386 Frank Carney 



ideal way of studying the origin of relief features, and it represents 

 a laudable desire to standardize physiographic processes. 



Nature, however, is not alw^ays so systematic. Oscillation 

 appears to be the habit of boundary lines betw^eenthe oceans and 

 the lands. It is not knowai that any land area ever has remained 

 static long enough for the theoretical development of a river 

 cycle. Instead of being able to point out a particular area where 

 the rivers have had this theoretical life history, we can with more 

 success find the three stages existing contemporaneously in one 

 and the same drainage basin. 'While the headwater parts of a 

 river system may be in youth, the portion near its mouth will 

 bear the earmarks of old age, and maturity will characterize the 

 parts between these limits. 



If the diastrophic movements of our continents were spaced 

 by drainage cycle periods, the land surfaces would then have 

 such roughening as would follow from one plain of erosion giving 

 place to the details of another plain of erosion. In that case, the 

 uplands would be peneplained; and beneath this level the new 

 drainage cycle would cut its valleys. What appears to happen 

 more often is that after a cycle has been under way , its develop- 

 ment is either retarded or hastened, and drainage irregularities 

 are introduced. These irregularities may result from the advan- 

 tage taken of weaker stream basins by the more vigorous drain- 

 age basins. Thus, when a land-tilt appreciably increases the 

 velocity of one river, it correspondingly retards the current of 

 the river flowing in an opposite direction from the same divide, 

 and, as a resjlt, the former encroaches on the drainage territory 

 of the latter. This is river jnracy. 



While piracy generally follows from land-tilts, as just mentioned , 

 it may also occur when one of two streams, which are balanced, 

 cuts into harder rock, w^hile the other stream continues in the 

 original homogeneous strata. The latter deepens its channel and 

 widens its valley, as both did formerly; but the other stream, 

 having to work against more resistant rock, is not able to cut 

 its channel correspondingly, and as a result loses drainage area 

 to the former. Such piracy is not based upon land-tilting, but 

 on rock structure. Furthermore, both land-tilting and variation 

 in the hardness of the rocks are usually operative at the same time. 



In some parts of the world normal drainage cycles have been 

 suddenly interrupted by lava flows. A flow of lava, taking a 



