596 W. M. DAVIS PENEPLAINS AND THE GEOGRAPHICAL CYCLE 



caiiie to my attention and understanding through observation in the 

 summer of 1883, when I was studying the series of rocks below the 

 Cretaceous coal horizon in Montana for the Northern Transcontinental 

 Survey, under the direction of Professor Pumpelly. Areas of the Great 

 Plains of nearly horizontal strata that I then traversed gradually came 

 to be understood as plains of degradation, because they were here and 

 there surmounted by lava-capped tables and dike-ridges, or even by 

 mountainous residuals held up by the extra resistance of a network of 

 dikes. It was several years after that beginning was made before the 

 idea of peneplanation was extended to regions of disordered rocks in 

 Pennsylvania and Xew England; and then again the facts observed, 

 compared, and generalized in inductions preceded the attainment of the 

 explanation by deduction. 



The same is true regarding the scheme of the cycles of marine abrasion 

 and of glacial erosion. As to the latter, it was the sight of the hanging 

 side valleys high above the broad floor of the valley of the Ticino, the 

 Val Leventina, in the southern Alps, which first led me to search for 

 their explanation by a strong deepening of the main valley by glacial 

 erosion in 1899, a. year after Gannett had, then unknown to me, seen 

 similar facts about Lake Chelan and reached a similar explanation for 

 them. But the disappointing thing about this advance is that it had 

 not been made deductively years before ; and the fact that it was not so 

 made must be explained chiefly by the inattention of geologists and 

 geographers to the immense aid thai deduction gives to observation when 

 the origin and the meaning of things are searched for. 



Deduction in the Scheme of the arid Cycle 



An exception to the statement that induction and deduction have 

 usually gone hand in hand, in so far as my own work on cycles of erosion 

 is concerned, must be made in the case of the arid cycle; for there, 

 following a clue suggested by Passarge's account of his observations in 

 South Africa, I deduced a whole sequence of changes far in advance 

 of their observational verification; and in spite of repeated search in 

 the records of explorers of desert regions, many of the deduced stages 

 are still without their observed counterparts. If the records of these 

 explorers gave specific examples of observed forms that contradicted the 

 features of the deduced forms, the scheme of the arid cycle might be 

 condemned as wrong; but as a matter of fact their descriptions are, as a 

 rule, so vague that it is impossible to know just what sort of forms they 

 arc describing. Their records neither prove nor disprove the arid cycle. 



