G. A. Cotton—Block Mountains in New Zealand. 253 



paper,^ attention will be directed principally to forms devel- 

 oped in a region in which the predeformational surface is a 

 plain of deposition, being the surface of a weak overinass 

 lying on a planed underraass. 



3. Possible Types of Drainage. 



The early stages of the drainage of a region in which the 

 initial forms have been produced by the faulting and folding 

 of a weak overmass on a planed undermass must be entirely 

 consequent if the overmass is continuous and if its surface is a 

 plain of marine deposition. Parts, however, of the Otago dis- 

 trict were probably emergent prior to the deformation, and 

 some of the streams may have persisted during and after the 

 deformation as antecedents. Such a preexisting drainage sys- 

 tem would be present also in any part of the area where the 

 upper layers of the covering strata were of subaerial origin, or 

 in any part where regional elevation antedating the deforma- 

 tion had brought the upper layers of marine strata above sea 

 level. Few streams are powerful enough to persist through 

 strong deformation, and though a few large streams may be 

 antecedent, the greater part of the drainage, including most if 

 not all of the minor and tributary streams, will be consequent. 

 As movements of deformation occupy some time, erosion can 

 accomplish much while uplift is in progress. A drainage sys- 

 tem will be established as soon as any portion of the region is 

 exposed as land, and this early consequent drainage tends to be 

 perpetuated during the continuance of the movements. As 

 movements probably will not go on continuously in all parts of 

 the region, if the consequent drainage resulting from the 

 deformation be alone considered, the early drainage pattern is 

 not necessarily the same as that which would have come into 

 existence had the final structure been instantaneously assumed. 

 In a deformed region may be found, therefore, true antecedent 

 streams, true consequent streams, and streams that were conse- 

 quent upon the form of the surface assumed as the result of 

 early movements but are antecedent to later movements of the 

 same series. Such streams might perhaps be appropriately 

 termed antecedent consequents or antecGonsequents. Spill-over 

 courses resulting from the overtopping of divides by alluvial 

 accumulations are also possible. 



After cutting down through the covering strata, streams of 

 any or all of these types will be superposed in places upon the 

 structures of the undermass. Besides, all may be expected to 

 develop insequent tributaries, and as the cycle progresses sub- 

 sequent streams will form upon weaker structures in the under- 

 raass and such portions of the covering strata as have escaped 

 complete destruction. 



