The extent to which a country suffers denudation at the present time is to be measured by the amount of mineral matter removed from its surface and carried into the sea. An attentive examination of this subject is calculated to throw some light on the vexed question of the origin of valleys and also on the value of geological time. Of the mineral substances received by the sea from the land, one portion, and by far the larger, is brought down by streams, the other is washed of by the waves of the sea itself.