58 GRAND CANON DISTRICT. 



detail the arguments upon which this deduction o!' a great denudation 



is based. 



If we stand before one of the great marginal el ills which hound the 

 several terraces, we shall speedily detect abundant evidence that time 

 and the elements are slowly robbing its face of the materials which com- 

 pose its mass. Fragments have spawled off and fallen, and they now 

 lie at its base in great quantities, forming a talus. Oliffand talus alike are 

 seamed and scored with rain-gullies, and if we are fortunate enough to 

 observe the effects of a shower we shall see the waters trickling, spout- 

 ing, or rushing down through every seam and gully, carrying sand, mud, 

 and small fragments with which they a re charged to their utmost capacity. 

 The meaning of this is that the oliff is wasting away, and its locus 

 through the ages is farther and farther back. This backward movement 

 of the line of frontage by slow waste is very happily named by Powell 

 the Recession of Cliffs. 



It may seem at first as if the rate of recession must be so exceedingly 

 slow that when we are asked to consider the possibility of a recession ot 

 thirty or forty miles the argument would break down under the weight 

 of its time-factor. But it *ill be shown hereafter that in the total pro- 

 cess of denudation the rate of recession is rapid enough to satisfy the 

 temper of such geologists as may be parsimonious or even very stingy 

 in their allowances of time. It is sufficient here to advert to the very 

 obvious fact that the cliffs are receding, and that at some former geo 

 logical period they once stood nearer the center of the denuded district 

 > T ow, it is sufficiently obvious that if we allow the imagination to range 

 back indefinitely into the past and reverse the process of recession, re- 

 storing the material which has been denuded, the continuity of argument 

 will at length bring us to an epoch in which the cliffs which now face the 

 center of the San Rafael Swell came together, and the strata which those 

 cliffs now terminate stretched unbroken from west to east across the 

 whole width of the Plateau Province. It is only a question of time and 

 continuity of the process. The geologist may, however, raise a very 

 pertinent inquiry. Admitting that the cliff'-bound strata once reached 

 out in advance of their present limits, may they not have grown thinner 

 as they approached the center; may they not have attenuated rapidly 

 so that their former thickness over the swell was but a small fraction of 

 the aggregate thickness disclosed in the present escarpments .' Ma v not 

 the higher beds have thinned out and disappeared entirely a few miles 

 from their present boundaries? In all other well-studied regions it is a 

 general and almost universal rule that the strata varv greatly in thick- 

 ness when traced from place to place, and attenuate as they extend away 

 from their shore-lines. May they not have done so here? 



Answering the questions directly, it may be said that the Permian 

 Trias, and Cretaceous certainly did not grow perceptiblv thinner as thev 

 approached the center of denudation. The Jurassic did thin out quite 

 notably from west to east, and it is possible that the Tertiary may have 



