116 Transactions. 



The trachytoid phonolite of the Signal Hill type does not occur over so- 

 great an area, nor, so far as is known, to so great a depth in the area 

 described, but a similar rock has a wide occurrence to the north, as 

 described by Dr. Marshall, and occurs also as a bedded flo^y at North 

 Head. It may, then, be supposed to originate at some point north of 

 Signal Hill. 



The acid basalt underlying the phonolites does not appear to have any 

 close allies in the neighbourhood, although I have compared it with a large 

 number of sections kindly lent for the purpose by Dr. Marshall. It cannot, 

 therefore, be traced to its origin. 



The main basalt-flow closely resembles a basalt occurring as a neck on 

 Mount Cargill. This may perhaps be its source. If so, the ancient valley 

 intervening must have been filled to a great depth. The occurrence of the 

 basalt at various levels on the inclined surface of the phonolites on Signal 

 Hill is more easily explained on the assumption that basalt overflow occurred 

 there, and the lava flowed down the slopes. 



It is difficult to draw conclusions as to the nature of the magmas from 

 which the rocks were derived, but they are best explained by supposing 

 the rocks to be derived from tw^o magmas, a basaltic and a phonolitic. The 

 basalts became progressively more basic as vulca'nism died out. The 

 interval between the first basaltic flow and the next must, however, as 

 before stated, have been great. 



The phonolitic magma which supplied the lava of the Logan's Point 

 type and the very much later nephelinitoid intrusion were similar in com- 

 position, and probably identical. The Signal Hill phonolite is chrono- 

 logically intermediate between these two, but not intermediate in composi- 

 tion. It is much poorer in alkalies and richer in iron, and appears to be 

 related to the trachydolerite of Mount Cargill.* It may have originated 

 from a mixture of the basaltic and phonolitic magmas. 



No explanation based on the theory of magmatic differentiation i& 

 applicable to the alternation of types, but the basalts, taken separately, 

 may be conceived to be derived from a differentiating magma in its final 

 stages. 



Petrography. 



Trachytoid Phonolite (Logan s Point Type). 



This rock is mentioned under the above name by Dr. Marshall. | At 

 the typical locality (Logan's Point Quarry) the rock is distinctly trachytoid, 

 and near the summit of Signal Hill even more so ; but in the south-east 

 peak feldspar is almost absent, and the rock should be classed as nephe- 

 linitoid. 



Macroscopic Characters. — In hand-specimens the rock is of a greenish- 

 grey colour, flecked with green streaks. It cleaves readily into flakes. 



Microscopic Characters. — The structure is uniformly fine-grained in 

 typical specimens, and holocrystalline. Flow structure is common wherever 

 there is a considerable development of feldspar. In the typical rock from 

 Logan's Point there are no phenocrysts. The chief constituent is ortho- 

 clase or sanidine, but in some places feldspar becomes scarce. It occurs 

 as bundles of fine needle-shaped crystals, some of which are of considerable 



* Marshall, " Geology of Dunedin," Quart. Journ. Gcol. See. Ixii, 1906. 

 t Marshall, " Geology of Dunedin." Quart. Journ. Geol. Soc, Ixii, 1906. 



