Bartrum. — Intrusive Igneous Rockx from Westport District. 265 



Many descriptions of rocks, based solely, however, on macroscopical 

 •examination, appear in various reports dealing with the geology of the 

 district. These cannot be ' considered to be of sufficient petrographical 

 interest to warrant the inclusion of the reports in which they are embraced 

 in the above list. 



The writer has been unable to gain access to a copy of Hutton's paper, 

 " Description of some Eruptive Rocks from the Neighbourhood of West- 

 port, New Zealand," but it is probable that the later paper, " The Eruptive 

 Rocks of New Zealand," embraces all the rock-types that Hutton discovered 

 near Westport. Amongst these, syenite, diorite, and dolerite — types 

 analogous to those described by the present writer — have been recorded. 



Description of Rock-types. 



(1 .) Biotite-hornbleride Granite. 



A biotite-hornblende granite outcrops on the beach north of the 

 Mokihinui River, at about 300 yards south of the Grenadier Rocks. In 

 hand-specimen it is a typical even-grained granite, showing dull, chloritized 

 ferro-magnesian minerals. Under the microscope the minerals observed are 

 quartz, orthoclase, oligoclase, brownish-green biotite, deep-green to pale- 

 yellow hornblende, subsidiary apatite, zircon, and muscovite, and secondary 

 chlorite, epidote, pyrite, calcite, sericite, and quartz. 



Orthoclase and quartz are very abundant, whilst plagioclase also is 

 common ; consequently the rock has a leucocratic appearance. The biotite 

 is almost invariably chloritized, or represented by pseudomorphs of chlorite, 

 epidote, and sphene, and is greatly in excess of the amphibole. 



The structure is not typically granitic, since the feldspar and amphibole 

 are idiomorphic, and the former mineral has preceded the latter in the 

 order of cessation of crystallization. Pressure has caused strain-shadows 

 to be exhibited in the quartz. 



(2.) Granitic Gneiss with Abundant Epidote. 



A granitic gneiss containing a great abundance of epidote, and pre- 

 senting some unusual features, was found near the debouchure of West's 

 Creek, a small tributary of the Buller River between Westport and the 

 Nine-mile Ferry. 



In its present state it is a gneissic rock with irregular mosaic bands of 

 coarse dentated crystals of quartz showing noteworthy undulose extinction. 

 Besides this indication of partial granulation and recrystallization, another 

 frequent and interesting characteristic of the quartz is a prominent fine 

 parallel banding, easily distinguished in ordinary light by reason of slight 

 differences in intensity. It is probable that a replacement of original 

 plagioclase is thus represented, for this striation exactly simulates albite 

 twinning, and in this event a part at least of the quartz must have been 

 introduced during the replacement of the plagioclase. Silicification of 

 plagioclase in schistose and gneissic rocks has been described by Mr. P. G. 

 Morgan from the Mikonui district* ; the writer examined several slides of 

 the rocks in question, but found that the silicification there shown was of 

 the ordinary micro -granular nature. 



In addition to quartz, the minerals present in the West's Creek gneiss 

 are plagioclase, cryptoperthite and orthoclase, abundant biotite, epidote, 

 and chlorite, accessory zircon and sphene, a few clusters of fine radiating 



* Bull. No. 6 (n.s.), N.Z. Geol. Surv., pp. 88 et seq., 1908 (see particularly Nos. 8 

 and 16). 



