266 Transactions. 



needles of tourmaline replacing feldspar, a little magnetite, and a moderate 

 amount of sericite derived from feldspar. The plagioclase — an oligoclase- 

 andesine variety — is in excess of the alkali-feldspar. The biotite is partially 

 chloritized, and is probably in part secondary ; its lamellae are frequently 

 bent. The epidote occurs abundantly in more or less idiomorphic crystals, 

 which exhibit faint pleochroism and are similar in general characters to 

 hypersthene, but can be distinguished from this mineral by the transverse 

 position of the axial plane compared with the elongation. Apart from the 

 induced gneissic character the structure is typically granitic. 



A rounded basic segregation or inclusion, of about 1 ft. in diameter, 

 occurring in this gneiss was examined and proved to be a diorite with 

 abundant amphibole. A moderate amount of epidote is again present, 

 with plentiful biotite and sphene. The constituent minerals are markedly 

 equidimensional. On the assumption that this is a segregation in the rock 

 now represented by the above-described gneiss it would appear that this 

 latter rock was originally related to the diorites, and perhaps could have 

 been classed as a monzonite, following Hatch's usage of that name.* 



(3.) Syenite-porphyry. 



The rock here described as a syenite-porphyry outcrops in a dyke at 

 least 20 ft. wide contained in granite in Slug Creek, a branch of the Pensini 

 Creek, Buller Valley. Macroscopically it is a fine-grained grey granitic 

 rock showing crystals of biotite, feldspar, hornblende, pyrite, and doubt- 

 ful quartz ; under the microscope, however, no quartz phenocrysts were 

 observed. 



This syenite-porphyry is essentially a fine granular mass of feldspar with 

 a little biotite and quartz enwrapping conspicuous phenocrysts of the 

 minerals noticeable in the hand-specimen. The plagioclase — an oligoclase- 

 andesine variety — is idiomorphic, whilst the orthoclase is commonly cor- 

 roded. The former feldspar is subsidiary to the orthoclase, particularly in 

 the groundmass. Perthitic intergrowths are occasional. 



Green, highly pleochroic hornblende, in long prismatic crystals, and 

 dirty-green to yellowish-brown biotite are subequal in amount. Yellowish 

 epidote and chlorite are common alteration-products of the amphibole. 

 Calcite and pyrite are other secondary minerals present. 



Although the apparent predominance of potash-feldspar warrants the 

 tentative classification of this rock as a syenite-porphyry, such classifica- 

 tion cannot be regarded as final until justified by chemical analysis. 



It must be noted that Huttonf has described a hornblende-syenite from 

 near the beach 16| miles north of Westport. The outcrop has not been 

 located by the present Geological Survey, although a similar type of rock 

 has been found in boulders up Stony Creek, at Waimangaroa. 



(4.) Diorite. 



Diorites and quartz-diorites are moderately common in the Westport 

 district, where their usual mode of occurrence is in dykes that transect 

 the general granite. A general description of them has not been attempted, 

 but an interesting type found in boulders in the Fox River has been selected 

 for description. 



Macroscopically this rock is a holocrystalline dark-green diorite showing 

 prominent coarse amphibole crystals. Under the microscope it is seen to 



* " Text-book of Petrology," 6th ed., London, 1910, p. IS.*?. 

 f " The Eruptive Rocks of New Zealand," 1889, p. 123. 



