320 Transactions. 



II. Description of the Bock Types and their Geological Occurrence. 



For the purpose of description the rocks of the Bluff district are 

 conveniently divided into two main divisions, distinguished here as 

 4 and B. These divisions are — A, the igneous rocks; B, the meta- 

 morphic rocks. 



Each of these divisions may be further divided into classes, the rocks 

 of each class in the division being entirely distinct. The igneous rocks, 

 Division A. consist of — I, igneous rocks of plutonic origin; II, igneous 

 rocks of hypabyssal origin. The term " hypabyssal ' is here used in 

 the same sense as Harker* uses it, hypabyssal rocks corresponding in a 

 general way, though not precisely, with the group " gangesteine or 

 " dyke-rocks " of Rosenbusch. 



The metamorphic rocks, Division B, contain two classes, as follows : 

 I, acid metamorphic rocks; II, basic metamorphic rocks. 



An attempt will be made below to show that there is an intimate 

 connection between the acid metamorphic rocks (Division B, Class I) 

 and the igneous rocks of hypabyssal origin, the metamorphic rocks being 

 derived from the hypabyssal ones. This is not the view taken by previous 

 investigations — e.g., Button and Park — but the present theory is the 

 outcome of the examination of material that had not previously been 

 brought under inspection. The point will be fully discussed in the course 

 of this paper. 



The relationship among some classes and the absence of relationship 

 among others thus briefly mentioned makes it difficult to suggest any 

 perfectly satisfactory scheme of classification. 



DIVISION A. THE IGNEOUS ROCKS. 



Glass 1. — Igneous Bocks of Plutonic Origin. 



1. Norite. 



As has been stated above, igneous rock of plutonic origin forms the 

 backbone of Bluff Peninsula. The mass is believed to lie essentially one 

 throughout as regards chemical and mineralogical composition. This 

 Parkt definitely states to be the case. 



The present paper will deal with the rock as it is typified in Bluff 

 Hill. There are numerous outcrops at the surface, as well as a continuous 

 outcrop at sea-level along the south-east end and the south-west side of 

 the range, except in the rare intervals occupied by sandy beaches. From 

 specimens collected from different parts of the mass some thirty sections 

 have been prepared and examined. 



Hand-specime?i (specific gravity, 2"68). — The rock varies somewhat in 

 grain in different parts of the mass, though this valuation is apparently 

 not systematic. In some parts it tends towards a pegmatitic structure; 

 sometimes a dense black rock of fine grain is found. 



The typical rock is a rather coarse-grained type, speckled black and 

 white. With a lens the black grains may be distinguished as pyroxene 

 or hornblende, according to the characteristic cleavage. The white specks 

 are feldspar. 



* Harker, A., " Petrology for Students," 4th ed. (1908), p. 108. 



t Rep. N.Z. Geol. Surv., " The Geology of Bluff Peninsula." 1887, p. 89. 



