THE ERUPTIVE ROCKS OF NEW ZEALAND. 131 



Thames District — 1. A dark greenish-black compact rock with 

 small black prisms scattered sparingly through it. S.G. 2-69. 

 Section : Ground-mass moderate, microgranitic, of felspar plates 

 and laths, from -002 to -006 in length, and chloritic infiltrations. 

 Porphyritic minerals are plagioclase, serpentine and chlorite. The ■ 

 plagioclase crystals go up to -04, zoned, partly decomposed. The 

 serpentine is brownish-green, in small prisms, striated, not pleo- 

 chroic. The chlorite is yellow-green, slightly pleochroic, and con- 

 tains inclusions of apatite. Magnetite is in fine specks but is not 

 abundant. Pyrites, also in fine specks, is less in quantity than 

 the magnetite. Forms a dyke-like mass near the mouth of 

 Tararu Creek. Hutton, Rept. Geol. Expl. 1868-9, p. 21 (Mela- 

 phyre); Hector, ditto, p. 40, No. viii. (Analysis). 



2. Greenish-grey rocks with large white crystals of felspar and 

 occasionally black prisms. S.G. about 2-66. Section : Ground- 

 mass abundant, microgranitic, felspar plates from -003 to "006. 

 Porphyritic minerals are plagioclase and chlorite with opacite after 

 hornblende. The plagioclase is much decomposed, and up to '20 

 in length. The hornblende crystals have been up to -40 in length. 

 The chlorite is green, sometimes pleochroic, changing from yellow- 

 brown to dark bluish-green, and with very low polarization 

 colours. Pyrite is common. Found as boulders on the beach 

 north of Waiohanga Point. 



Trachytic rocks composed of a 

 microlites of felspar, containing porphyritic crystals of plagioclas , 

 (rarely with sanidine) and augite, often with hornblende or eustatite 

 and occasionally with olivine in addition. 



Hornblende Anj)ESITE. — (a) With quartz. 

 (Dacite.) 



Thames ZJis^Hci.— Greyish-green rocks with translucent colour- 

 lfs.s felspars and black prisms of hornblende thickly scattered 

 through them. S.G. 2-62 to 2-76. Section : Ground-mass 

 moderate, colourless, chiefly crystallitic but occasionally felsitic, 

 ^^'ith scattered felspar microlites and grains of magnetite. The 

 I'orphyritic minerals are quartz, plagioclase and hornblende. The 

 quartz is in small quantity, in allotriomorphic grains, from "02 to 

 ■U< in diameter, rarely more than one on a slide. The plagioclase 

 ^'oes to -08 ; it is idiomorphic, zoned, and occasionally shews 

 iniiary twins. The hornblende is brown, rather pale, idiomorphic, 

 up to -04 in length. Secondary minerals are chlorite, calcite, 

 pyrit^js, and leucoxe^e. Hutton, Reports Geol. Explorations, 

 1^68-9, p. 21 (Timazite) ; Hector, ditto, p. 40, Nos. x. and xi. 

 (Analyses). 



