392 PKOF. P. MAKSHALL ON THE [Aug. 1906^. 



A. B. 0. 



Per cent. Per cent. Per cent. 



SiO„ 53-54 56-71 53*09 



A1 2 3 22-52 22-49 21-16 



Feb j 350 340 J 2-04 



CaO 301 2-22 330 



MgO 0-43 1-19 0-32 



Na 2 8-80 7-37 6"86 



K>0 5-26 5-87 8-42 



P 2 6 0-34 ... 0-15 



H 2 330 0-45 328* 



Totals 100-70 99-70 100-51 



* This includes 0O 2 , Ti0 2 , Zr0 2 , S0 3 , Cl 2 , S, MnO, and BaO. 



A = Hornblende-foyaite, Harbour Cone, Dunedin. Anal. D. B. Waters. 



B = Foyaite, Bratholmen (Southern Norway). Rosenbusch, ' Elemente der 



Gesteinslehre' 2nd ed. (1901) p. 129, No. 2. 

 O = Foyaite, Diamond Jo Quarry, Magnet Cave. Journ. Geol. Chicago, 



vol. ix (1901) p. 645 ; anal. H. S. Washington. 



The close agreement of these analyses supports the classification 

 of the rock as foyaite, which was based on the mineralogical com- 

 position. The matter has been fully discussed by Mr. C. N. Boult, in 

 a paper that is now being published by the New Zealand Institute. 



Several other varieties of nepheline-syenite are represented in 

 the Port-Chalmers Breccia. They vary considerably, but as all the 

 fragments are small — not more than 20 centimetres in diameter — 

 it is inadvisable to attempt a classification of them, as none of them 

 is necessarily typical of the rock-mass from which they are derived. 

 In all cases they contain much felspar — always orthoclase — and its 

 structure usually is completely allotriomorphic. Nepheline is 

 present in some quantity, but is considerably decomposed. It seldom 

 shows any crystallographic boundaries. iEgirine is the most frequent 

 dark constituent. It is of a very bright green, with an extinction- 

 angle of 15°. It is intensely pleochroic, and has the usual formula 

 — a deep green, fo yellowish green, c dirty yellow. One crystal is cut 

 almost at right angles to an optic axis, and indicates a strong 

 inclined dispersion of the bisectrices. In several slices a biotite is 

 associated with the segirine. It is intensely pleochroic — fc and c 

 dark brown, a bright golden-yellow. No hornblende or other 

 coloured constituent is present, except in one slice a core of brown 

 augite closely surrounded by golden mica and segirine. Some of 

 the mica-plates are crowded with apatite-inclusions, when the 

 mineral closely resembles the crystal of pseudobrookite illustrated 

 by Prof. Brogger, 1 if it be cut nearly parallel to the base, and fails 

 to show the pleochroism that is so noticeable in all other sections. 

 This golden mica is not found in any of the volcanic rocks of the 

 district, although some of the phonolites must be their volcanic 

 equivalents. The absence of amphibole and sodalite in these plutonic 



1 ' Die Eruptivgesteine de3 Kristianiagebietes : I — Die Gesteine der Grorudit- 

 Tinguait-Serie ' Christiania, 1894, p. 44 & pi. ii, fig. 1. 



