192 STRUCTURAL GEOLOGY. 



alternation of softer and harder beds not being an absolute, but only an 

 accessory condition. In those mountain regions where the rocks are granites, 

 schists, or extravasated masses, every little stream engaged in deepening its 

 channel furnishes conditions favorable to sapping; and so these walls are ever 

 yielding, in small fragments that tumble down from time to time, in large 

 fragments when cliffs or crags topple over, and in great masses by land 

 slides. So wherever cliffs are formed, whether by deep corrosion or unequal 

 erosion, the cliffs themselves are degraded, disintegration beginning with the 

 breaking of the rocks above through atmospheric causes, aided by gravity. 

 Then the rocks are transported by gravity from the higher to lower levels 

 when the disintegration is increased by the fall, and the rocks are left in a 

 condition to be readily transported by the wash of rains or the flow of 

 streams. 



The extent to which degradation is carried on by this sapping though 

 much less than either of the others, is so great that it must not be neglected 

 in any consideration of this subject. 



In a mountain region every stream forms cliffs by deep corrasion, and 

 every cliff has a talus in evidence of the efficiency of this method, and we 

 see its effects exhibited in the most remarkable manner in the long lines of 

 cliffs or towering escarpments which stand athwart the Plateau Province. 



In this method of degradation petrologic conditions may be more or 

 less favorable, but within these conditions the principal factor in determin- 

 ing rate of degradation is declivity. The declivity must be very great for 

 the initiation of the process. With rocks already disintegrated by atmos- 

 pheric agencies, the declivity must be greater than what is usually denomi- 

 nated natural slope ; that is, it must be so great that rock power can Over- 

 come friction so as to transport the material to a lower level ; and again, 

 that gravity may be used as a force of disintegration, the declivity must be 

 still further increased, and with this increase of declivity the rate of dis- 



t/ 



integration and transportation by falling is increased at a great ratio until 

 the rocks are undermined, when the conditions again multiply the power. 

 But rapid corrasion, which depends on declivity, increases sapping ; and 

 sapping, which depends on declivity, increases corrasion by adding to the 



