Dtrrrox.l WEATHERING OF CLIFFS. 161 



just beyond the rapid. But this tendency is quickly checked and brought 

 to a stop by the increased power of the current due to the increased slope. 

 The body of fragments brought into the river laterally is vast in amount. 

 But on the whole it is insufficient at the present epoch to prevent the 

 river from corrading its channel, though corrasion is greatly retarded by 

 it. There are many stretches where there is an equilibrium between the 

 tendency to cut deeper and the tendency to build up the bottom by the 

 accumulation of d6bris; where the whole energy of the river is consumed 

 in dissipating the fragments brought into it. But there are other por- 

 tions where the river bed is in the bare rock of Palaeozoic and Archaean 

 strata, and wherever it is so corrasion is proceeding rapidly. 



WEATHERING. 



The work of corrasion is limited to the cutting of narrow gashes in 

 the strata, and the grinding up of the fragments brought into the river 

 channels. The widening of these cuts into the present configuration of 

 the chasm is the work of weathering. The common notion is that 

 "solid rock" is but little affected by any natural agents such as water 

 and air, and though it is acknowledged that water and carbonic acid 

 exert a certain nominal solvent and chemical action upon rock material, 

 yet these are usually esteemed so feeble that even the enormous periods 

 of time which the geologist invokes seem quite insufficient to warrant 

 us in ascribing to them any very important effects. Our observation 

 upon the works of human construction which have been exposed for 

 many centuries to the action of the elements confirms this notion. 

 The structures of Egypt, Greece, and Italy have been thus exposed for 

 periods which are nearly or even exactly known. They bear evidence 

 that this action is a real one, and that their final dissipation would, in 

 the event of indefinite exposure, be a mere question of time. But they 

 also indicate that so far as their own materials are concerned the pro- 

 cess is exceedingly slow. Their rate of decay by solution, if applied as 

 a factor to the recession of the walls of the Grand Canon, would give a 

 period of time so vast that the mind would promptly reject it because 

 of its very enormity. But we shall find that the recession of those walls 

 goes on, slowly indeed, but at a rate very much greater than would be 

 inferred from an inspection and comparison of the works of human an- 

 tiquity. 



It is at once obvious that the building stones are not a fair criterion. 

 They are selected for their durability, and of all rocks they represent 

 those which offer the greatest possible resistance to weathering. Taking 

 the common rocks only— those of frequent and world wide occurrence — 

 there is reason to believe that their rates of decay under equal condi- 

 tions vary among themselves enormously. Leaving out of the account 

 the unconsolidated or loosely consolidated strata, it cannot be doubted 

 11 G A 



