Maclaren. — On the Geology of Te MoeJiau. 497 



chroic. Chlorite and magnetite abundant. Chalcedony, 

 showing a black spherulitic cross under crossed nieols, ap- 

 pears along planes of fracture. 



Dyke Bock from Eastern Slope of Motmtain. 



Hornblende Porphyrite. — A coarse-grained porphyritic 

 grey rock with large black crystals of hornblende. Under 

 the microscope the base is seen to be completely holocrystal- 

 line. It is abundant, and is formed of feldspar grains, laths, 

 and plates up to 0-02 mm. in length. Feldspars are porphy- 

 ritic, and range from 3 mm. long by 2 mm. broad to 7 mm. 

 long. They show marked polysynthetic twinning both on the 

 albite and pericline types, the latter crossing the former at 

 right angles. Phenocrysts of feldspar idiomorphic and zoned. 

 From their extinction angle of about 30° they must be placed 

 in the labradorite group. Hornblende is highly porphyritic, 

 as may be seen from hand specimens, ranging up to 12 mm. 

 (■|-in.) long and 4 mm. (^in.) broad. They are strongly pleo- 

 chroic, and show alteration to chlorite. Ophitic plates are not 

 uncommon. Magnetite abundant. The amphiboles and feld- 

 spars, from their corroded outlines, evidently crystallized long 

 before the base, or at great depths. Specific gravity, 2-71. 

 Considering, then, the three main features of the rock — -viz., 

 its holocrystalline base, its basic plagioclase feldspars, and its 

 porphyritic ferro-magnesian silicate — the rock must be classed 

 as a "hornblende porphyrite." I use the term "porphyrite" 

 very reluctantly, as the use of this term is one of the moot 

 points in the nomenclature of the igneous rocks. Continental 

 petrologists include under the name " porphyrite " the " older " 

 andesites, while some British authors apply the name to ande- 

 sites altered by atmospheric action, and others again use it 

 in the sense in which I have used it above. 



Saddle hetioeen Waikaivau and Cabbage Bay. 



Hornblende Andesite. — A compact greenish-black rock 

 showing no porphyritic minerals. Section : Ground-mass 

 abundant, microlitic, chiefly feldspar plates ranging up to 

 02 mm. in length. Porphyritic minerals are hornblende and 

 feldspar. The amphiboles reach 2 mm. in length, and show 

 remarkable multiple twinning, a feature very rare indeed in 

 hornblende. In one crystal alone there are as many as thirty 

 lamellae present. Phenocrysts slightly decomposed, showing 

 resorption border and alteration to magnetite along cleavage- 

 planes. Feldspars reach 5 mm. in length, showing polysyn- 

 thetic twinning both on the albite and perichne types. 

 Zonal bands strongly marked. Phenocrysts much corroded. 

 One section of the rock contains an included fragment of grey- 

 "wacke. This Waikawau rock differs from that from the 

 32 



