398 PROF. P. MARSHALL ON THE [Aug. I906, 



porphyritic rock that appears to be intrusive. Olivine, augite, 

 and nepheline are present as in the effusive type, but are in rather 

 larger crystals. There are many big crystals of felspar : anorthoclase 

 and sanidine. Sodalite is occasionally present in crystals of mode- 

 rate size. The groundmass contains a large number of felspar- 

 microliths, much ssgirine, and some cossyrite and magnetite. 



At Pine Hill another type of this rock occurs. Olivine is less 

 abundant, and there is but little brown augite, but nepheline and 

 felspar are abundant as phenocrysts. There are numbers of com- 

 pletely-resorbed crystals of hornblende, the form of which has not 

 been lost. In the groundmass there is some dull-green aegirine- 

 augite, no cossyrite, and a very fine-grained development of felspar 

 and nepheline. I have made no analysis of either of these rocks. 



There is no evidence of this trachydolerite overlying or being 

 covered by any other of the rock-masses near it ; and the sudden 

 change from the coarse variety to the volcanic rocks around, suggests 

 its intrusive nature. The summit of Flagstaff is one of the highest 

 points of the whole district, and may well represent the volcanic 

 pipe from which much volcanic material was emitted in the southern 

 portion of the Dunedin volcanic area. At Pine Hill the rock-mass 

 is much smaller, but it is as sharply separated from the surrounding 

 rocks as is the occurrence at Mount Flagstaff. 



6. Camptonite. — The rock here described is not a normal 

 member of the group, although it shows closer affinities to it than to 

 any other group of rocks. It is a light- grey rock, with large 

 steam-cavities lined with analcite-crystals. In sections big pheno- 

 crysts of oligoclase are seen. There are smaller crystals of horn- 

 blende, and these are embedded in a groundmass of felspar, analcite, 

 hornblende, and segirine ; (see PI. XXXVIII, fig. 2 & PI. XXXIX, 

 fig. 1). The dyke is 15 feet wide, and juts out into Portobello Bay. 

 The following quantitative analysis of the rock was obtained : — 



SiO Q 51-48 



A1 2 3 16-37 



Fe 3 5-71 



FeO 4-64 



CaO 3-60 



MgO 1-81 



JS T a 2 5-86 



K 2 409 



P 5 0-21 



H 2 6 5-82 



Total 99-59 



This camptonite evidently is closely related to the ulrichite 

 described above, but differs from it in the presence of triclinic felspar, 

 in |the greater abundance of hornblende, and in the absence of 

 nepheline and olivine. 



£; A rock rather similar to this, but not so fresh, occurs in a dyke 

 at Port Chalmers. Much calcite is found in it ; there is also much 

 less segirine and more analcite. 



