G. A. Cotton — Block Mountains in New Zealand. 259 



have been cliffed by wave action (^N'oble, 1914, p. 62), and 

 fault-line scarps with a very similar form may be present on 

 one or more sides of a small block. Moreover, salients of 

 both kinds may be dissected by inseqnent ravines, and thus 

 soon lose their initial form. 



In New Zealand remnants of cover are common as salients 

 projecting above stripped plateaus. In northern Nelson these 

 are generally limestone mesas ; in eastern Central Otago vol- 

 canic rocks cap many small buttes and protect large areas. On 

 the more level plateaus are small salients of indurated quartz 

 conglomerate. The writer has not recognized with certainty 

 any monadnocks. 



Mature Dissection of the TJnderm.ass. — Stripped plateau 

 surfaces traversed by ravines of moderate depth will persist 

 for a long period if the surface slope is rightly adjusted to the 

 volumes and grade of the streams. In a region of small rain- 

 fall an initial slope of a block surface as high as 10° may have 

 small consequent streams and a large number of subequal, 

 graded, shallow ravines occupied by intermittent streams. 

 Though these break up the stripped surface to some extent, 

 unless the stream spacing is very close, the plateau remnants 

 will be relatively stable. 



Grading of the ravine sides by soil creep will cause the sharp 

 shoulders bounding the plateau remnants to disappear and the 

 interfluvial areas to be reduced. But still their summits will 

 be accordant with one another and suggest the reconstruction 

 of the tangent surface of the undermass. Such conditions 

 exist in the stripped plateaus of South Canterbury and Otago 

 (see figs. 3 and 4). Later, a surface tends to waste away very 

 slowly unless destroyed by erosion working back from initially 

 steep portions of tbe same block. 



Under other conditions plateau remnants are relatively short- 

 lived. If, owing to abundant rainfall, to steepness of initial 

 slope, or to initial irregularities of surface which have resulted 

 in concentration of consequent drainage along a few channels, 

 the graded profile for such streams lies far below the stripped 

 plateau surface, the plateau remnants will be cut up by conse- 

 quent, insequent, and perhaps subsequent ravines. An early 

 and complete dissection of the surface over the whole block 

 will result, and the block will become an asymmetrical moun- 

 tain ridge with strong relief throughout (see fig. 5). 



Block surfaces in this stage of dissection are common in 

 northern Nelson, examples being the surfaces sloping easterly 

 from the Pikikiruna Mountains to Tasman Bay. Of the same 

 kind are the northwestern slopes of the Kaikoura and Seaward 

 Kaikoura Mountains, in Marlborough. 



A special case of large stream volume leading to deep dis- 

 section of a plateau surface is that in which the descent from 



