BUTTON. 



THE GREAT DENUDATION. 101 



Colorado River and its tributaries entering the Grand Canon had their 

 origin at the time the country emerged from the waters and became 

 land. This was in early Tertiary time. The rivers then must have had 

 their courses laid out in conformity with the very feeble slopes of the 

 newly risen country and in conformity with the surfaces of the newest 

 strata. In the progress of Tertiary time this surface, originally as level 

 as the prairies of Illinois, or more so, began to deform by unequal up- 

 lifting; but the rivv rs remained unchanged, and some of them are How 

 ing to-day along the same routes as of old. Others have dried up, and 

 the very strata which contained their troughs have been swept away. 

 Those which remain occupy a very different relation to the strata from 

 that which they held at first. The tributaries on the north side now 

 run against the dips; those on the south side run icith the dips, or nearly 

 so. But the change has been in the attitudes of the strata and not in 

 the positions of the rivers. And if we theoretically reconstruct the at- 

 titudes of the strata to conform to the courses of the drainage channels 

 we reach a reconstruction exactly the same as that which we deduced 

 from a restoration of the faults and flexures. 



Thus the stratification, the outliers, the faults and flexures, and the 

 drainage all yield their quotas of testimony to the great fact of denuda- 

 tion, and indicate that at some initial epoch the whole Mesozoic system 

 and the lower Eocene once extended over the entire platform of the Grand 

 Canon district, with a thickness varying somewhat, no doubt, but on the 

 whole differing but little, from that which we now find in the terraces of 

 the High Plateaus. It is to be noted that the evidence of this former 

 extension is more complete in the older formations than in the younger 

 ones. In the case of the Permian it is quite perfect; in the case of the 

 Trias very nearly so; in the case of the Jura it is very little less cogent 

 than in that of the Trias; and in the Cretaceous practical certainty is 

 exchanged for a very high degree of probability barely distinguishable 

 from certainty. In the case of the Eocene there still remains a strong 

 probability, but there is room for reservation. Xo reason to the con- 

 trary can be shown at present, and it may be regarded as one of those 

 cases where "the tail goes with the hide"; but we cannot promise that 

 future research will not develop reasons for a different conclusion. As 

 the evidence now stands we are impelled to accept the full extension of 

 the Eocene with some reservations, arising not from conflicting evidence, 

 but from want of perfection in the evidence known to us. 



1JASE LEVELS OF EROSION. 



In his popular narrative of Explorations of the Colorado River, Powell 

 has employed the above term to give precision to an idea which is ol 

 much importance in physical geology. The idea in some form or other 



