96 GRAND CANON DISTRICT. 



from which their materials wore in part derived. The distance of that 

 shore-line, from the summit of the Pink Cliffs, is from 130 to 180 miles, 

 and the width of the denuded district is from 120 to 140 miles. From 

 the base of the Vermilion Cliffs the distance is 25 to 30 miles less. The 

 area of maximum denudation is from 13,000 to 15,000 square miles, and 

 the average thickness of the strata removed from it was about 10,000 

 feet. 



The general reader will no doubt feel a strong aversion to such pro- 

 digious figures on their first presentation, and even the geologist whose 

 credulity has been shocked so often that he has gotten used to it may 

 wince once more. It is not from a love of the marvelous or dramatic; 

 it is not without a full sense of the oppression of unaccustomed mag- 

 nitudes that these assertions are made. They are made because they 

 follow inexorably from the facts, and because they are necessary conclu- 

 sions from clear premises. But, in order that the reader may not bo 

 obliged to carry a heavy burden of prejudice as he follows the various 

 steps of the argument, it is well to anticipate some part of the discussion, 

 and thus relieve him of a great part of the load at the outset, for it can 

 be shown that the figures, while they are certainly very large, are in 

 no respect abnormal, and in only one respect are they at all unusual. 



Erosion, viewed in oneway, is the supplement of the process by which 

 strata are accumulated. The materials which constitute the stratified 

 rocks were derived from the degradation of the land. This proposition 

 is fundamental in geology — nay, it is the broadest and moat comprehen- 

 sive proposition with which that science deals. It is to geology what 

 the law of gravitation is to astronomy. We can conceive no other origin 

 for the materials of the strata, and no other is needed, for this one is 

 sufficient and its verity a thousand times proven. Erosion and "sedi- 

 mentation" are the two half phases of one cycle of causation — the debit 

 and credit sides of one system of transactions. The quantity of mate- 

 rial which the agents of erosion deal with is in the long run exactly 

 the same as the quantity dealt with by the agencies of deposition; or, 

 rather, the materials thus spoken of are one and the same. If, then, we 

 would know how great have been the quantities of material removed in 

 any given geological age from the land by erosion, we have only to esti 

 mate the mass of the strata deposited in that age. Constrained by this 

 reasoning, the mind has no escape from the conclusion that the effects 

 of erosion have indeed been vast. If, then, these operations have 

 achieved such results, our wonder is transferred to the immensity of the 

 periods of time required to accomplish them ; for the processes are so 

 slow that the span of a life-time seems too small to render those results 

 directly visible. As we stand before the terrace cliffs and try to con- 

 ceive of them receding scores of miles by secular waste, we find the en- 

 deavor quite useless. There is, however, one error against which we must 

 guard ourselves. We must not conceive of erosion as merely sapping the 

 face of a straight serried wall a hundred miles long; the locus of the wall 



