DUTTOH. 



THE GREAT DENUDATION. 97 



receding parallel to its former position at the rate of a foot or a few feet 

 in a thousand years; the terrace back of its crest line remaining solid and 

 nncnt; the beds fchus dissolving edgewise nntii after the lapseof millions 

 ol centuries their terminal dills stand a hundred miles or more back of 

 their initial positions. The true story is told bv the Triassic terrace 

 ending in the Vermilion Cliffs. This terrace is literally sawed to pieces 

 with canons. There are dozens of these chasms opening at intervals o! 

 two or three miles along the front of the escarpment and setting far 

 back into its mass. Every one of them ramifies again and again until 

 they become an intricate net-work, like the libers of a leaf. Every canon 

 wall, throughout its trunk, branches, and fcwigs,and every alcove and 

 niche, becomes a dissolving face. Thus the lines and area of attack are 

 enormously multiplied. The front wall of the terrace is cut into prom- 

 ontories and bays. The interlacing of branch canons back of the wall 

 cuts off the promontories into detached buttes, and the buttes, attacked 

 on all sides, molder away. The rate of recession therefore is corre- 

 spondingly accelerated in its total effect. 



The largeness of the area presents really no difficulty. The forces 

 which break up the rocks are of meteoric origin. The agency which car 

 ries off' the debris is the water running in the drainage channels. Surely 

 the meteoric forces which ravage the rocks of a township may ravage 

 equally the rocks of the county or state, provided only the conditions 

 are uniform over the larger and smaller areas. And what is the limit 

 to the length of a stream, the number of its branches and rills, and to 

 the quantity of water it may carry? It is not the area, then, which op- 

 presses us by its magnitude, but the vertical factor— the thickness of 

 the mass removed. But upon closer inspection the aspect of this factor 

 also will cease to be forbidding. 



For if the rate of recession of a wall iifty feet high is one foot in a given 

 nnmberof years, what will be (ceteris paribus) therateof recession ina wall 

 a thousand feet high? Very plainly the rate will be the same.* If we 

 suppose two walls of equal length, composed of the same kind of rocks, 

 and situated under the same climate, but one of them much higher than 

 the other, it is obvious that the areas of wall-face will be proportional to 

 i heir altitudes. Tn order that the rates of recession may be equal, the 

 amount of material removed from*the higher one must be double that 

 removed from the other, and since the forces operating on the higher one 

 have twice the area of attack, they ought to remove from it a double 

 quantity, thus making the rates of recession equal. Iu the same way it 



"The geologist will no doubt recognize that this is a simple and unqualified state- 

 ment of a result whioh is in reality very complex, and sometimes requiring qualifica- 

 tion. But a candid review of it in the light of established laws governing erosion will, 

 1 am confident, justify it for all purposes here contemplated Though some qualifying 

 conditions will appear when the subject is analyzed thoroughly, they are of no appli- 

 cation to this particular stage of the argument. The statement is amply true for the 

 proposition in hand, and it would be hardly practicable, and certainly very prolix, to 

 give here the full analysis of it. 

 7 G A 



