foot of water, multiplied by the number of cubic feet of water
discharged per year, gives the total load carried in suspension
during that time. Adding to this the estimated amount of sand and
gravel rolled along the bed, which in many swift rivers greatly
exceeds the lighter material held in suspension, and adding also
the total amount of dissolved solids, we reach the exceedingly
important result of the total load of waste discharged by the
river. Dividing the volume of this load by the area of the river
basin gives another result of the greatest geological interest,--
the rate at which the region is being lowered by the combined
action of weathering and erosion, or the rate of denudation.

THE RATE OF DENUDATION OF RIVER BASINS. This rate varies widely.
The Mississippi basin may be taken as a representative land
surface because of the varieties of surface, altitude and slope,
climate, and underlying rocks which are included in its great
extent. Careful measurements show that the Mississippi basin is
now being lowered at a rate of one four-thousandth of a foot a
year, or one foot in four thousand years. Taking this as the
average rate of denudation for the land surfaces of the globe,
estimates have been made of the length of time required at this
rate to wash and wear the continents to the level of the sea. As
the average elevation of the lands of the globe is reckoned at
2411 feet, this result would occur in nine or ten million years,
if the present rate of denudation should remain unchanged. But
even if no movements of the earth's crust should lift or depress
the continents, the rate of wear and the removal of waste from
their surfaces will not remain the same. It must constantly
decrease as the lands are worn nearer to sea level and their
slopes become more gentle. The length of time required to wear