 
block of nearly two tons in weight about a quar- 
ter of a’mile. During the Moray flood of 1829, 
the river Nairn carried a block of sandstone rock, 
fourteen feet in length, three feet in width, and 
one foot in thickness, upwards of two hundred 
yards down its channel; and the river Don drove 
about 450 tons of stones, many of which were 
each from 200 to 300 pounds in weight, up an 
inclined plane of about ten yards in length, and 
six feet in ascent, and left them on a piece of 
flat ground in a heap of about three feet in 
height. The Niagara river in America wears so 
rapidly the precipice at its celebrated falls, as to 
have made the cataract recede about 150 feet 
during the last forty or forty-five years; and it 
flows for seven miles below the falls in a channel 
of about 150 feet in depth, and 480 feet in width, 
before debouching into a plain, and has evidently 
formed the whole of this long sweep of channel 
by the same process of erosion which continues 
to go on at the falls. The river Simeto, the 
largest stream in Sicily, was dammed up by a 
mass of lava from Mount Etna in 1603; and 
though the lava formed a rock nearly as hard 
and compact as basalt, the river has already 
worn through it a channel of from forty to fifty 
feet in depth, and from fifty to several hundred 
feet in width. The Nerbuddah, a river of Hin- 
dostan, has worn in a basaltic rock a passage of 
about an hundred feet in depth. The rivers of 
the eastern Alps have, in various instances, cut 
defiles and ravines through masses of coarse, 
horizontal conglomerate to the depth of 600 or 
700 feet. The river Inn near Innspruck, and 
the river Drave between Klagenfurt and Mar- 
burg, have also cut enormously deep ravines 
through stupendous masses of solid rock. A 
temporary stream formed by the bursting of the 
barrier of a lake near Martigny in the Vallais, 
passed over a distance of forty-five miles to the 
lake of Geneva in five hours and a half; it swept 
away trees, bridges, and human habitations with 
CC ee 
