418 



PROP. P. MARSHALL ON THE 



[Aug. 1906, 



in hornblende, of the St. Leonard's type, with two basalts above it. 

 Finally the crest is crowned by another trachytoid phonolite, less 

 alkaline than the earlier one. 



Pig. 2. — Section at North Otago Head. 



N.W, 



[Scales, vertical and horizontal : 4 inches =1 mile.] 



1= Trachyte. 



2= Trachytoid phonolite (Logan's-Point 



type). 

 3= Trachytoid phonolite (St. Leonard's 

 type). 



4= Trachytoid phonolite (andesitic 



type). 

 5=Basanite. 

 6 = Basalt (twenty lava-flows). 



No analyses have yet been made of this series of rocks, but 

 sections fail to show any resemblance whatever between the 

 different rocks or their component minerals ; except that the basalt 

 immediately above the lower trachytoid phonolite contains segirine- 

 augite, and is certainly more alkaline than the higher basalts. 

 A basanite with large nepheline-crystals is exposed at the northern 

 end of the cliff, and appears to extend throughout it as a lava 

 among the basalts, but it has not yet been definitely traced in that 

 precipitous portion where the succession is clear. 



The general section (fig. 3, p. 420) across the whole district, from 

 west to east, is less instructive. The coarse trachydolerite that forms 

 the southern portion of Mount Flagstaff: is believed to be intrusive, 

 and to represent the neck of a volcano, from which much trachy- 

 doleritic lava was erupted. 



On the east side of Mount Flagstaff a succession of lavas of very 

 various character is found, and an attempt has been made to repre- 

 sent their probable arrangement. The trachydolerite which occurs 

 over some portion of the western side of the Leith is supposed to 

 have flowed from Mount Cargill into a valley that was already eroded. 

 The Pine-Hill mass flowed from Mount Cargill apparently over a 

 great thickness of trachytoid phonolite, of which all the central and 

 southern part of Signal Hill is composed. The capping of basalt on 

 the top of Signal Hill flowed from a vent farther north. Trachyte 

 is found near the shore of Otago Harbour, and forms the main mass 

 of that portion of the peninsula which lies opposite to it ; but the 

 occurrence of basalt on this shore-line indicates that it lies beneath 

 the mass of trachyte. A dolerite occurs at the top of Harbour 

 Cone, and seems to have been intruded through the trachyte at 

 the eastern base of the Cone, where the foyaite occurs. Mount 

 Charles is almost entirely composed of the type of dolerite named 

 after it. 



