ntJTTOH.] BASE LEVELS OF EROSION. 103 



During the progress of the great denudation of the Grand Canon Dis- 

 trict the indications are abundant that its interior spaces have occupied 

 for a time the relation of an approximate base-level of erosion. Through- 

 out almost the entire stretch of Tertiary and Quaternary time the region 

 lias been rising, and in the aggregate the elevation has become immense, 

 varying from 11,000 to 18,000 feet in different portions. But it seems 

 that the movement has not been at a uniform rate. It appears to have 

 proceeded through alternations of activity and repose. Whether we can 

 point to more than one period of quiescence may be somewhat doubtful. 

 but we can point decisively to one. It occurred probably in late Miocene 

 or early Pliocene time, and while it prevailed the great Carboniferous 

 platform was denuded of most of its inequalities, and was planed down 

 to a very flat expanse. Since that period the relation has been destroyed 

 by a general upheaval of the entire region several thousands of feet 

 The indications of this will appear when we come to the study of the 

 interior spaces of the Grand Canon District and of the Grand Canon 

 itself. To this study we now proceed. 



