Wild.— Geolor,!/ of the Bluff. 327 



below. B is a type mineralogically similar to the Bluff porphyry; 

 C and D are analyses of typical porphyries, one from ' Analyses of 

 Rocks," U.S. Geol. Surv.. the other from Rosenbusch. 



Si0 2 



Ti0 2 



A1 2 0, 



Fe 2 3 



FeO 



MnO 



CaO 



MgO 



K 2 



Na 2 



Loss on ignition 



Totals .. .. 99-88 99-73 100-12 99-35 



Specific gravity, 2-5. 



A. Porphyry, Tewaewae Point, Bluff. 



B. Porphyry, Missouri. Composed principally of orthoclase and 

 quartz, with some microline, plagioclase, and biotite, and minor accessory 

 minerals. ("Analyses of Rocks," U.S. Geol. Surv., F. W. Clarke, 

 1904.) 



C. Porphyry, Mount Zion. Contains orthoclase, plagioclase, quartz, 

 biotite, apatite, magnetite, and zircon. (Anal., L. G. Eakins.) 



D. Alkaligranitporphyr mit Einsprengl. von Orthoklas und Oligo- 

 klas; grundmasse wesentlich Quartz und Anorthoklas. (Rosenbusch, H., 

 "Elemente der Gesteinslehre," 1901, p. 205.) 



A study of these analyses shows that compared with typical porphyries 

 the Bluff type is relatively rich in the oxides of the bases calcium, mag- 

 nesium, and iron, and correspondingly poor in silica, while the pro- 

 portion of alumina and alkalies is about normal. Further, it compares 

 favourably with the analyses of the quartz-porphyrites except as regards 

 the percentage of alumina. In other words, the rock is mineralogically 

 a porphyry, but chemically it tends towards the porphyrites. 



We conclude, therefore, that from the evidence of both chemical and 

 mineralogical composition the rock is a porphyry which has been enriched 

 with the bases calcium, magnesium, and iron. 



Further considerations are necessary before the method of this enrich- 

 ment can be studied. (See p. .334, "The Origin of the Hornblende 

 Schists.") 



DIVISION B. THE METAMORPHIC ROCKS. 



These are found along the southern shore of the harbour. The out- 

 crop is exposed between the levels of high and low water. Above high- 

 water mark, as has been pointed out above, is a plain of marine 

 denudation covered now by recent alluvium. On the other hand, the 

 lowest ebb of the tide fails to disclose the limits of the outcrop. The 

 main outcrop begins at Henderson Street, and strikes 15° S. of E. for 

 a distance of 15 chains. The strike then varies to E.S E., and continues 

 so for other 22 chains, when a southerly bend of the coast cuts off the 

 outcrop. 



