JOHN WESLEY POWELL. 47 



ley. The second treatise makes a volume by itself, and has for 

 its theme the Geology of the Eastern Portion of the Uinta Mountains. 

 In these works the details of observation are not recited. The 

 features of the country and the geologic structure are set forth in 

 comprehensive statements, and are treated as texts for the discus- 

 sion of the departments of geologic v philosophy to which their ex- 

 planation belongs. The principal generalisations are: (i) a defini- 

 tion of the "plateau province," (2) a classification of mountain 

 types, (3) a classification of valleys, and (4) a classification of the 

 forms of displacement of the plateau province, with a demonstra- 

 tion of the equivalence of the fault and the monoclinal flexure. The 

 chief additions to geologic theory appear in discussions of the 

 physics of erosion and of the production of topographic forms by 

 the joint action of upheaval and erosion. The term " base-level of 

 erosion," first used in these discussions and now current wherever 

 the forms of the land are studied, carries with it an idea of appar- 

 ent simplicity but of far-reaching importance. A stream cannot 

 wear down below its base-level, and the rate and manner of degra- 

 dation of a region depend on the relation of the region to the base- 

 levels of its streams. 



It was shown that the degradation of mountains is many times 

 more rapid than that of lowlands, and that mountains are therefore 

 temporary elevations unless continuously renewed by uplift. All 

 great mountains are young. 



When the strata deposited by the sea are lifted into land, riv- 

 ers begin to flow over them. The initial direction of the rivers is 

 down the slope, and this is also the direction of the dip. It is 

 found, however, that many drainage systems are quite independent 

 of the direction of the dip, and, still more strange, that rivers often 

 cut their way through mountain ranges instead of going around 

 them. A generation of geologists observed this and wondered at 

 it without finding an adequate explanation, but the present genera- 

 tion has discovered three different ways in which "inconsequent" 

 drainage may arise and has arisen. Two of these ways were dis- 

 covered by Powell, and to characterise them he introduced the 

 terms "superimposed drainage" and "antecedent drainage." 



When a region of disturbed strata has in long ages been de- 

 graded nearly to base-level, then sinks below water level and re- 

 ceives a coating of sediments, and then is lifted into land, its new 

 drainage conforms to the overlying strata. With continued uplift 

 and continued degradation the newer deposits are destroyed and 

 the drainage system sinks into the underlying disturbed strata. 



