EFFECTS OF GREAT DENUDATIONS. 21 



the earth, behaving as a quasi-plastic body, have reasserted its equilibrium 

 of figure by making good a great part of the loss by drawing upon its 

 whole mass beneath ? Few geologists question that great masses of sedi- 

 mentary deposits displace the earth beneath them and subside. Surely 

 the inverse aspect of the problem is a priori equally palpable. That some 

 such process as this has operated in the Plateau Country looks at least 

 plausible, and if there could be found independent reasons for believing in 

 its adequacy the facts certainly bear it out. Yet its application is not 

 without some difficulties, and the explanation is not quite complete. Grant- 

 ing the principle, it will still be difficult to explain how these local uplifts 

 were inaugurated, and we can only refer them to the operations of that 

 mysterious plutonic force which seems to have been always at work, and 

 the operations of which constitute the darkest and most momentous problem 

 of dynamical geology. On the whole, it seems to me that we are almost 

 driven to appeal to this mysterious agency to at least inaugurate and in part 

 to perpetuate the upward movement, but that we must also recognize the 

 co-operation of that tendency which indubitably exists within the earth to 

 maintain the statical equilibrium of its levels The only question is whether 

 that tendency is merely potential or becomes in part kinetic, and this again 

 turns upon the rigidity of the earth. But it is easy to believe that where 

 the masses involved are so vast as those which have been stripped from the 

 Kaibabs and from the San Rafael Swell, the rigidity of the earth may be- 

 come a vanishing quantity. 



The great erosion of the Plateau Province was most probably accom- 

 plished mainly in Miocene time, but continued with diminishing rapidity 

 throughout the Pliocene. But it is necessary to say that the terms Mio- 

 cene and Pliocene have here no definition. They cannot be correlated 

 except in a very general manner with events occurring outside the province. 

 We have only a vast stretch of time, with an initial epoch near the close of 

 the local Eocene. The greater part of the denudation is assigned to the Mio- 

 cene, because the conditions appear to have been more favorable to a rapid 

 rate of destruction in that age than subsequently. The climate appears to 

 have been humid, while the elevation was at the same time gradually increas- 

 ing, both conditions being favorable to a rapid disintegration and removal 



