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about which, when properly verified, no further question can possibly 
arise. In this manner the subject is at once disencumbered of 
difficulty in fixing the relative importance of rain, rivers, frost, 
glaciers, &c., considered as denuding agents. We have simply to 
deal with the sum-total of results achieved by all these forces 
acting severally and conjointly. ‘Thus considered, this subject casts 
a new light on the origin of existing land-surfaces, and affords 
some fresh data for approximating to a measure of past geological 
time. 
Of the mineral substances received by the sea from the land, 
vastly the larger portion is brought down by streams; a relatively 
small amount is washed off by the waves of tlie sea itself. It is the 
former, or stream-borne part, which is at present to be considered. 
The quantity of mineral matter carried every year into the ocean by 
the rivers ofa continent represents the amount by which the general 
surface of that continent is annually lowered. Much has been 
written of the vastness of the yearly tribute of silt borne to the 
ocean by such streams as the Ganges and Mississippi; but “the 
mere consideration of the number of cubic feet of detritus annually 
removed from any tract of land by its rivers does not produce so 
striking an impression upon the mind as the statement of how much 
the mean surface-level of the district in question would be reduced 
by such a removal.”+ ‘his method of inquiry is so obvious and 
instructive that it probably received attention from early geologists, 
though data were still wanting for its proper application. Playfair, 
for instance, in speaking of the transference of material from the © 
surface of the land to the bottom of the sea, remarks that ‘the 
time requisite for taking away by waste and erosion two feet from 
the surtace of all our continents and depositing it at the bottom of 
the sea, cannot be reckoned less than two hundred years.” This 
estimate does not appear to have been based on any actual measure- 
ments, and must greatly exceed the truth ; but it serves to indicate 
how broad was the view that Playfair held of the theory which he 
undertook to illustrate. The first geologist who appears to have 
attempted to form any estimate on this subject from actually 
ascertained data, was Mr. Alfred Tylor, who, in the year 1850, 
published a paper in which he estimated the probable amount of 
solid matter annually brought into the ocean by rivers and other 
agents. He inferred that the quantity of detritus now distributed 
freee, Ap 
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a 
- 
2. 
over the sea bottom every year would, at the end of 10,000 years, - 
cause an elevation of the ocean-level to the extent of at least three 
inches.* ‘lhe subject was afterwards taken up by Dr. Croll, who 
1 Tylor, Phil. Mug. 4th series, v. p. 268, 1850. 
2 « [)lustrations,” p. 424. Manfredi had previously made a calculation of the amount 
of rain that falls over the globe, and of the quantity of earthy matter carried into the sea 
by rivers. He estimated that this earthy matter distributed over the sea bed must 
raise the level of the latter five inches in 348 years. Von Hoff, “ Veranderungen der 
Erdoberfliche,” Band i. p. 232. See the other autuorities there cited. 
3 Phil. Mag. loc. cit. 


y 
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442 ~~ DYNAMICAL GEOLOGY. — [Beow HII: 
