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a - DYNAMICAL GEOLOGY. ———_[Boox TIT. ; 
gives the subjoined table of the power of transport of different 
velocities of river currents :— | 
In. per Mile per 
Second. Hour. 
3 = 0°170 will just begin to work on fine clay. 
6 = 0°340 will lift fine sand. 
8 = 0-4545 will lift sand as coarse as linseed. 
12 = 0°6819 will sweep along fine gravel. 
24 = 1°3638 will roll along rounded pebbles 1 inch in diameter. 
36 = 2:°045 will sweep along slippery angular stones of the size of an egg. 
It is not the surface velocity, nor even the mean velocity, of a 
river which can be taken as the measure of its power of transport, but 
the bottom velocity—that is, the rate at which the stream overcomes 
the friction of its channel. (b) The average specific gravity of the 
stones in a river ranges between two and three times that of pure 
fresh water; hence these stones when borne along by the river lose 
from a half to a third of their weight in air. Huge blocks which 
could not be moved by the same amount of energy applied to them 
on dry ground are swept along when they have found their way into 
a strong river current. The shape of the fragments greatly affects 
their portability, when they are too large and heavy to be carried in 
mechanical suspension. Rounded stones are of course most easily 
transported; flat and angular ones are moved with comparative 
difficulty. (See p. 372.) 
Besides inorganic sediment, rivers sweep seaward the remains of 
land animals and vegetation. The great rafts of the Mississippi and 
its tributaries are signal examples of this part of river action. The 
Atchafalaya has been so obstructed by drift-wood as to be fordable 
like dry land, and the Red River for more than a hundred miles 
flows under a matted cover of dead and living vegetation. The 
Amazon, Ganges, and other tropical rivers furnish abundant examples 
of the transport of a terrestrial fauna and flora to the sea. 
Besides their ordinary powers of transport, rivers gain at times 
considerable additional force from several causes. Those liable to 
sudden and heavy falls of rain acquire by flooding an enormous 
increase of transporting and excavating power. More work may 
thus be done by a stream in a day than could be accomplished by it 
during years of its ordinary condition. Another cause of sudden 
increase in river-action is provided when, from landslips formed by 
earthquakes, by the undermining influence of springs, or otherwise, 
a stream is temporarily dammed back, and the barrier subsequently 
gives way. ‘The bursting out of the arrested waters produces great 
destruction in the valley. Blocks as big as houses may be set in 
motion, and carried down for considerable distances. Again, the - 
transporting power of rivers may be greatly augmented by frost (see 
postea, p. 401). Ice forming along the banks or on the bottom en- 
1 The extent to which heavy rains can alter the usual characters of rivers is fereibly 
exemplified in the graphic account of “'The Morayshire Floods,” by the late Sir T. Dick 
Lauder. In tho year 1629 the rivers of that region rose 10, 18, and in one case even 50 
feet above their common summer level, producing almost incredible hayoc. 
