WHAT IS TERRA FIRMA ? WILLIS. 403 



have been large and mountain chains both numerous and high. At 

 present continents are unusually large and mountains are unusually 

 elevated. During other much longer periods erosion has exceeded 

 uiDlift. Then continents have become low and featureless; great 

 plains have prevailed ; and in consequence of slight subsidence exten- 

 sive lands have been submerged. These are facts of the geological 

 record which admit of no doubt. 



In this play of processes any particular part of the earth's surface 

 may reach just that altitude at which it is in perfect isostatic balance, 

 but it is not probable that the equilibrium can be long maintained. 

 If the High Plateaus of Utah be in general in isostatic balance, then 

 the Grand Canyon of the Colorado must be too light by the weight 

 of the rock removed in carving it out of the plateau. It is, further- 

 more, certain that the Grand Canyon is but the beginning of that 

 erosion which will eventually remove as much of the mass of the 

 High Plateaus as lies above a plain, which will slope gently from 

 no great altitude to sea level. If the region is now in isostatic balance, 

 it will then be out of balance. Or, to consider another case: It is a 

 commonly accepted fact among physiographers of the present day 

 that ttie Appalachian region of the eastern United States was a low 

 plain during the Cretaceous and early Tertiary periods. The plain 

 is now warped up to 4,000 feet, more or less, above sea. If it is now 

 in isostatic balance, it was out of balance during the long lapse of 

 time of the periods named. 



It is reasonable to link the movements which are expressed in the 

 warped surfaces of continents with the stresses that are set up by 

 disturbance of isostatic balance. It is probable that the stresses 

 directly or indirectly cause the movements. But the effect is neither 

 immediate nor constant. The disturbing process, erosion, is a very 

 slow process. The jDlains which it produces endure during a geologic 

 age. The earth is sufficiently rigid to be very slow in responding to 

 the stress. 



However, if the hypothetical relation of cause and effect exists 

 between isostatic stress and warping, it is highly probable that equi- 

 librium is most nearly perfect at the culmination of movements of ele- 

 vation, such as the existing relief presumably represents. Valleys 

 excavated by erosion represent disturbances of that equilibrium, 

 which therefore can not be perfect in detail, or even very nearly so, 

 as Hayford assumes and calculates, but the mass of any large area, 

 such as the Great Plains of central North America, or the High 

 Plateaus of Utah, is very probably nearly in equilibrium, considered 

 as a mass and reduced to " mean plain." 



Geological considerations thus afford reason to prefer the method of 

 reduction employed by Putnam and Gilbert, the Faye reduction, 

 rather than that used by Hayford. 



