NATIONAL ACADEMY BIOGRAPHICAL MEMOIRS VOL. VIII 



Europe, Powell's published paper (Science, XII, 1888, pp. 

 229-233) does not contain a single citation. As a correlative 

 of the transportation of load by stream, its deposition was 

 also considered, and a good beginning made toward recog- 

 nizing the great importance of fluviatile deposits in the case 

 of an extensive conglomerate (Uinta, 170). In connection 

 with the transportation of large boulders from mountains uy 

 storm floods, a curious suggestion is made (Colorado River, 

 208) regarding a possible similar interpretation of parts of 

 the "Drift" in the upper Mississippi Valley, which Powell had 

 studied while he was a professor in Illinois. 



BASEXEVIX OF EROSION. 



Among all Powell's many generalizations none has been 

 more broadly applied than his conception of the base level 

 (now better printed as a single word, baselevel) of erosion. 

 He wrote : "We may consider the level of the sea to be a grand 

 base level, below which the dry lands cannot be eroded ; but we 

 may also have, for local or temporary purposes, other/ base 

 levels of erosion, which are the levels of the beds of the prin- 

 cipal streams which carry away the products of erosion. . . . 

 What I have called the base level would, in fact, be an imag- 

 inary surface, inclining slightly in all its parts toward the 

 lower end of the principal stream draining the area through 

 which the level is supposed to extend, or having the inclination 

 of. its parts varied in direction as determined by tributary 

 streams." Where a "stream crosses a series of rocks in its 

 course, some of which are hard and others soft, the harder 

 rocks form a series of temporary dams; . . . and thus 

 we may have a series of base levels of erosion" (Colorado 

 River, 203). 



It is to be noted that baselevel as thus defined seems to have 

 two meanings. One meaning is very simple; ft is simply the 

 level of the sea, extending in imagination under the lands. 

 The other is much more complicated ; it is an imaginary, undu- 

 lating, and inclined surface passing through a river and its 

 tributaries, but passing beneath the intervening land surfaces. 

 And as thus defined baselevel must be conceived with diffi- 

 culty because of the vagueness as to the stage of river devel- 



32 



