400 PROF. P. MARSHALL OST THE [Aug. I906, 



The remarkable similarity of analyses A & B shows that the rocks 

 are almost identical. The lestiwarite, however, is a dyke-rock, 

 whereas the trachyte is for the most part a lava-flow, although dykes 

 that differ in no respect from the lava are found. It is interesting 

 to compare these analyses with that of anorthoclase placed beside 

 them. The very great similarity is remarkable, and is perhaps suffi- 

 cient in itself to justify us in regarding the Portobello trachyte as 

 really an anorthoclase-rock. 



Trachyte is found over a considerable area around the Quarantine 

 Islands. It is usually associated with scoriaceous formations, and 

 often shows a conspicuous flow-structure: therefore, it was un- 

 doubtedly a lava in most of its occurrences. At times it is 

 porphyritic, with large crystals of anorthoclase embedded in the fine 

 white base ; more usually it is fine-grained. The large crystals can 

 be detached from the decomposed base in places. They are bounded 

 by planes inclined almost at right angles, but they have not yet 

 been accurately measured with a reflecting-goniometer. At two 

 places on the Portobello Peninsula this trachyte forms dykes that 

 penetrate the coarse Mount-Charles Dolerite which forms the main 

 mass of the peninsula. The contrast between the white and the 

 black rocks is very striking. The trachyte is pierced by tinguaite- 

 and camptonite-dykes at Portobello, as well as by basaltic dykes 

 at Portobello and Omihi, and no dykes of trachyte have been 

 observed traversing any rocks except the Mount-Charles Dolerite. 

 The trachyte-lavas underlie basalts, trachytoid phonolites, and all 

 the other lavas described, except the Mount-Charles Dolerite. The 

 trachyte was, therefore, evidently emitted after the dolerite in 

 question, and before any of the other lavas of the district. 



9. Kaiwekite. — Hand-specimens are of a dull-green colour, and 

 show numerous bright cleavage-planes of large felspar-crystals, 

 which have the same dark coloration as that which is so noticeable 

 in the Norwegian rhomb-porphyries. Slices show that the pheno- 

 crysts are composed of anorthoclase (see PL XXXIX, fig. 2), and 

 contain numerous inclusions of glass, aegirine-augite, and other sub- 

 stances. Smaller crystals of segirine-augite are frequent, and there 

 are occasional serpentine-pseudomorphs after olivine. Many of 

 the smaller felspar-crystals are almost perfect squares, and they are 

 embedded in a groundmass of felspar-laths and grains of segirine- 

 augite. I have been unable to distinguish nepheline m the slices or 

 by staining methods, although treatment of the rock-powder with 

 acid and subsequent evaporation produce numerous crystals of 

 sodium- chloride. 



In some examples of this rock (Long Beach and "Wiekliffe Bay) 

 there is much triclinic felspar (oligoclase), but it is always bordered 

 by untwinned mineral. In the same examples, irregular grains of 

 pale-brown augite are common ; they always have a margin of 

 segirine-augite, containing many grains of magnetite. The segirine- 

 augite fills up the irregularities of the crystal, and gives it a sharp 

 idiomorphic outline. Brown hornblende is occasionally present 

 (Long Beach), of tenin large crystals, 5 centimetres long. 



