1903I Petrography of some Igneous Rocks. 91 



The hand specimen shows a compact fine-grained pale- 

 greenish-grey rock. 



Under the microscope, it is seen to be very highly altered and 

 to consist of a trachytic groundmass of plagioclase laths inter- 

 spersed with grains of epidote and chlorite, in which are imbedded 

 larger individuals of plagioclase, and forms which have once been 

 occupied by phenocrysts of some ferromagnesian constituent now 

 entirely altered to chlorite, epidote, zoisite, quartz and calcite. 



The rock is too much altered to enable a determination of its 

 precise character to be made, but it is evidently some basic igneous 

 rock allied to andesite. 



No. 30. — From the country rock between West Bridge and 

 Stuart's hotel. 



Hand specimen shows fine-grained pink-coloured rock, in 

 which may be distinguished phenocrysts of hornblende, biotite 

 and plagioclase. 



Under the microscope, the rock is seen to be composed of a 

 microcrystalline groundmass of felspar and quartz, in which are 

 imbedded phenocrysts of plagioclase containing inclusions of some 

 alteration products ; also biotite altered in many cases to chlorite, 

 associated with which may often be seen sphene. There are also 

 a few fresh-looking quartz individuals, and outlines now entirely 

 filled with quartz, calcite, chlorite and epidote, which were once 

 occupied by some ferromagnesian constituent, probably augite, 

 as they often show forms indicative of that mineral. Magnetite 

 and apatite are present as accessory constituents. 



The rock is an altered andesite. 



It will be seen from the foregoing descriptions that most of 

 the rocks described are andesites or closely related rocks. No. 14 

 presents a striking contrast to the others in that, while they are 

 much altered, it is fresh and has every appearance of being a com- 

 paratively lecent eruptive. 



The occurrence of native copper in No. 125 is interesting, 

 and its secondary nature is very plainly shown. It has no doubt 

 been derived from the chalcopyrite by reduction. /C^^'^^/N^ 



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