THE GEOLOGY OF NEW ZEALAND 



13 



complex. Somewhat later were the eruptions of basalt, etc., 

 which formed Banks Peninsula, described by Speight, and the 

 basalts and varied alkaline rocks about Dunedin, which Marshall 

 has studied. In Upper Tertiary times volcanic activity broke out 

 in the center of the North Island, where it has continued to the 

 present time. Andesites, and more or less pumiceous rhyohtes, 

 are the chief products of this activity, the principal centers of which 

 are arranged on a line following the northeasterly "grain" of the 

 country. Mount Egmont, the great isolated cone in the west of 

 the island, also consists of andesite. 



Pleistocene diastrophism. — The period which followed the 

 cessation of Notocene sedimentation, though comparatively short, 

 was that during which New Zealand assumed its present form as 

 a result of a great series of differential movements, warping or 

 tilting of crust-blocks, and the denudation of the surface so pro- 

 duced. The nature of these processes has been elucidated by 

 Cotton. The boundaries of the several blocks are now marked by 

 fault-scarps in homogeneous structures, or by faulted contacts of 

 older and younger strata, at which the latter are often steeply 

 upturned. Usually the faults are oblique to the strike of the 

 folded strata they truncate, and very frequently extend in a north- 

 easterly direction. The movement was not all due to simple tension 

 and differential subsidence of blocks, but strong compressive 

 lateral thrusts also occurred, with the occasional production of 

 folding passing into faults, of overthrusting and even overfolding 

 (Fig- 4)- 



The faults present the following characteristics. The movement even 

 along the same dislocation, may be concentrated in a single fracture with 

 walls close together, or perhaps several chains apart, the intervening space 

 being fiUed with comminuted rock. Again the fault may be a shear-zone 



One type of fault constantly recurs: narrow trough-faults in which the 

 rock between the main fault-waUs belongs to a higher horizon than the waUs 

 themselves. When the Tertiary beds which overlie the more ancient sediments 

 and graywackes are involved, the recognition of this type of fault is very 

 easy. [Henderson.] 



The surface of New Zealand at the close of the Pleistocene 

 orogeny was thus that of a group of variously elevated earth-blocks 



