798 GKOGRAPHY OF BLUE MTS. AND SYDNF.Y DISTRICT, 



of the upper hard and soft layers until the underlying bench is 

 cut through, the upper surface of the harder layer forming a wide 

 terrace or esplanade. In this way the mesas of a former period 

 may suffer great degradation in the earlier stages of a new cycle, 

 their own powers of resistance to erosion being of no value when 

 "sapping" takes place. Should the rocks be of massive homo- 

 geneous nature, as granite, or highly inclined hard varieties such 

 as indurated slates and quartzites, then, provided the mesas are 

 far removed from the incising action of the revived streams, the 

 younger valleys may attain a considerable width before the 

 residual blocks suffer any appreciable reduction. If the rejuve- 

 nated streams flow, however, at the foot of a large mesa, the mass 

 suffers material reduction during canon development. For 

 example, before the elevation of a plain to form a plateau, an old 

 sluggish river may in its lateral migrations be actually under- 

 cutting a large unreduced mass; on revival of the stream by 

 elevation, a canon quickly becomes the expression of the upward 

 movement, and, as it Ijroadens its valley, the monadnock suffers 

 considerably even during the youthful stages of the cycle. Fre- 

 quently mesas or buttes are favourably situated for their own 

 preservation, being far removed from strong stream courses after 

 elevation; their very existence, also, after the second cycle of 

 erosion, implies their excessive obduracy of resistance to erosion; 

 lateral corrasion, also, has but little effect on them, since the 

 streams now have forsaken the plains for canons sunken therein. 

 Thus monadnocks (residuals) occupying the centre of elevation 

 in areas of dense homogeneous rocks will suffer but little until 

 the new cycle of erosion has progressed to such a stage that the 

 main and tributary streams have lowered their courses to a point 

 near base-level and lateral erosion ensues, with consequent 

 broadening of the valleys and concomitant disappearance of the 

 plateau or esplanade encircling the base of the monadnock. 



In New England the writer has made special studies of the 

 loss suffered by residuals at the hands of erosive activities. In 

 the granite area of that district may be found remnants of at least 

 four successive cycles of reduction. Only the most siliceous 



