THE A PRIORI AND A POSTERIORI ARGUMENTS. 395 



an especially good illustration of the manner in which large continental 

 surroundings ma}^ affect the opinions of an observer who, from certain 

 associations, might be expected to follow the insular school. 



Although mature deliberation and good judgment may lead through 

 a priori argument to a safe conclusion in many problems, the method is 

 of difficult application here on account of the great number of variable 

 factors whose appropriate values can be hardly determined. It is prob- 

 ably by reason of assigning diff'erent values to variable factors that the 

 opposite conclusions summarized above have been reached. 



Statement of the a posteriori Argument. 



In attempting to decide by arguing from effect to cause whether evenly 

 denuded regions have been worn down by subaerial or marine agencies, 

 let us try to stand on a provisional Atlantis, hoping that it may give 

 steady support long enough for us to gain an unprejudiced view of the 

 opinions that are so generally accepted on the lands to the east and west. 

 From this neutral ground let us attempt to deduce from the essential 

 conditions of each explanation of the problem as many as possible of its 

 essential consequences, and then confront these consequences with the 

 facts. The measure of accordance between consequences of theory and 

 facts of observation will then serve as a measure of the verity of the theory 

 from which the consequences are derived. No final decision can be 

 reached in many cases ; for, however clearly the consequences may be 

 deduced, the facts with which they should be compared are often beyond 

 the reach of observation. In such cases it is advisable to announce in- 

 decision as clearly as decision is announced in the others. 



As far as I have been able to carry the analysis of the problems, it is 

 more difficult to find positive criteria characteristic of plains of marine 

 denudation than of plains of subaerial denudation ; hence I will take up 

 the latter class first. It should be remembered, however, that in each 

 class of plains both classes of agencies may have some share, one pre- 

 ponderating over the other. 



Consequences of subaerial Denudation. 



Imagine a region of deformed harder and softer strata raised to a con- 

 siderable elevation. Then let the land stand essentially still, or oscillate 

 slightly above and below a mean position. The rivers deepen their val- 

 leys, the valleys widen by the wasting of their slopes, and the hills are 

 slowly consumed. During this long process a most patient and thorough 

 examination of the structure is made by the destructive forces, ^ and 

 whatever is the drainage arrangement when the rivers begin to cut their 



*See Bearing of physiography on uniformitarianisrn. Bull. Geol. Soc. Am., vol. 7, 1895, pp. 8-11. 



