Bartrum. — Intrusive Igneous Rocks from Westport District. 267 



consist of approximately equal proportions of andesite-labradorite on the 

 one hand, and of hornblende and biotite on the other. The texture is very 

 coarse, and the hornblende presents idiomorphic outlines to the feldspar 

 and biotite. This amphibole is a deeply pleochroic bluish-green to pale 

 yellowish-green variety, and commonly shows separation of iron-ore along 

 its cleavage-traces. Twinning is very common. The form of alteration is 

 to a fine fibrous mineral that resembles actiniolite, with the exception that 

 the strong pleochroism of the unaltered amphibole is retained. Minute 

 strings of sphene are associated with and appear to be derived from the 

 primary amphibole. The feldspar, commonly altered to sericitic aggre- 

 gates, forms coarse irregular crystals, which enwrap the ferro-magnesian 

 and other earlier minerals. 



The deep-green pleochroic biotite suggests by its association with the 

 amphibole that it is derived therefrom ; it is commonly chloritized. Fre- 

 quently in association with the biotite and its alteration-products are nests 

 of secondary tremolite. Further minerals present in the diorite are sphene, 

 plentiful coarse apatite, pyrite, iron-ore, and epidote. The sphene is in 

 most curious coarse strings, helping to fill cracks which are partially occu- 

 pied by feldspar that has been dissected by the original fractures. The 

 origin of the titanium required for the formation of the sphene is obscure ; 

 much of it, however, may have been derived from the biotite and amphi- 

 bole. The epidote forms coarse crystals with very bright yellow tints and 

 strong pleochroism. In the central portions of most of these yellow crystals 

 are well-defined and moderately extensive cores of a strongly pleochroic 

 mineral showing colours varying from sepia-brown to faint grey. The 

 optical character is negative, and the axial plane at right angles to the 

 elongation. The birefringence is high. The writer was unable to obtain 

 more conclusive tests, but considers that the mineral must be classed as 

 an epidote allied to orthite. A similar association of epidote and orthite 

 is fairly well known with granites from America and elsewhere ; the primary 

 origin of suchf epidote has frequently^be en s uggested. 



(5.) Lamprophyres. 



As already indicated, no record exists of lamprophyres having previously 

 been found in situ in the Westport district, unless, as is possibly the case, 

 a rock from near Lyell described by Hutton as a dolerite* should correctly 

 be termed a camptonite. 



The lamprophyres are chiefly camptonites, but sections made from 

 stream-boulders and from one dyke reveal the presence also of monchiquite. 



Camptonites. — The Westport rocks classed as camptonites agree well with 

 the type as defined by Rosen busch.f Sections from several dykes in the 

 Blackwater River watershed exhibit differences one from another which at 

 first appear important, but, to a large extent, gradations in texture and 

 in weathering account for these. The freshest most typical camptonite 

 section prepared represents a portion of a narrow dyke in the basal breccias 

 of the coal-measures near the mouth of Haggard Creek, a tributary of 

 Blackwater River, Buller Valley. Sections were cut from other portions 

 of the dyke, and show a passage from the typical camptonite to a monchi- 

 quite, in which, in place of the feldspar of the camptonite, there is a mode- 

 rate amount of colourless isotropic base crowded with crystallites and 

 enclosing minute prisms of augite and amphibole. 



* " Note on the Geology of the Country about Lyell," 1889. 



t " Microskopische Physiographie," Band 2, 1 Halfte, pp. 684 et seq. 



