PHYSICAL GEOGRAPHY IN THE UNIVERSITY. 75 



deepen their valleys beneath the constructional surface, it often 

 happens that they discover structures of unequal hardness. If, 

 in passing down stream, a weak structure succeeds a hard struc- 

 ture, the valley will be quickly deepened in the former and 

 slowly in the latter ; a local increase of slope appears and a fall 

 or cascade is the result. This is a subsequent fall on a conse- 

 quent stream. It endures until the harder structure is worn 

 down or back so far that it overtakes the deepening of the stream 

 bed below the fall. The extinction of falls is accomplished in 

 adolescence on large streams and on tilted rocks ; but it may 

 not be reached until maturity on the smaller streams in regions 

 of horizontal strata. 



A further consequence of the discovery of the variable 

 resistance of internal structure is the variable rate at which the 

 narrow young consequent valley widens into the more mature 

 open valley. If the consequent stream crosses a local trans- 

 verse belt of hard rocks, the gorge-like form of the valley walls 

 may there be retained into the maturity of the region as a whole. 

 If it crosses a belt of weak rocks, the consequent valley may 

 there widen so greatly as to develop other valleys on either side 

 of its path. Thus many a transverse consequent stream, cutting 

 its valleys across belts of harder and softer structures, allows the 

 development of longitudinal valleys on every belt of weak struc- 

 ture that it traverses, while the intermediate belts of harder 

 structure stand up as longitudinal dividing ridges. The longi- 

 tudinal streams and valleys are then called subsequent branches 

 of the transverse consequent streams and valleys. Each 

 of the subsequent streams deepens its valley only as 

 fast as the down-stream deepening of the consequent valley 

 permits. 



It is extremely important to recognize the difference thus 

 indicated between consequent and subsequent streams. The first 

 control the drainage of a region in its early stages of develop- 

 ment. The second are of increasing importance in the secondary 

 and later stages of growth, when they share the drainage of the 

 region with the surviving consequent streams. Subsequent falls 



