EFFECTS OF EAI^^FxlLL ON EOCK DECAY. 55 



tlie abrasive forces keep pace with tlie agents of decay; and iinallT, the 

 conglomerates, formed when the abrasive actionh go forward more rapidlj" 

 than tliose which lead to the general softening of the rocks. 



RELATION OF ARKOSES TO EROSION. 



The above-mentioned facts concerning the snccession of 1)eds in the 

 Narragansett Basin clearly point to the existence of what may be termed a 

 cycle of erosion dependent on the relative rate at which the two diverse 

 processes connected with the decay of tlie lands go forward. This equation, 

 in a general way, seems to be as follows: Where the rainfall is so slight 

 that a vegetal covering is not established in a country, the chemical assault 

 on the rocks, which is due in the main to the OO2 and other products which 

 the decaying organic matter in the soil contributes to the ground water, is 

 probably wanting. The result is that the erosive work, or that which oper- 

 ates to remove the detritus in the form of visible sediment, though it may 

 be very slight, is likely to be enough to keep the rocks which have been 

 softened well cleared away. In such an arid country the rainfall is apt to 

 be irregularly distributed, so that the ton-ent action is at times exceedingly 

 strong. The result of this is that the valleys become encumbered with 

 angular breccia-like debris, such as now exists in the valleys of the arid 

 districts of the Cordilleras. 



An increase in the rainfeU to the point where an ample mantle of 

 vegetation is supplied, but short of the point where torrent action is made 

 excessive, tends to produce a greater amount of decayed rock than can 

 be cleared away. The result is that the coating of what we may term the 

 the nontransported arkose steadily increases in amount. The thicker this 

 porous layer becomes the more the rate of the toiTent action approaches 

 uniformity, for the reason that the open structure of the decayed rock causes 

 the corroded zone to be an effective storehouse for the ground water, whence 

 it is slowly yielded to the streams. With an increase in the rate of precipi- 

 tation beyond the point where the water can be taken into the unoccupied 

 detrital layer a critical point is soon reached where the mechanical erosion 

 will rapidly hicrease and will gain on the process of interstitial decay. If 

 the rate of mechanical wear much exceeds that of the decay, the result will 

 be the deportation of solid waste in the form of pebbles, the process being 

 marked in a geological way by the production of conglomerates. 



