252 G. A. Cotton — Block Mountains in JSleiv Zealand. 



uplift, (2) the relative strength of, or resistance offered to, 

 erosion by the imdermass and the cover, and (3) the nature of 

 the structures in the undermass where the overmass is rela- 

 tively weak. 



The covering strata in New Zealand are relatively weak, 

 consisting in great part of mudstones and incompletely indu- 

 rated sandstones, also of thin limestones, relatively resistant 

 but soluble, interbedded with weak elastics. A few thick 

 masses of indurated conglomerate which occur in places at the 

 base of the cover offer great resistance to erosion. In some 

 districts thin lava flows occur ; but these like the liuiestones 

 are weakened by interbedding with elastics. 



In contrast with the overmass, the rocks of the undermass 

 are generally highly resistant. In northern IN^elson these are 

 indurated argillite, quartzite, quartz schist, crystalline lime- 

 stone, and intrusive granite. In western Otago, gneissic and 

 plutonic rocks occur, and in Central Otago, the undermass 

 consists entirely of schist — relatively a very resistant rock com- 

 pared with the unconsolidated sands and clays largely devel- 

 oped in the overmass of that district. 



2. The Initial Surface. 



The form of the initial surface depends upon the nature of 

 the uplift. Two types of uplift may be distinguished : {a) The 

 blocks are differentially elevated, depressed, or tilted — the dis- 

 placement being solely by faulting or by faulting replaced to a 

 minor extent by monoclinal flexures. The initial surface must 

 be a mosaic of plane areas at various attitudes, some perhaps 

 horizontal and many inclined, separated by initial fault scarps 

 facing in different directions. (5) Strong warping — perhaps 

 better termed folding — attains considerable development; 

 faults though present pass into or replace the limbs of folds, 

 the uplifted blocks being in part anticlinal and the fault angles 

 and trough depressions being in part synclinal. The surfaces 

 of the structural units, which here as well as in {a) may be 

 termed "blocks," will be in part warped or flexed, though there 

 may still be notable plane areas, and the fault-scarp boundaries 

 between adjacent blocks will be replaced in part by monoclinal 

 slopes. This is the type of deformation concerned in producing 

 many of the New Zealand block mountains, and, according to 

 Gilbert (1874), a somewhat similar type is not unknown in the 

 North American Great Basin. 



The types of initial and sequential forms for regions of 

 uplifted and tilted blocks without cover have been deduced by 

 Davis (1903, 1905, 1912) and matched with examples from the 

 Basin Ranges and elsewhere. Louderback (1901)'has described 

 faulted blocks with a cover of resistant lava. In the present 



