222 ISLAND LIFE part i 



are forming at the present time, along with a large but 

 unknown proportion of surface where the deposits were far 

 below the maximum thickness. This follows, if we con- 

 sider that deposit must go on very unequally along- 

 different parts of a coast, owing to the distance from each 

 other of the mouths of great rivers and the limitations of 

 ocean currents ; and because, compared with the areas over 

 which a thick deposit is forming annually, those where 

 there is little or none are probably at least twice as exten- 

 sive. If, therefore, we take a width of thirty miles along 

 the whole coast-line of the globe as representing the area 

 over which deposits are forming, corresjDonding to the 

 maximum thickness as measured by geologists, 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 



^ 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 tlie 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 and 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 viaximum 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 lew 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 ]\Iississippi 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. " 



