By far the most trustworthy determinations of the whole are those of the Mississippi by Messrs. Humphreys and Abbot, which may be relied upon as not far from the truth. But, supposing the estimates in the foregoing table to be perfectly correct, can we assume that their mean may be safely taken as probably representing the average rate of denudation of the whole earth? I would most unhesitatingly reply, Certainly not. The Rhone and Po are full of glacier mud from the Alps; and the amount of sediment which they carry down may give us the rate of denudation of Switzerland, but certainly not that of the whole earth, or even of Europe. The same may be said of the Ganges, which is charged with the mud which it brings down from the Himalaya Mountains. The Hoang-Ho, or Yellow River, is an exceptionally muddy river; in fact, it derives its name from the vast quantity of yellow mud held by its waters in a state of solution. It was probably the exceptionally muddy character of the Po, the Rhone, the Ganges, and the Yellow River which attracted attention, and led to observations being made of the sediment they contain. Rivers more unsuitable than these to give us the average denudation of the earth’s surface could not well be selected. Among the seven rivers in the table, leaving out of account the small Scottish stream, the Nith, with its basin of only 200 square miles, there are only two, the Mississippi and the Danube, that drain countries which may be regarded as in every way resembling the average condition of the earth’s surface. I would choose the Mississippi as being superior to the Danube, for two reasons: (1) because the rate of denudation of its basin has been more accurately determined; and (2) because the area of its basin not only exceeds that of the Danube as five to one, but better fulfils the necessary conditions, as Sir Charles Lyell has so clearly shown. “That river,” says Sir Charles, “drains a country equal to more than half the continent of Europe, extends through twenty degrees of latitude, and therefore through regions enjoying a great variety of climate, and some of its tributaries descend from mountains of great height. The Mississippi is also more likely to afford us a fair test of ordinary denudation, because, unlike the St. Lawrence and its tributaries, there are no great lakes in which the fluviatile sediment is thrown down and arrested on its way to the sea.”[27] There is no other river in the globe which to my mind better fulfils the required conditions. It is no doubt true that the rate of denudation of the basin of the Mississippi is probably less than that of Switzerland, Norway, and the Himalayas, where glaciers abound, and certainly less than that of Greenland and the Antarctic continent; but, on the other hand, this rate is certainly much greater than that of the whole continent of Africa, Australia, and large tracts of Asia, where the rainfall is much smaller.