The quantity of matter carried away from the land by a river is greater than at first sight appears, and is more likely to be under- than over-estimated. By taking samples of water near the mouth of a river (but above the influence of the tide) at a sufficient number of points in its channel and at different depths, and repeating this daily or at other short intervals throughout the year, it is easy to determine the quantity of solid matter held in suspension and solution; and if corresponding observations determine the quantity of water that is discharged, the total amount of solid matter brought down annually may be calculated. But besides this, a considerable quantity of sand or even gravel is carried along the bottom or bed of the river, and this has rarely been estimated, so that the figures hitherto obtained are usually under the real quantities. There is also another source of error caused by the quantity of matter the river may deposit in lakes or in flooded lands during its course, for this adds to the amount of denudation performed by the river, although {104} the matter so deposited does not come down to the sea. After a careful examination of all the best records, Sir A. Geikie arrives at the following results, as to the quantity of matter removed by seven rivers from their basins, estimated by the number of years required to lower the whole surface an average of one foot: The Mississippi removes one foot in 6,000 years. ,, Ganges ,, ,, 2,358 ,, ,, Hoang Ho ,, ,, 1,464 ,, ,, Rhone ,, ,, 1,528 ,, ,, Danube ,, ,, 6,846 ,, ,, Po ,, ,, 729 ,, ,, Nith ,, ,, 4,723 ,,