334 Transaction*. 



has taken in producing or altering the various rock types. Perhaps the 

 best way of opening up these questions will be to discuss independently 

 the origin of the metamorphic rocks, and a subsequent paragraph will 

 deal with the relative age of all the rock types. 



An attempt will now be made to deal with these problems. 



A. Origin of the Hornblende Schists. 



To an observer traversing the schist area described above, the solution 

 of the problem seems evident. The rocks are hornblende schists, 

 apparently well bedded, and inclined at various high angles. Intruded 

 into them is a mass of igneous rock. The suggestion at once occurs that 

 the rocks are the result of the metamorphism of a series of sediments, 

 produced by the igneous intrustion. 



Hutton* first put forward this view in 1872, referring to the rocks 

 as slates and sandstones, some argillaceous and some arenaceous. 



Parkf made similar statements in 1887, and added, " Tewaewae 

 Point, on the mainland opposite to the Pilot-station, appears also to be 

 formed of sedimentary rocks, but I had no opportunity of determining 

 this." This is the view at present held as to the origin of the schists. 



A visit to Tewaewae Point, however, and an examination of the rocks 

 that actually do occur there, at once raises grave doubts as to the correct- 

 ness of this view. For there we find not a sandstone, but a typical 

 porphyry. Microscopical examination shows evidence of strain in the 

 undulose extinction of some of the feldspars, and metamorphism is 

 indicated in other ways. The feldspar phenocrysts are cloudy, they 

 have irregular outlines, and new minerals are closely associated with 

 their decomposition. The chief of these are hornblende, mica, and iron- 

 ores. The rocks are traversed by joints striking in the same direction 

 as those found in the schists. The outcrop disappears below low-water 

 mark, and presumably reappears on the other side of the harbour. At 

 any rate, I have the assurance of the Engineer of the Bluff Harbour 

 Board that rocks outcrop in situ right across the channel. 



Microscopical examination of the members of the schist series shows 

 that the outermost members contain comparatively large phenocrysts of 

 feldspar. These are much broken down, and are quite surrounded by 

 microlites and crystals of hornblende. Associated with this is the 

 separation of iron-ores, especially magnetite. As the norite is ap- 

 proached these residual feldspars are found to decrease in size, until 

 finally with the innermost series they disappear completely. This 

 gradual disappearance of the feldspars is found to correspond to a 

 gradual increase in the amount of ferro-magnesian minerals, especially 

 hornblende and iron-ores. 



The groundmass of the porphyry consists of feldspar and quartz, and 

 small flakes of hornblende and brown mica. The groundmass of the 

 schist is essentially similar. The feldspar grains have been considerably 

 comminuted, while an opposite process has taken place in the case of the 

 hornblende and mica. The schists also contain secondary minerals, such 

 as apatite and epidote. 



* Hutton, P. W.. " Report on Geology of Southland," Rep. N.Z. Geol. Surv , 

 1871-72, p. 89. 



fPark. J.. "The Geology of Bluff Peninsula," Rep. N.Z. Geol. Surv., 1887-88 

 •p. 72. 



