250 C. A. Cotton — Block Mountains in New Zealand. 



Introduction. 



The geologic structure of I^ew Zealand has been described 

 by the writer in a recent article (1916(2, pp. 319, 320) as "a 

 concourse of earth blocks of varying size and shape, in places 

 compressed ; the highest blocks lying in the northeast and 

 southwest axis of the land masses, so that the whole strncture 

 may be termed a geanticline ; the blocks initially consisting of 

 an older mass of generally complex structure much denuded 

 and largely planed, and concealed over the greater part of the 

 area by covering strata which had not been disturbed before 

 the ' blocking ' took place ; the whole since these movements 

 considerably modified b}^ erosion somewhat complicated by the 

 effects of later movements of uplift and subsidence." 



In another article (1916 &), the block mountains and asso- 

 ciated features in a small area in northern Nelson have been 

 described. This paper deals with Central Otago, where the 

 " block" features are unusually well preserved. 



As an aid in presentation, the description and interpreta- 

 tion of selected types of New Zealand mountains is preceded 

 by a discussion of the physiographic development of block 

 mountains in general based on the established principles of 

 geomorphology. 



Part I. Block Mountains and Related Forms. 

 1. Structure. 



The structure is postulated as an uudermass of rocks with 

 varied structures which before being covered were denuded 

 enormously and reduced to small relief. Though the final 

 planing was accomplished in some parts of New Zealand by 

 the sea, subaerial agencies probably prepared the large planed 

 areas for the final marine planation. 



Remnants prove the presence of a former widespread over- 

 mass or cover upon the eroded surface of the undermass. It 

 is not necessary to assume that this overmass was laid down as 

 a continuous sheet over the region or that it is entirely of 

 marine origin. It may be postulated, indeed, that the cover- 

 ing beds are in part iiuviatile, resting upon a peneplain,* and 

 that other parts of such a surface may never have been covered. 



Perhaps the most important factors to be considered in the 

 processes of uplift and denudation are : (1) the nature of the 



* It is not to be inferred from the above statements concerning covering 

 strata in New Zealand that the writer believes submergence and the initia- 

 tion of sedimentation to have been everywhere simultaneous. It is probable 

 that overlap in the covering straia resulted from submergence of successive 

 "blocks" of the undermass. Further, the statement of Speight (1915), 

 based on the results of prolonged study, that in North Canterbury islands 

 rose through the Upper Cretaceous and Tertiary sea, must not be overlooked. 



