Barthum. — Rocks of Mount Cargill, Dunedin. 



173 



tion has effected a rounding of these crystals, and an edging of aegirine- 

 augite. 



Unless these minerals have been caught up from elsewhere, their occur- 

 rence shows that a close relationship to the trachydolerites exists. 



Comparison with other Logan's Point trachytoid phonolites shows that 

 the rock from Logan's Point itself is much denser and has less mossy cossy- 

 rite and less idiomorphism of the feldspar phenocrysts. There are no 

 feldspar phenocrysts, but abundant intensely green almost unpleochroic 

 pyroxenes, in a trachytoid phonolite from the foot of the North-east Valley 

 to Junction School Road. 



In the same type of rock from the North Head there are conspicuous 

 phenocrysts of sanidine, but none of aegirine-augite. The nepheline of 

 the groundmass can only be detected by staining. 



A knob on Signal Hill, above Burke's, is composed of a rock almost 

 identical with that of Mount Cargill : few or no feldspars show up in the 



first generation. 



Chemical Gha meters. 



An analysis of the Logan's Point trachytoid phonolite is compared 

 below with analyses of similar rocks. 



Reference to the analyses of nephelinitokl phonolites on page 171 will 

 show how closely the Mount Cargill Logan's Point phonolite resembles in 

 chemical composition the nephelinitoid types. 



A. 

 SiO, . . . . 56-12 



21-32 

 2-59 

 3-29 

 0-56 

 2-30 

 4-81 

 5-79 

 1-54 

 0-34 



A1,0 3 



Fe 2 3 



FeO 



MgO 



CaO 



K 2 



Na 2 



H 2 



CI 



98-66 



98-9 



99-79 



A. Logan's Point trachytoid phonolite. Mount Zion, Mount Cargill, 

 Dunedin. (Analysis by J. Bartrum.) 



B. Trachytoid phonolite from East Lothian, Scotland.* 



C. Trachytoid phonolite from Logan's Point. f 



(1).) Signal Hill Trachytoid Phonolite. 



This rock does not occupy any important area on Mount Cargill itself, 

 but is extensive across the North-east Valley, on Signal Hill, and also 

 covers a large portion of Pine Hill. Occasional sections cut from the 

 Mount Zion phonolite show examples of this type, but these seem to be 

 far from typical. 



Basalts apparently underlie this rock towards the headwaters of the 

 North-east Valley Stream. This agrees with the succession described by 

 Professor Marshall at the North Otago Head.:!: 



* Rosenbusch, " Elemente tier Gestemslehre," p. 292, 1901 ed. 



f Marshall, "Geology of Dunedin," Quart. Journ. Geol. Soc, vol. 62, Aug., 1906, 

 p. 402. 



X " Geology of Dunedin,'' Quart. .Journ. Geol. Soc, vol. 61, 1906, p. 41S. 



