SECOND EVENING DISCOURSE. 949 



than that required fur the erosion of the caiiyou. The tilthig of the cfyatalliue 

 foundation and its heavy stratified cover and the erosion of the tilted mass to a 

 plain that truncates its two parts required a third vast period of time. The 

 deposition of the horizontal plateau series, 3,000 feet or naore in thickness, required 

 a fourth vast period of time. 



A first view of the region would suggest that the erosion of the canyon 

 followed the deposition of the plateau series and the uplift of the total mass ; but 

 an examination of the area to the north and east shows the retreating escarp- 

 ments of overlying series of Mesozoic strata, which seem once to have stretched 

 over the whole plateau area with a thickness of several thousand feet. Their 

 deposition required a fifth vast period of time, and their removal a sixth vast 

 period, and only then was the region uplifted to its present altitude and the 

 erosion of the canyon begun. 



At first sight of the huge canj^on one is disposed to think that its erosion 

 must have occupied an enormously long chapter of geological time. The chapter 

 was certainly long if measured in years ; but if rated on a geological scale it 

 appears to be much shorter than any one of the six chapters into which the 

 geological history of the region is naturally divided. Huge as the canyon is, it is 

 appropriately regarded as a young geographical feature ; its magnitude indicates 

 a precocious development rather than a venerable antiquity. Ileally venerable 

 features are in the buried plains of erosion revealed in the canyon walls. 



