deepening southward and westward." Yet
these deposits are always arranged successively, and their several
thicknesses added together to obtain the total thickness of the formations
of the country. (See Presidential Address, Sect. C. British Association,
1876.)

[93] Mr. John Murray in his more careful estimate makes it about 51-1/2
millions.

[94] 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 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 _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."

[95] From the same data Professor Haughton estimates a minimum of 200
million years for the duration of geological time; but he arrives at this
conclusion by supposing the product