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Parr IL. Sucr. ii. § 3] RIVER EROSION. 371 
Yang-tse the proportion of sediment by weight is estimated by Mr. 
H. B. Guppy at 44,,' but according to Dr. A. Woeikof this estimate 
is much under the truth. 
The most extensive and accurate determinations upon this subject 
yet made, are those of the United States Government upon the physics 
and hydraulics of the Mississippi river. As the mean of many obser- 
vations carried on continuously at different parts of the river for 
months together, Humphreys and Abbot, the engineers charged 
with the investigation, found that the average proportion of sediment 
contained in the water of the Mississippi is 7,5 by weight, or 
sooo by volume.” But besides the matter held in suspension, they 
observed that a large amount of coarse detritus is constantly being 
pushed along the bottom of the river. They estimated that this 
moving stratum carries every year into the Gulf of Mexico about 
750,000,000 cubic feet of sand, earth, and gravel. Their observations 
‘led them to conclude that the annual discharge of water by the 
‘Mississippi is 19,500,000,000,000 cubic feet, and consequently, that 
the weight of mud annually carried into the sea by this river must 
~ reach the sum of 812,500,000,000 pounds. ‘Taking the total annual 
contributions of earthy matter, whether in suspension or moving 
along the bottom, they found them to equal a prism 268 feet in 
height with a base of one square mile. 
The value of these data to the geologist consists mainly in the 
fact that they furnish him with an approximate measurement of the 
rate at which the surface of the land is lowered by subaerial waste. 
This subject is discussed at p. 441. 
2. Excavating Power.—It was a prominent part of the teaching 
of Hutton and Playfair, that rivers have excavated the channels 
in which they flow. Experience in all parts of the world has con- 
firmed this doctrine. The erosive work of running water depends 
for its rate and character upon (a) the friction of the detritus driven 
by the current against the sides and bottom of a watercourse, 
modified by (6) the geological structure of the ground. 
__ (a) Driven downward by the descending water of a river, the 
loose grains and stones are rubbed against each other, as well as upon 
the rocky bed, until they are reduced to fine sand and mud, and the 
sides and bottom of the channel are smoothed, widened, and deepened. 
The familiar effect of running water upon fragments of rock, in 
reducing them to rounded pebbles, is expressed by the common 
phrase “water-worn.” <A stream which descends from high rocky 
ground may be compared to a grinding mill; large boulders and 
angular blocks of rock, disengaged by frosts, springs, and general 
atmospheric waste, fall into its upper end; fine sand and silt are 
discharged into the sea. In the series of experiments already 
referred to, Daubrée, using fragments of granite and quartz, caused 
them to slide over each other in a hollow cylinder partially filled with 
1 Nature, xxii. p. 486, xxiii. p. 9. 
2 Report, p. 148. ‘The specific gravity of the silt of the Mississippi 5 as as 1°9. 
B 
