CHAP. X.] 
THE EARTH’S AGE. 
215 
we shall certainly over- rather than under-estimate the possible 
rate of deposit. 1 
Now a coast-line of 100,000 miles with a width of 30 gives 
an area of 3,000,000 square miles, on which the denuded matter 
of the whole land-area of 57,000,000 square miles is deposited. 
As these two areas are as 1 to 19, it follows that de- 
position, as measured by maximum thickness, goes on at least 
nineteen times as fast as denudation — probably very much 
faster. But the mean rate of denudation over the whole earth 
is about one foot in three thousand years ; therefore the rate of 
maximum deposition will be at least 19 feet in the same 
time ; and as the total maximum thickness of all the stratified 
rocks of the globe is, according to Professor Haughton, 177,200 
feet, the time required to produce this thickness of rock, at the 
1 As by far the larger portion of the denuded matter of the globe passes 
to the sea through comparatively few great rivers, the deposits must 
often be confined to very limited areas. Thus the denudation of the vast 
Mississippi basin must be almost all deposited in a limited portion of the 
Gulf of Mexico, that of the Nile within* a small area of the Eastern 
Mediterranean, and that of the great rivers of China — the Hoang Ho and 
Yang-tse-kiang, in a small portion of the Eastern Sea. Enormous lengths 
of coast, like those of Western America andj Eastern Africa, receive very 
scanty deposits ; so that thirty miles in width along the whole of the coasts 
of the globe will probably give an area greater than that of the area of 
average deposit, and certainly greater than that of maximum deposit, which 
is the basis on which I have here made my estimates. In the case of the 
Mississippi, it is stated by Count Pourtales that along the plateau between 
the mouth of the river and the southern extremity of Florida for two 
hundred and fifty miles in width the bottom consists of clay with some 
sand and but few Rhizopods ; but beyond this distance the soundings 
brought up either Rhizopod shells alone, or these mixed with coral sand, 
Nullipores, and other calcareous organisms (Dana’s Manual of Geology , 
2nd Ed. p. 671). It is probable, therefore, that a large proportion of the 
entire mass of sediment brought down by the Mississippi is deposited on 
the limited area above indicated. 
Professor Dana further remarks : “ Over interior oceanic basins as well 
as off a coast in quiet depths, fifteen or twenty fathoms and beyond, the 
deposits are mostly of fine silt, fitted for making fine argillaceous rocks, 
as shales or slates. When, however, the depth of the ocean falls off 
below a hundred fathoms, the deposition of silt in our existing oceans 
mostly ceases, unless in the case of a great bank along the border of 
a continent.” 
