178 Transactions. 



Source and Sequence of the Mount Cargill Rocks. 



In his paper on the " Geology of Signal Hill,"* Cotton deduces that the 

 Logan's Point phonolite is earlier than the Signal Hill type. His statement 

 is based on evidence brought forward by Professor Marshall showing the 

 relative sequence of the two rocks at the North Head. It is probable that 

 the Mount Cargill and Signal Hill occurrences of the two phonolites are 

 portions of the same flows, and, if this is the case, field evidence at Mount 

 Cargill makes it certain that the flow of the later of the two trachy- 

 toid phonolites must have been south-west, down a steep slope of the 

 earlier. 



Basaltic rocks in the North-east Valley Stream, near its headwaters, 

 may represent basic outpourings intermediate between the two phonolites 

 — a supposition in accordance with the sequence noted by Professor 

 Marshall at the North Head.f 



The trachydolerite seems to overlie a surface of Signal' Hill phonolite 

 that slopes gently south-west. This, together with the fact that the Mount 

 Holmes basalt is the ' north-east boundary of the trachydolerite. strongly 

 supports the contention that the flow of this latter rock was in a south-west 

 direction from Mount Cargill. 



A series of rough joints, very steeply inclined, and running approxi- 

 mately east and west along the strike of the prominent rock ridge of the 

 Main Peak and of Butter's Peaks, together with petrological evidence show- 

 ing differences in the rates of cooling, tends to indicate that the eruption 

 of trachydolerite was from a fissure occupying the site of the present ridge. 



The North-east Valley seems to have begun its existence after the extru- 

 sion of the trachydolerite and before the ejection of the Mount Holmes 

 basalt, for basalt remnants are found on both Mount Cargill and Signal 

 Hill slopes, and a simple explanation of this is that the basalt from Mount 

 Holmes flowed down the already formed valley. 



The origin of the basanites is uncertain ; possibly many of them are 

 more of the nature of intrusions than flows, but it is probable that they are 

 in some way connected with the basanitic outpourings that were frequent 

 in the district north-east of the Mount Cargill area. 



The nephelinitoid phonolite of Butter's Peaks may be a dyke. The 

 other types outcropping near it are simply modifications of the main trachy- 

 dolerite flow. Al! probably originate from the one magma. 



Quantative Classification. 



The quantative classification of this series of Mount Cargill rocks has 

 been worked out by the method of Cross, Iddings, Pirsson, and Washing- 

 ton.! The following is the result : — 



1. General lava trachydolerite — 

 Class II. Dosalane. 

 Order 6. Norgare. 

 Rang 3. Salemase. 

 Subrang 4 Salemose. 



* Trans. N.Z. last., vol. 41, 1909, p. 113. 



+ "U.-ology of Dunedin," Quart. Journ. Geol. .Soc, vol. 62, 1900, p. 418. 



% " Quantative Classification of Igneous Rooks." 



