CHAPTER IV. 



THE GREAT DENUDATION. 



Before leaving the terraces we may with advantage pause to contem- 

 plate the great lesson in geology which they lay open to us. The sub- 

 ject of the lesson is Erosion. In a preliminary way we examined the 

 type of it in the San Rafael district, which was brielly treated of in the 

 first chapter. The same fact confronts us again in the Grand Canon 

 district. Here, however, the attendant facts are more complex, more 

 difficult to grasp, and less easy to summarize. And vast as the erosion 

 has been in the San Rafael it has been many times greater in the Grand 

 ( 5aSon district. In this discussion three classes of facts will be utilized : 

 1st, the stratification; 2d, the faults and tlexures, or vertical displace- 

 ments; 3d, the drainage. Each class furnishes its quota of evidence. 

 Yet, so intimately are the several threads of argument interwoven, that 

 it is almost impossible to separate them and view each independently 

 of the others. Hence if the argument skips about from one to another 

 before the one is fully developed, it is because no other method of treat- 

 ment seems practicable. 



The geologist seeing the series of Mesozoic and Eocene strata sud- 

 denly terminating in the terraces in the faces of the cliffs, would at 

 once say that these strata formerly extended farther southward. For 

 he is ever mindful of the fact that in the lapse of long periods the rocks 

 decay, and the rains and rills gather up the debris and carry it away. 

 He also has had impressed upon his mind the general fact that the most 

 rapid waste takes place on the edges of the strata exposed in vertical 

 wall-faces. Every year the rains wash away something from the mural 

 fronts. In a single year it may be a mere film, but in the lapse of thou- 

 sands of centuries the amount whittled off becomes a vast aggregate. 

 Like the motion of the fixed stars the change is not perceptible to a 

 generation; but a million years would change the aspect of a denuding 

 country as profoundly as they would change the aspect of the heavens. 

 How long in terms of years this "Recession of the Cliff's" has been going 

 on, the geologist does not know, though he presumes the period to have 

 been certainly hundreds of thousands of years and very probably some 

 millions. Feeling assured, then, that the terraces once projected further 

 south, the inquiry arises, how far? Let me answer at once. They ex- 

 tended southward over the entire Grand Canon district, into cen- 

 tral Arizona, where they ended along the shore of the ancient mainland 



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