470 



THE RATIONAL ELEMENT IN GEOGRAPHY 



Structural description recognizes a correlation between the 

 horizontal strata of which the canyoned plateau is built and the 

 attitude of the belted cliffs and slopes in the canyon walls, and 

 between the relative resistance of the various layers and their 

 surface expression, for the harder strata determine the cliffs and 

 the weaker ones determine the slopes ; yet the consideration of 

 structure alone will not- lead to a just comparison of the two 

 diagrams, inasmuch as the structures are alike in both, and the 

 forms represented depend on the less development of one and 

 the greater development of the other canyon ; but structural de- 

 scription takes no account of development or of the resulting 

 correlations of form and time. 



DIAGRAM OF NARROW CANYON 



From Davis and Snyder's Physical Geography, by courtesy of Mess7-s Ginn & Co. 



Genetic or explanatory description recognizes and employs all 

 that has gone before, and goes further. The variety of form in 

 the canyon walls is seen to be a necessary consequence of the 

 action of the weather on horizontal layers of unequal hardness. 

 The narrower canyon is soon perceived to be only a younger 

 stage of the wider one, and the platforms of the wider canyon 

 are recognized as characteristic features of an approaching ma- 

 turity of development. Moreover, the platforms are found to be 

 systematically placed between the slope from a weaker, faster- 

 retreating cliff and a stronger cliff next below it, and not vice 

 versa. Structure, process, and time are thus all rationally cor- 

 related with form, and ail these elements interact most suggest- 

 ively in framing a verbal description. It cannot be doubted that 

 a student who has gained an understanding of such correlations 

 will give a much better account of forms like those here illus- 



