386 W. M. DAVIS 



of weak structures that are laid bare on the valley sides of the larger 

 consequent streams. In the arid cycle subsequent streams have a 

 smaller opportunity for development; first because all the belts of 

 weak structure under the basin deposits are buried out of reach; 

 second, because, in the absence of deep-cutting trunk rivers, many 

 belts of weak structure are but little exposed. In so far, however, 

 as the highlands are dissected by their headwater consequent streams,, 

 subsequent branches may grow out and diversify the slopes and 

 rearrange the drainage. 



Mature stage. — Continued erosion of the highlands and divides, 

 and continued deposition in the basins, may here and there produce 

 a slope from a higher basin floor across a reduced part of its initial 

 rim to a lower basin floor. Headward erosion by the consequent or 

 subsequent streams of the lower basin will favor this change, which 

 might then be described as a capture of the higher drainage area. 

 Aggradation of the higher basin is equally important, and a change 

 thus effected might be described as an invasion of the lower basin by 

 waste from the higher one ; this corresponds in a belated way to the 

 overflow of a lake in a normal cycle. There may still be no persistent 

 stream connecting the two basins, but whenever rain falls on the- 

 slope that crosses the original divide, the wash will carry waste from 

 the higher to the lower basin. Thus the drainage systems of two 

 adjacent basins coalesce, and with this a beginning is made of the 

 confluence and integration of drainage lines which, when more fully 

 developed, characterize maturity. The intermittent drainage that 

 is established across the former divide may have for a time a rather 

 strong fall; as this is graded down to an even slope, an impulse of 

 revival and deeper erosion makes its way, wave-like, across the floor 

 of the higher basin and up all its centripetal slopes. The previously 

 aggraded floor will thus for a time be dissected with a bad-land 

 expression and then smoothed at a lower level; the bordering waste 

 slopes will be trenched and degraded. At the same time, the lower 

 basin floor will be more actively aggraded. If there is a sufficient 

 difference of altitude between the two basins, all the waste that had 

 been, in a preliminary or youthful view of the case, gathered in the 

 higher basin, will in time be transferred to the lower basin; and thus 

 a larger relation of drainage lines, a longer distance of intermittent 



