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



would appear, therefore, that the schist is more susceptible to 

 weathering in the interior areas on account of some peculiarity 

 in the jointing, and it may be that the surface was deeply and 

 irregularly weathered prior to the deposition of the covering 

 strata. The occurrence of scattered logs of timber on the 

 block surfaces (Park, 1908, p. 23; Speight, 1911) where the 

 climate now borders on aridity, proves the existence of forest 

 established in an earlier, moister period. If the forest cover- 



FiG. 25. 



Fig. 25. Schist tors on Eough Eidge. 



ing was continuous, deep and irregular weathering may have 

 taken place even in the current cycle. 



Whether differential weathering is or is not still going on, 

 weathei'ed material is being rapidly removed from the surface 

 of poorly protected soil, and it is thus that the tor pattern has 

 been etched out. 



As greywacke does not occur in the most arid area, the con- 

 trast between greywacke and schist surfaces may be seen only 

 in the vicinity of the scarp of the northern highland where the 

 climate is semiarid. It may be that tors would form on a 

 greywacke surface in the arid district. But as the greywacke 

 is always thoroughly jointed, possibly no portions of the rock 

 are sufficiently free from joints to survive as tors. 



It is to be noted that the )3resence of tors on the plateaus 

 and sloping uplands indicates a general lowering of the sur- 

 face. Tors 20 and 30 feet high are very common, and on the 

 higher plateaus some reach a height of 70 to 80 feet (Park, 



