OHAr. X.] 



THE EARTH'S AGE. 



215 



we shall certainly over- rather than under-estimate the possible 

 rate of deposit.-^ 



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 oft" 

 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." 



