690 



SUBANTAECTIC ISLANDS OF NEW ZEALAND. 



[Geology. 



Directly the glaciers had dwindled away the surface of the island was occupied 

 by peat-forming plants, and by their agency the whole island has become covered 

 with a mantle of peat. The origin of the peat-forming plants hardly comes within 

 the scope of this paper, but it may here be emphasized that we know but little of 

 the depth of the surrounding ocean. That it is shallow is known, and it is quite 

 possible that portions are so shallow that a relatively small elevation would allow 

 of a land connection between lands which are now widely separated. It is, how- 

 ever, quite possible that the glaciation was not severe enough to prevent vegetable 

 growth from continuing to exist on the lowlands. 



Since the disappearance of the glaciers the island does not appear to have had 

 its level lowered to an important extent ; but wave-action has had an important 

 effect in modifying the coastal features ; the western coast in particular has 

 suffered, and the removal of much material has been effected and high cliffs have 

 been formed. 



PETROGRAPHY. 



Gabbro. (Fig. 8.) 



Hand-specimen grey, without any conspicuously large crystals, though feldspar 

 and augite are distinct. 



Section : Feldspar much twinned ; the maximum angle of extinction is 27°, 

 so the species is an acid labradorite. Augite light brown, with slight schiller 

 structure, and often twinned. Olivine in many instances included poecilitically in 

 the augite. A good deal of ilmenite. The normal order of crystallisation was 

 followed. 



The rock occurs at Mount Menhir, and on the west coast, south of North-west 

 Bay. 



SiO, 



AI263 



FcOg 



FeO 



CaO 



MgO 



TiO, 



Na,0 



K,0 



h;o 



10013 

 Chemically the rock is a fairly typical gabbro, though rather acid, as would be ex- 

 pected from the large quantity of feldspar. 



Porphyry. (Fig. 10.) 



A light-grey rock, showing large crystals of feldspar and cleavage-plates of 

 black mica. 



Section : Feldspar crystals, with a core of oligoclase, usually with a mantle of 

 anorthoclase. The mica is biotite. A little brown hornblende (barkevicite). A 



Analysis. 



5176 



1754 



540 



694 



7-85 



2-89 



0-96 



351 



0-87 



241 



