BY W. H. TWELVETREES AND W. F. PETTERD. 15 



porphyritic pseuclomovphs of liebenerite (?) aggregates 

 after haiiyne in rectangular and rounded sections. 



Sharply defined sections of a cubic mineral decomposed 

 to limonite are plentiful. The determination of the 

 ■original mineral is difficult, as we have not much beyond 

 the forms to guide us. 



Dana says* — "Garnets containing ferrous iron often 

 l^ecome rusty and disintegrated through the oxidation of 

 the iron, and sometimes are altered more or less completely 

 to limonite, magnetite, or hematite." In one of our sections 

 we detected a crystal of melanite-garnet undergoing this 

 change, but we have not been able to discover any further 

 instances of partial change. Haiiyne also suffers a some- 

 what similar change, and the choice here appears to be 

 between the two minerals, haiij'ne and garnet, with proba- 

 bilities stronger in favour of the latter. 



The gi'oundmass is rather obscure, but appears to con- 

 sist of prisms of straight extinction felspar. Iron ore in 

 minute grains. 



Hauyne-Trachyte. 



This is another trachyte from the top of the Livingstone 

 Hill. It is a grey rock, with the faint bluish tinge, which 

 in the Port Cygnet trachytes we have found associated 

 with the presence of haiiyne. Groundmass compact lava- 

 like, with numerous porphj^ritic crj'stals of dull white 

 orthoclase ^" to ^" in length. These crystals are tabular in 

 habit. The other visible porphyritic constituent is the 

 limonite to which we have alluded above as being probably 

 pseudomor]3hous after melanite-garnet. This is in hex- 

 agonal and other familiar sections of the isometric system. 



Mineral constituents. 

 Orthoclase : secondary limonite, muscovite, iron oxide. 



Microscop ica I cJi aracte) 's. 

 The large orthoclase crystals are turbid, and enclose 

 -occasional sections of nosean, noAv replaced by micaceous 

 aggregates in confused flakes, polarising in the vivid 

 colours of the second order. These remind one of the 

 secondary muscovite (liebenerite) in liebenerite-porphyry. 

 Mingled with them is a mineral giving soft grey inter- 

 ference tints, and this may be natrolite. The same aggre- 

 gates are frequent throughout, the rock, filling up the 

 interiors of the porphyritic haiiyne (nosean) crystals which 



* System of Mineralogy, 1898, p. UQ. 



