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Show 214 ISLAND LIFE. [PAR'r I. How to estimate the average rate of Deposition of the Sedimentary Rocks.-But if we take the estimate of Professor Haughton (177,200 feet), whic.h, as we have seen, is probably excessive, for the maximum thickness of the sedimentary rocks of the globe of all known geological ages, can we arrive at any estimate of the rate at which they were formed ? Dr. Croll has attempted to make such an estimate, but he has taken for his basis the mean thickne.ss of the rocks, which we have no means whatever of arriving at, and which he guesses, allowing for denudation, to be equal to the maxin~J~~Jm thickness as :measured by geologists. The land-area of the globe is, according to Dr. Croll, 57,000,000 square miles, and he gives the coast-line as 116,000 miles. This, however, is, for our purpose, rather too much, as it allows for bays, inlets, and the smaller islands. An approximate measurement on a globe shows that 100,000 miles will be nearer · the mark, and this has the advantage of being an easily remembered even number. The distance from the coast, to which shore-deposits usually extend, may be reckoned at about 100 or 150 miles, but by far the larger portion of the matter brought down from the land will be deposited comparatively close to the shore; that is, within twenty or thirty miles. If we suppose the portion deposited beyond thirty miles to be added to the deposits within that distance, and the whole reduced to a uniform thickness in a direction at right ano'les to the coast we b ' should probably include all areas where deposits of the maxi-mum thickness 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 consider 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 extensive. If, therefore, we take a width o~ thirty miles along the whole coast-line of the globe as representmg the area over which deposits are forminO' corre-d' bl spon mg to the maximum thickness as measured by geologists, 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 deposition, as measured by maxim~(;m ~hickness, 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 s~me time . and as the total maximum thickness of all the stratified rock; 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 n:attcr of the glob.o passes to the sea through cbmparatively few great nvers, the. deposits must often be confined to very limited areas. Thus .the d~n~datwn o~ the vast Mississippi basin must be almost all deposited m a hmited portion of the Gulf of Mexico, that of the Nile within· a sm~ll area of the Eastern Mediterranean and that of the great rivers of Chma-the Hoang Ho and Yang-tse-kian~, in a small portion of the Eastern Sea. E.normous. lengt~s of coast, like those of Western America andj Eastern Afnca, receive ve1y scanty deposits; 80 that thirty miles in width along the whole _of the ~oasts of the globe will probably give an area greater than that of the. atea. of average deposit, and certainly greater than tha~ 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 alo.ng the plat~au between the mouth of the river and the southern extremity of Flonda .for two } undred and fifty miles in width the bottom consists of clay with s?me stand and but few Rhizopoda ; but beyond this di~tance .the soundmgs brought up either Rhizopod shells alone, or these mixed with coral sand, Nullipores, and other calcareous organisms (Dana's Manual o.:( Geology, 2nd Ed. p. 671). It is probable, therefore, ibat a .la~·g~ p:o.portwn _of the entire mass of sediment brought down by the MisSISSippi IS deposited on the limited area above indicated. . . . Professor Dana further remarks : " Over intenor oceamc basms as we 11 fE t · quiet depths fifteen or twenty fathoms and beyond, the as o . a coas m tl f fine silt fitted for making fine argillaceous rocks, depositls are mlost y o When h'owever the depth of the ocean falls off as sha es or s a es. , ' . . . . b 1 h ndred fathoms the deposition of silt m our existmg oceans meo ostwly a ceaus es, unless I·n 'the case of a great bank along the bordor of a con tin en t." |