82 GEOLOGY. 



the final result of stream erosion is the reduction of the land to base- 

 level. The base-leveled surface, as before, would not be absolutely 

 fiat. The area reduced by each stream will have a slight gradient down- 

 stream, and from each lateral divide toward the axis of the valley. 

 The crests of the scarcely perceptible elevations which remain will be 

 in the position of the former divides, and these will be highest where 

 most distant from the sea by the course which this part of the drainage 

 took. Even the insensible divides between streams flowing in a common 

 direction may disappear, for when valleys have reached their limits in 

 depth, their streams do not cease to cut laterally. Meandering in their 

 flat-bottomed valleys, they often reach and undercut the divides (PI. 

 VII), whether they be high or low. By lateral planation, therefore, 

 the divides between streams may be entirely eaten away. 



Fig. 66. — Diagram showing tributaries of several orders developed from the condi- 

 tions sketched in the text. 



It has now been seen that by whatever method erosion by running 

 water proceeds, whether there be many valleys, or few or none, the 

 final result of subaerial erosion must be the production of a base-level. 

 It has also been seen that the base-level is first developed at the lower 

 ends of the main streams, and that it extends itself systematically up 

 the main valleys and up all tributaries. The time involved in the reduc- 

 tion of a land area to base-level is a cycle of erosion. 



It will have been evident from the preceding pages that the terms 

 ''grade,'' ''graded plain," and ''base-level" and " base-leveled plain,'' 

 are somewhat variously, and therefore somewhat confusingly, used. 

 "Grade is a condition of essential balance between corrasion and deposi- 



