I (/neons Rocks. 267 



by a prismatic- variety. Latlis of clear, zoned, acid plagioclase, 

 apparently all)ite. are very al)undant in the altered rock . 



The granodiorites, from the other localities mentioned, are very 

 eiinilar to these rocks described above, and they do not call for 

 further attention. 



B. — Dacites. 

 Tlie majoi- portion of the area desci'ibed in this paper is covered 

 by a series of fairly normal dacites. They show considerable 

 variation in colour and granularity, although their mineral com- 

 position is usually very uniform. Nevertheless, in the dacites from 

 the Blacks' Spur, quartz phenocrysts are abundant and garnet is 

 often present, and hypersthene is apparently wanting, Avhilst in the 

 dacites from the summit of Mount Juliet and from the ranges at the 

 head of the Acheron river, hypersthene is relatively abundant and 

 macro.scopic quartz phenocrysts are absent. Flow structure is rare 

 in hand specimens of the dacites, althougli not infrequently visible 

 in thin sections of the Blacks' Spur dacite. Vesicles are usually 

 absent, and mineralisers were apparently rare, as these rocks are 

 relatively little altered. These facts, considered in conjunction 

 with the great thickness of these lavas, probably point to a high 

 degree of viscosity for the extruded magma. Numerous xenoliths of 

 fine-grained andesites resembling liornfels, and white porcelainous 

 rocks occur in the cTacites from the Blacks' Spur. Lenticular 

 patches of more coarsely crystallised dacite, often rich in garnets, 

 are also common in the finer grained rock from this locality. 

 Jointing is Avell developed, both in the dacites and in the rhyolites. 

 Sections along Blacks' Spur road in dacite ,and at Archer's 

 Look-out in rhyolite, illustrate this jointing very Avell. The aver- 

 age specific gravity of the dacites is 2.71. A chemical analysis 

 by Mr. F. Stone of a specimen of dacite from the Blacks' Spur 

 gave the following result : — 



SiOs 65.80 



AI265 16.87 



FeoOs 3.97 



FeO 1.08 



MgO 1.76 



TaO 3.16 



KoO 3.46 



Na^O 2.54 



I-I.6 1.05 



MnO tr. 



Total 99^68 



