THE GEOLOGY OF NEW ZEALAND 3 



with limestones, graywackes, and quartzites and pass with appar- 

 ently gradual transition into mica-schists, which are invaded 

 by gneissic diorites. A narrow zone of such schistose rocks extends 

 intermittently down the west coast of the island. In the southwest 

 the gneissic diorites become dominant and are associated in one 

 region with "granitic" gneisses. In addition, there are massive 

 plutonic rocks, chiefly granites, occurring especially in the two 

 extremities of the island and surrounded by zones of contact 

 metamorphism, which is naturally more marked where the regional 

 metamorphism is least pronounced. The first hypothesis is 

 that adopted by the Geological Survey. The gneissic rocks and 

 invaded schists are considered to be of pre-Ordovician, possibly 

 pre-Cambrian age, and covered unconformably by the Ordovician 

 sediments. No sharp line of separation has, however, been found 

 between them, nor other evidence of difference of age, such as the 

 inclusion of gneissic pebbles in the Ordovician rocks. The massive 

 plutonic rocks are considered to be of much later date, and prob- 

 ably later Paleozoic or even in part Mesozoic. 



The second hypothesis regards the more or less gneissic dioritic 

 rocks as having invaded the Ordovician sediments in Paleozoic 

 or possibly Mesozoic times, before or during the climax of an 

 orogenic movement, while the intrusion of the massive plutonic 

 rocks followed after that climax. Within the present year massive 

 diorite has been found to invade Lower Triassic ( ?) annelid- 

 bearing graywacke. In a variant of this hypothesis advocated by 

 Park, the subordinate mass of "granitic" gneiss is held to be 

 intensely altered sedimentary rocks and regarded as probably 

 pre-Ordovician, merging upward into Ordovician slates. The abun- 

 dance of sillimanite in the gneiss accords with this view. Petro- 

 graphical examination of the gneissic diorites on Grubenmann's 

 principles, first applied hereto by Speight, indicates that they form 

 the core of a folded range, for structures characteristic of deep 

 seated metamorphism are found in the center of the massif, while 

 those produced at lesser depths occur on either side. 



Bartrum has found that pebbles of similar metamorphic rocks 

 are widely distributed in the Mesozoic and Cainozoic rocks of the 

 North Island, suggesting the existence of an ancient Paleozoic 



