162 GRAND CANON DISTRICT. 



that some indurated rocks decay fifty times faster than others, the 

 conditions being identical as to climate and exposure. Wo have, it 

 is true, no experimental or laboratory data upon which this assertion 

 can be based, but it is, I am confident, quite defensible, and will appear 

 to be so when we examine the results of weathering in the rocks in 

 place. For there is another consideration which is not apparent in the 

 decay of building-stones. The strata are disintegrated by a process 

 which includes something far more efficient than mere solution or chem- 

 ical decomposition. 



At the base of every cliff in the Plateau country we find a large talus 

 consisting of fragments fallen from the rock faces above. The fragments 

 vary in size from bowlders of many tons' weight down to the finest 

 gravel, sand, and clay. Here is proof at once that the decay of cliffs 

 goes on chiefly through the breaking off of fragments. It soon appears 

 that the amount of material removed from the wall in solution is but a 

 very trifling fraction of the quantity which has spalled off from the face 

 of the wall. As the large fragments fall off from the vertical front they 

 are dashed to pieces below. In this fragmental condition they exposo 

 a much greater surface to weathering and are dissipated with corre- 

 spondingly increased rapidity. And now we come to the key of the 

 problem. The explanation of those persistent profiles of the Grand 

 Canon is found when we analyze the formation and decay of talus. It is 

 one of the most charming studies in the whole range of physical geology. 



In the Carboniferous strata of the Grand Canon we have a mass of 

 rocks widely varying in lithological characters, but which on the aver- 

 age are just about as obdurate to weathering as the average of rocks 

 found in other regions. So far as can be seen or inferred, in this respect 

 they differ not at all from the strata of other regions. Some of them 

 weather easily, some are very obdurate. Perhaps the only qualification 

 to this comparative statement is that there are no extremely perishable 

 strata in the whole series. The softest beds are still firm and perfectly 

 indurated. The degrees of obduracy, however, appear to vary greatly 

 in the series. 



The upper stratum in the caiion wall is a cherty limestone, which is 

 harder* than the average, though not extreme in that respect. It forms 

 usually a precipitous face, though it is frequently breached and broken 

 down. It is out of this series that the rows of pinnacles in the crest 

 of the canon are carved.t Beneath it is another series of limestones 

 of less than average hardness. They are sometimes a little cherty 



* In speaking here of relative hardness and softness, I wish to he understood as 

 meaning the resistance which the rock opposes to destruction by direct -weathering, 

 and not hardness in the mineralogical sense, nor the resistance which the rocks might 

 offer to the tools of the stone-cutter. 



t The cherty limestones are full of silicious nodules. They occur in vast numhers 

 and show a tendency to arrange themselves in hands parallel with the hedding. The 

 inclosing matrix, though mainly calcareous, has much disseminated silica in the 

 cherty form. In the weathering tho nodules are dissolved out of the matrix and fall 

 down the cliffs. 



