September 1, 1892.] 



KNOWLEDGE. 



163 



is at present being worn down by " rain and rivers." 

 Take the great area drained by the Mississippi, which is 

 what geographers call its " basin." The area of this basin 

 is reckoned to be 1,147,000 S(iuare miles. It is clear that 

 all the mud, sand, Ac, brought don-n by this great river 

 to the Gulf of Mexico must be derived from the rocks and 

 soil in that area ; the nest step is to find out how much 

 solid matter is brought down every year. Most extensive 

 and accurate determinations upon this subject have been 

 made by the United States Government. As the mean of 

 many observations carried on continuously at different 

 parts of the river for months together, Messrs. Humphreys 

 and Abbot, the engineers employed to investigate the 

 physics and hydraulics of the Mississippi, found that the 

 average proportion of sediment contained in the water is 

 __Ljjth by weight, or jr^Votli ^J volume. But, besides the 

 matter held in suspension, they observed that a large 

 amount of coarse detritus is constantly being pushed along 

 the bottom of the river. They estimated that this mo^•ing 

 stratum carries every year into the Gulf of Mexico about 

 750,000,000 cubic feet of sand, earth, and gravel. Their 

 observations led them to conclude that the annual dis- 

 charge of water by the Mississippi is 19,500,000,000,000 

 cubic feet, and consequently that the weight of mud 

 annually carried into the sea by this river must reach the 

 sum of" 812,500,000,000 pounds. Then, taking the total 

 annual contributions of solid matter, whether in suspension 

 or moving along the bottom, they found them to equal a 

 prism '2GS feet high, with a base of one square mile. 



But, besides all this, there is in every river a large 

 amount of matter chemically dissolved. This consists 

 chiefly of carbonate of lime, dissolved by rain-water in 

 filtering through rocks before it reaches the river. 

 Properly to estimate the loss sustained by the surface of a 

 river basin, we ought to know the amount of mineral 

 matter thus removed, as well as that referred to above ; and 

 to make sure of good results, we ought to have the total 

 volume of water discharged, from measurements made at 

 different seasons nnd extending over a series of years. 

 Such data have not been fully collected from any river, 

 though some of them have been ascertained with approxi- 

 mate accuracy, as in the cases . of the survey of the 

 Mississippi and the Danube. As a rule, more attention has 

 been paid to the amount of mechanically suspended matter 

 than to the amount in solution. We must therefore 

 confine ourselves to the former, but it must be borne in 

 mind that the following estimates are under-statements of 

 the truth, because the amount of dissolved matter is left 

 out. Some of the results obtained are as follows : — The 

 Mississippi, with a basin of 1,147,000 square miles, 

 discharges annually 7,459,267,200 cubic feet of solid 

 matter; the Rhone, with a basin of 25,000 square mUes, 

 discharges 600,381,800 cubic feet of solid matter; the 

 Danube, with a basin of 234,000 square miles, discharges 

 1,253,738,600 cubic feet ; the Po, with a basin of 30,000 

 square miles, discharges 1,510,137,000 cubic feet. Now 

 as all this solid matter comes off" the surface of so much 

 land, the area of which is known, it can easily be calculated 

 what thickness of rock must have been removed (on an 

 average) to produce the amount brought down to the sea, as 

 given in cubic feet. On elevated laud, where moimtain 

 streams run faster, more rock is removed than over low 

 plains or gentle slopes, where rivers run slowly. But we only 

 want a general average for the whole area. An illustration 

 may serve to make this clear. Given a lump of butter, 

 containing so many cubic inches, and a slice of bread, 

 with area so many square mches ; any schoolboy could 

 find what the thickness of the butter would be when spread 

 evenly over the bread. The results for the great rivers were 



as follows : — The Mississippi removes ^ J^^ foot from its 

 area in one year, or one foot in 6000 years ; the Rhone 

 removes i-Vs, or one foot in 1528 years; the Danube 

 removes ^ sV'e- o^' o^^ foot in 6846 years ; the Po removes 

 yig, or one foot in 729 years. Now these are very 

 important results, and since the physics of the Mississippi 

 have been more carefully studied than those of perhaps 

 any other river, and as that river drains so extensive a 

 region, embracing so many varieties of climate, rock and 

 soil, we shall probably get the best restilts by taking the 

 Mississippi rate of denudation as a fair one. Let us see, 

 then, what that rate means. It means that the surface of 

 its basin will be lowered 10 feet (generally) in 60,000 

 years ; supposing the rate to continue, 100 feet in 600,000 

 years, and 1000 feet in 6,000,000 years. Apply this to the 

 whole of North America, the mean height of which, 

 according to Humboldt, is 748 feet above sea-level, and we 

 find that this continent would be worn away in about 4^ 

 millions of years. The same kind of calculation, based 

 upon the rate of denudation by the Upper Ganges, has 

 been applied to the continent of Asia, and a shorter length 

 of time was found to be required to wear it all down to 

 sea-level. But the Ganges rate seems to be hardly a fair one ; 

 so we wUl keep to the Mississippi. Such calculations are 

 made on the assumption that no serious changes take place 

 in the way of earth-movtments, raising or depressing a 

 continent. Upheaval would undoubtedly quicken the rate of 

 denudation, by giving greater velocity to the rivers (on 

 account of increased fall), and in the same way depression 

 would check the rate of denudation. But in spite of this 

 possible element of disturbance, the result above given is 

 an important one. Now the amount of denudation that 

 might thus take place over the North American continent 

 is a mere trific compared to the vast denudation which 

 must have taken place in order to provide the prodigious 

 amount of solid matter contained in the whole series of 

 stratified rocks. Their total estimated thickness is about 

 100,000 feet ! It is clear, then, that a much greater 

 number of millions of years was required to lay down this 

 great series of sedimentary rocks on ocean beds, especially 

 when we reflect that such material had to be distributed 

 by ocean currents over vast areas, and also that many of 

 these rocks were built up very slowly in the deeper parts 

 of oceans by the slow accumulation of organic remains. 

 This applies, for example, to the carboniferous limestone, 

 the oolites, and the chalk formation. 



Evidences of great denudation abound both in Great 

 Britain and in Europe, and in all parts of the world. 

 Thousands and thousands of feet of solid rock have been 

 removed, and yet such phenomena were by no means 

 spread over the whole of geological time. We can often 

 prove that even in the interval between two successive 

 periods enormous denudation took place, and the mind is 

 bewildered in endeavouring to combine with any reasonable 

 amount of time required for such intervals the much 

 greater periods required for the accumulation of the sub- 

 sequent or overlying strata. Any student who is familiar 

 with geological sections can call to mind numerous 

 examples of great denudation. For instance, what a vast 

 period of time is indicated by the upheaval and subsequent 

 denudation of the pre-Cambrian (or ArchiBan) rocks before 

 those of the PaliEOzoic era were deposited on their upturned 

 edges ! No attempt has been made to estimate in years 

 this interval. Or, to take another case, it is found that in 

 many parts of our country a great thickness of the carbon- 

 iferous rocks, especially the coal measures, was denuded 

 away before the advent of the Secondary or Mesozoic era. 

 Sir Andrew Ramsey has calculated (from sections drawn 

 to scale) that a covering of rock to the depth of one mile 



