PHYSICAL GEOGRAPHY IN THE UNIVERSITY. 77 



the structures on which they are developed ; and thenceforward 

 further change is slow. 



Stream profiles. — Let us 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 attain- 

 a tolerably systematic curve of descent, steeper near the head- 

 waters, 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. Follow- 

 ing certain French writers, the profile of the stream when this 

 balanced condition is reached has been called i\\c profile of equilib- 

 rium. 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 rocks, 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. 



