This gives a mean of 3,378 years to remove one foot, or a little over one-half the time taken by the Mississippi. This mean appears to be generally taken as representing the average rate of subaërial denudation of the whole earth, but it has, I fear, been rather too hastily adopted. To estimate correctly the quantity of sediment annually discharged by a large river is a most difficult and laborious undertaking. A perusal of the voluminous report of Messrs. Humphreys and Abbot, extending over 690 pages, which Dr. Geikie justly styles a model of patient and exhaustive research, will clearly show this, and at the same time prove how skilfully and accurately the task allotted to them was performed. [Footnote 26: _Trans. of Geol. Soc. of Glasgow_, vol. iii.; Jukes & Geikie’s _Manual of Geology_, chap. xxv.; _Text Book of Geology_, p. 441.] The risk of making very serious errors in computing the amount of sediment discharged, unless proper precautions are taken, is well illustrated in the case of the determinations made by Messrs. Brown and Dickson, to which reference has already been made. Although their report shows that they took great pains in order to arrive at correct results—in fact, they computed the total annual quantity of sediment discharged to within a cubic foot—after all, instead of being correct to this minute quantity, they gave a total more than fourfold what it ought to be. A somewhat similar discrepancy exists in reference to the denudation of the basin of the Ganges. The time required to lower its surface by one foot is, according to one estimate, 2,358 years; according to another, 1,751; and according to a third, only 1,146 years. The first figure is probably nearest the truth. Still, these differences show both the difficulty of the problem and the necessity of caution in adopting any of these results as correct.