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the structures on which they are developed; and thenceforward further change is slow.
Stream profiles.—Let is next examine the changes produced in the initial profile of the troughs where the first constructional streams settled. The irregularities of constructional profile which determine lakes and falls are in most cases soon extinguished. The profile of a consequent stream may for a time possess unequal slopes at its subsequent falls, but it soon attaina tolerably systematic curve of descent, steeper near the headwaters, flatter near the mouth. While the young stream has abundant fall and rapid current, with moderate load delivered from the relatively simple constructional and consequent slopes of its basin, it deepens its trench rapidly. But as the profile becomes flatter and the current runs slower, and as the area of wasting slopes increases by the deepening of the consequent valleys and the development of subsequent valleys, a time will soon arrive when the carrying power is reduced to equality with the load; and from this time on the deepening of the valley is very much slower than before. It is only as the load from the wasting slopes decreases in amount that the deepening can go on. Following certain French writers, the profile of the stream when this balanced condition is reached has been called the profile of equilibrium. The term is inconveniently long; but the idea is of essential importance. Mr. Gilbert has recently suggested to me that a stream in this condition of balance between degrading and aggrading might be called a graded stream; and its slope, a graded slope.
It is sometimes said that streams in this condition have reached baselevel; but this introduces a confusion of ideas that should be avoided. For example: given two constructional areas of similar form and altitude, and under equivalent climatic conditions; but let one be made of resistant rock, and the other of weak rocks. The baselevel is the same for both. The streams will cut deep into the harder mass, producing strong relief before reaching an equilibrium profile; because its waste is shed so slowly that the streams can carry it on a faint slope.