114 
ELEMENTS OF GEOLOGY. 
the effects wnich causes in every-day action must produce 
when the multiplying power of time is taken into account. 
Attempts were made by Manfredi in 1736, and afterwards 
by Playfair in 1802, to calculate the time which it would 
require to enable the rivers to deliver over the whole of the 
land into the basin of the ocean. The data were at first 
too imperfect and vague to allow them even to approximate 
to safe conclusions. But in our own time similar investiga¬ 
tions have been renewed with more prospect of success, the 
amount brought down by many large rivers to the sea hav¬ 
ing been more accurately ascertained. Mr. Alfred Tylor, 
in 1850, inferred that the quantity of detritus now being dis¬ 
tributed over the sea-bottom would, at the end of 10,000 
years, cause an elevation of the sea-level to the extent of at 
least three inches.* Subsequently Mr. Croll, in 1867, and 
again, with more exactness, in 1868, deduced from the latest 
measurement of the sediment transported by European and 
American rivers the rate of subaerial denudation to which 
the surface of large continents is exposed, taking especially 
the hydrographical basin of the Mississippi as affording the 
best available measure of the average waste of the land. 
The conclusion arrived at in his able memoirf was that the 
whole terrestrial surface is denuded at the rate of one foot 
in 6000 years, and this opinion was simultaneously enforced 
by his fellow-laborer, Mr. Geikie, who, being jointly engaged 
in the same line of inquiry, published a luminous essay on 
the subject in 1868. 
The student, by referring to my “ Principles of Geology,”J 
may see that Messrs. Humphrey and Abbot, during their 
survey of the Mississippi, attempted to make accurate meas¬ 
urements of the proportion of sediment carried down •annual¬ 
ly to the sea by that river, including not only the mud held 
in suspension, but also the sand and gravel forced along the 
bottom. 
It is evident that when we know the dimensions of the 
area which is drained, and the annual quantity of earthy 
matter taken from it and borne into the sea, we can affirm 
how much on an average has been removed from the general 
surface in one year, and there seems no danger of our over¬ 
rating the mean rate of waste by selecting the Mississippi as 
our example, for that river drains a country equal to more 
than half the continent of Europe, extends through twenty 
degrees of latitude, and therefore through regions enjoying 
a great variety of climate, and some of its tributaries de- 
* Tylor, Phil. Mag., 4th series, p. 268. 1850. 
t doll, Phil. Mag., 1868, p. 381. f Vol. i., p. 442. 1867. 
