174 Transactions. 



Macroscopically the Signal Hill phonolite is a greasy green fine-grained 

 rock of platy nature, showing occasional feldspar crystals. 



Under the microscope there is a noticeable scarcity of phenocrysts as 

 compared with the typical rock from Signal Hill. In the rock outcropping 

 in the North-east Valley Stream, however, there are plentiful conspicuous 

 amphiboles up to \\ in. by £ in. in size. Resorption of this amphibole 

 is noticeable, and its pleochroism is marked — a, pale golden-yellow; 

 t>, rich brown ; c, dark opaque-brown. Though the tests made were 

 scarcely satisfactory, apparently c A h = 6°. A = d, B = b, and mineral 

 is positive. This amphibole is probably barkevicite, and it is considered 

 that it is the same as that in the various other allied rocks of Mount Cargill. 



Sanidine, in much corroded crystals of small size, is persistent, but 

 never plentiful. There are occasional corroded crystals of anorthoclase, 

 of oligoclase, and of a more basic feldspar that is apparently andesine. 



The only other phenocrysts are small scarce ragged crystals of greenish 

 and pink augite. 



The groundmass is the most characteristic feature of the type. 



A dense web of small feldspar laths, showing remarkable flow structure, 

 constitutes the greatest part of the groundmass, and entangles fairly 

 plentiful augite granules, very minute nepheline prisms, and a little 

 scattered magnetite. 



Most of the augite is the greenish soda-bearing variety, but in many 

 .sections pink augite also is common. 



The chief feldspar of the groundmass is sanidine. 



Cossyrite is absent. 



The typical rock from Signal Hill shows in comparison with the above 

 an abundance of resorbed amphibole and of coarse feldspars, amongst 

 which oligoclase and anorthoclase are prominent. A little serpentinized 

 olivine also is present. 



No chemical analysis of this rock was made. 



D. Basalts. 



Of three main basaltic areas to be described, the most important is 

 the old neck of Mount Holmes. There a good example of columnar joint- 

 ing is shown. The disposition of the columns is irregular, but indicates 

 that the vent from which the basalt flowed was of the nature of a fissure. 

 This Mount Holmes basalt has apparently burst its way through the 

 Logan's Point phonolite outcropping on Mount Zion. 



A basalt covers a considerable area near the Junction School, and is 

 the same as that described from there by C. A. Cotton.* Mineralogicallv 

 it agrees with the Mount Holmes basalt, but. as one would expect, textural 

 differences are marked. It is very probable that this lava flowed from 

 the former vent of Mount Holmes. 



A basaltic-scoria bed of an amygdaloidal nature is found on a branch 

 road leading north-west from the junction School, and apparently is part 

 or the surrounding Junction basalt. 



On a knoll close to the west side of the Main North Road, about half 

 a mile north-west of its junction with the Port Chalmers Road, is a diffe- 

 rent type of basaltic rock. It is similar in general characters to basanites 

 found to the north-east of the district. Professor Park, in his paper on 

 the geology of North Head. YVaikouaiti.f mentions Mount Cargill as the 



* " Geology of Signal Hill," Trans. N.Z. Inst., vol. 4J, 1909, p. 121. 

 t Trans. N.Z. Inst., vol. 36, J 90.3, pp. 423, 424. 



