
i 
‘Parr IL. Sucr. ii.§7.] SUBAERIAL DENUDATION. 443 
specially drew attention to the Mississippi as a measure of denudation 
and thereby of geological time.> __ 
When the annual discharge of mineral matter carried seaward by 
a river and the area of country drained by that river are both known, 
the one sum divided by the other gives the amount by which the 
drainage area has its mean general level reduced in one year. For 
it is clear that if a river carries so many millions of cubic feet of 
sediment every year into the sea, the area drained by it must have 
lost that quantity of solid material, and if we could restore the 
sediment so as to spread it over the basin, the layer so laid down 
would represent the fraction of a foot by which the surface of the 
basin had been lowered during a year. 
It has been already shown that the material removed from the 
land by streams is twofold—one portion is chemically dissolved, the 
- other is mechanically suspended in the water or pushed along the 
bottom. Properly to estimate the loss sustained by the surface 
of a drainage basin, we ought to know the amount of mineral 
‘matter removed in each of these conditions, and also the volume of 
water discharged, from measurements and estimates made at different 
seasons and extending over a succession of years. These data have 
not yet been fully collected from any river, though some of them 
have been ascertained with approximate accuracy, as in the Mississippi 
Survey of Messrs. Humphreys and Abbot, and the Danube Survey of 
the International Commission. Asa rule, more attention has been 
shown to the amount of mechanically suspended matter than to that 
of the other ingredients. For the present, therefore, we may confine 
ourselves to this part of the earthy substances removed trom the 
land by running water. It will be borne in mind, however, that the 
following estimates, in so far as they are based upon only one portion 
of the waste of the land, are under-statements of the truth. 
The proportion of mineral substances held in suspension in the 
water of rivers has been already (p. 370) discussed. It was pointed 
out that it is most advantageous to determine the amount of mineral 
matter by weight, and then from its average specific gravity to 
estimate its bulk as an ingredient in river water. The proportion by 
weight is probably, on an average, about half that by bulk. 
It may seem superfluous to insist that the earthy matter borne 
into the sea from any given area represents so much actual loss from 
the surface of that area. Yet this self-evident statement is probably 
not realized by many geologists to the extent which it deserves. 
If a stream removes in one year one million of cubic yards of earth 
from its drainage basin, that basin must have lost one million of 
cubic yards from its surface. From the data and authorities which 
have already been adduced (pp. 370, 371), the subjoined table has been 
constructed, in which are given the results of the measurement of 
the proportion of sediment in a few rivers. The last column shows 
1 Phil. Mag. for February 1867, and May 1868. See also his “ Climate and Time.” 
Geikie, Geol. Mag. June 1868; Trans. Geol. Soc. Glasgow, iii. p. 153. 
