Finlayson. — Schists of Central Otago. 



73 



and these are seen to face generally to the north — i.e., to the 

 midday sun. The formation of these cavities is, therefore, pro- 

 bably due to changes of temperature, and to freezing, and con- 

 sequent disintegration. 



No writer, however, has sufficiently emphasized the import- 

 ance of joints in the production of these hummocks, for to this< 

 factor their formation is chiefly due. This is evident from a 



OTS! 





Fig. 2. 



Fig. 1. 



study of them in any one locality, where they are seen to 

 be roughly square in plan, with their corresponding sides 

 parallel. 



Castle Rock, on the Dunstan Range, in the form of two large 



turrets, shows very con- 

 spicuously the effect of 

 the jointing (fig. 2). 



The buttes are seen to 

 best advantage in dis- 

 tricts of horizontal strata, 

 and this is why they 

 are so conspicuous along 

 the summits of many 

 of the ranges, where the 

 bedding-planes of the schist are generally horizontal. Where the 

 dip increases they become irregular ; and with a nearly vertical 

 dip, as on the fault- line at the 

 south end of the Pisa Range, they 

 appear as nearly upright minarets 

 or " bayonet peaks " (fig. 3). 



It thus appears that the 

 amount of dip is the chief cause 

 of their varying form, the joint- 

 ing of the rocks the cause of 

 their existence. 



The combined effects of spher- 

 oidal weathering and of split- 

 ting along joint-planes have been 

 the cause of the numerous resemblances in the rock-hummocks 

 to} human forms, such as the Monk on the Carrick Range, 

 and the Celebrities on the Skipper's Road. 



Fig. 3. 



