402 PROF. P. MARSHALL ON THE [Aug. 1906, 



augite is very pale. Nepheline cannot be distinguished, but when 

 it is treated with dilute hydrochloric acid a large portion at once 

 dissolves, and it contains a considerable percentage of alkali, as 

 stated below. The evaporated solution deposits a great number of 

 crystals of sodium-chloride. Staining does not take place readily, 

 but with careful treatment a large portion of the groundmass 

 becomes coloured. Its arrangement shows that the nepheline is 

 allotriomorphic, and is in very small grains. 



(D) Green phonolite. — Small crystals of brown augite, fringed 

 with a bright-green border of aegirine. Smaller crystals of sodalite, 

 sharply idiomorphic. Minute microliths of felspar, with segirine- 

 granules, in the groundmass. Sometimes the felspar shows a feeble 

 spherulitic structure. There is some transparent glassy base, which 

 probably is strongly alkaline. Evidently this rock is closely related 

 to the Mopanui type of nephelinitoid phonolite. It occurs in a 

 neighbouring locality. There is no cossyrite present, but there are 

 occasionally crystals of nepheline. This rock is in all respects (except 

 for the presence of sodalite) exactly similar to the green rock that I 

 consider to be a marginal, rapidly-cooled portion of the alkaline 

 intrusion at Sea View. 



A. B. C. D. E. F. 



Per cent. Per cent. Per cent. Per cent. Per cent. Per cent. 



Si0 2 60-00 57-42 57"00 55-66 54-52 53-56 



A1 2 3 16-26 18-83 18-56 17'18 15*84 15-28 



Fe 2 3 5-72 4-89 4'58 4-43 6-42 736 



FeO 3-52 3-56 2-76 3-56 4-53 542 



CaO 3-30 1-75 1-05 1-14 420 430 



MgO 1-05 0-59 0-41 0-73 0-98 1-29 



Na,0 4-08 6-23 6-34 7"02 4-38 5-51 



K 2 4-17 5-46 6-13 5-30 4-23 4-88 



H 2 2-64 2-36 2-96 4'53 3-64 312 



Totals 100-74 101 -09 99-79 99-55 98-74 100-72 



A=Trachytoid phonolite, Au Koraki. ^ 



B = Traclrytoid phonolite, Signal Hill. 



C=Trachytoid phonolite, Logan's Point. I Dunedin (N.Z.). Anal. 



D = Green phonolite, Purakanui. [ P.Marshall. 



E=Trachytoid phonolite, See House. j 



F=rTrachytoid phonolite, See House. J 



Chemically there is a moderately-close relationship between all these 

 rocks, although those with easily-detected nepheline are markedly 

 more alkaline than the others. The high percentage of iron-oxides 

 is characteristic of the whole group, as developed in Dunedin. The 

 rock from Au Koraki (A) contains very little nepheline, but a large 

 quantity of felspar, much of which is distinctly anorthoclase. This 

 rock appears to be a connecting-link between the trachytoid 

 phonolites and the trachytes. The green glassy type (D) is evidently 

 closely related to the nephelinitoid type from Mopanui. This rock 

 has some chlorine, but the amount was not estimated. The lava- 

 flows at the See House (E and E) are different in texture, but they 

 represent a very common type (andesitic) iu this district. They 



