256 C. A. Cotton — Block Mountains in Neic Zealand. 



covering strata must rapidly take place, maturity of dissection 

 will be rapidly attained, iirst in the middle parts of the steepest 

 slopes and later over the whole area of sloping upland. On 

 level upland surfaces, where streams may be widely spaced, on 

 gentle slopes, where stream grade may be attained at no great 

 depth below the initial surface, and also on low-lying blocks, 

 where the surface is at no great height above base-level and 

 where, consequently, deep dissection is impossible, maturity of 

 dissection may be relatively long delayed. 



Fig. 2. 



Fig, 2. Diagram of ihe development of the back slope of a gently tilted 

 block with a relatively resistant undermass and weak overmass from the 

 initial form A through sequential forms B, C, and D to a stripped floor 

 (earlier denudation plain) with shallow dissection, E. 



During this stage of the cycle, the troughs will generally be 

 aggraded on account of an enormous quantity of waste from 

 the upland surfaces. During the whole period of deformation, 

 the troughs will be filled as they sink (see fig. 2, B, C, D). 

 The waste may be laid down in part on the floors of lakes, 

 wholly or in part conformable to the deposits of the prede- 

 formational period, or it may be deposited wholly subaerially 

 as fans growing outward from the margins of the surrounding 

 blocks, coalescing and forming an aggraded plain, the deposits 

 of which will, in general, accumulate upon a maturely dissected 

 surface developed as a sequel to the deforming movements. 



Stripping. — When the consequent and other streams of the 

 sloping uplands cut thi'ough the cover and become superposed 



