academy o F Sciences] POWELL'S SURVEY 101 



of some small animal was to be magnified by projection upon tbe lantern screen so as to exhibit 

 heart beats to the audience. Unfortunately, something interfered with the mechanism at the 

 critical moment and the lever shadow remained stationary, until James himself gave it a series 

 of gentle periodic touches, whereupon the words of the lecturer were admirably verified by the 

 pulsating shadow; and as James remarked afterwards, "The audience was entirely satisfied." 

 So, it may be assumed, was the reading public in the other case. 



INTERDEPENDENCE OF DRAINAGE LINES 



Of much greater importance than these small matters is the law of the interdependence 

 of drainage lines, which is certainly one of the most beautiful of Gilbert's generalizations. 

 Given "the tendency to equality of action, or to the establishment of a dynamic equilibrium" 

 in the streams of a well-dissected land surface, it must come to pass that every valley-side slope 

 will be — 



a member of a series, receiving the water and the waste of the slope above it, and discharging its own water 

 and waste upon the slope below. If one member of the series is eroded with exceptional rapidity, two things 

 immediately result: first, the member above has its level of discharge lowered, and its rate of erosion is thereby 

 increased; and second, the member below, being clogged by an exceptional load of detritus, has its rate of erosion 

 diminished. The acceleration above and the retardation below, diminish the declivity of the member in which 

 the disturbance originated; and as the declivity is reduced the rate of erosion is likewise reduced. But the 

 effect does not stop here. The disturbance which has been transferred from one member of the series to the 

 two which adjoin it, is by them transmitted to others, and does not cease until it has reached the confines of 

 the drainage basin. For in each basin all lines of drainage unite in a main line, and a disturbance upon any 

 line is communicated through it to the main line and thence to every tributary. And as any member of the 

 system may influence all the others, so each member is influenced by every other. There is an interdependence 

 throughout the system (124). 



How alive, how organic a landscape becomes when the truth of this extraordinary principle 

 is realized. How infinite the patience of the processes by which the orderly interdependence of 

 slope lines is established. And when once established, how persistent is the interdependence 

 in the further progress of degradation, as long as no crustal or climatic disturbance intervenes. 



As if to illustrate the manner in which the interdependence of drainage lines is evolved, 

 several pages are devoted to classifying streams according to their origin and to explaining the 

 changes that they and their divides afterwards suffer by reason of reactions between external 

 erosion and internal structure. Many examples of such changes are taken from the Henry 

 Mountains district. It was indeed from these pages that most American students of land 

 sculpture gained their first introduction to the fascinating problem of river "abstraction" or 

 capture, a subdivision of the rational treatment of rivers that has been much advanced in later 

 years, largely because of the great impulse that Gilbert gave to it. It should therefore be under- 

 stood that his analysis of the conditions by which river courses are established in their initial 

 stage and by which they are diverted or shifted in their later stages of development, although 

 incomplete, shows that he apprehended even more fully than Powell the responsibility of the 

 physiographer to give a reasonable account of drainage systems, as well as of land forms. Physi- 

 ographers were soon united in feeling their great indebtedness to him and in striving to learn 

 and to apply the principles that he taught. 



TERMINOLOGY OF DRAINAGE LINES 



It is instructive to trace Gilbert's progress in the matter of stream terminology. Attention 

 has already been called in the review of his early Maumee Valley study to the fact that he 

 considered the origin of stream courses as a problem for investigation; he there explicitly noted 

 that "the smaller streams follow and indicate the slopes," and that the larger ones are guided 

 by the morainic ridges; but he proposed no genetic name for streams of this kind. Again, it 

 has been noted in the review of his two reports for the Wheeler survey, that he there, while 

 using such a term as monoclinal valley appropriately enough, employed no systematic series of 

 descriptive or genetic terms to indicate the origin of river and valley courses. The same is 



