22 ox HAUYXE-TRACHYTE AND ALLIED ROCKS. 



]Maci'Oscoi)ically it is a very light-coloured stone^ 

 weathering easily, and resembling a coarse trachyte rather 

 than a syenite. It may l)e compared with those elaeolitic 

 syenites which have a tendency to trachytic structure. 



Under tlie microscope this pseudo-trachytic appearance 

 shows itself by the larger felspars (sometimes idio- 

 mor})hic) l)eing cemented or surrounded l)y a holocrys- 

 talline groundmass of smaller hypidiomorphic and allo- 

 triomorphic felspars. There is, however, great variation in 

 the size of the latter. There is a remarkable absence of 

 coloured constituents. A flake or two of biotite is the 

 only ferro-magnesian mineral which we can detect... 

 Sphene, zircon, and apatite are accessories. 



Orthoclase felspar preponderates. It is fresh and often, 

 zonal. Oligoclase is freely intergrown with it. Some of 

 it is streaky, like the orthoclase of the Norwegian elaeolite 

 syenites, and has an undulose extinction. There is 

 much residual felspar (albite) in its clearness resembling 

 quartz. 



In the rock are certain irregular and imperfect forms of 

 elaeolite, a few basal sections being isotropic. This deter- 

 mination was confirmed by digesting the rock in HCl and 

 obtaining a fair quantity of gelatinous silica. 



There is not sufficient of the felspathoid to constitute 

 the rock a true elaeolite syenite, besides which the 

 pyroxene and hornblende so abundant in elaeolite syenites 

 are here conspicuously absent. It is rather one of 

 those alkali sj'enites which occasionally carry subordinate 

 elaeolite. 



Trachyte. 



We have examined three varieties of trachyte rock from 

 Little Oyster Cove. They all appear to belong to the 

 igneous complex, which embraces both the Port Cygnet 

 and Oyster Cove Districts. They carry identical minerals, 

 viz : — sanidine and oligoclase, cataphoritic hornblendes, 

 green augite, sphene, zircon, and apatite ; and, from gar- 

 netiferous gold-bearing sand found in the neighbourhood, 

 we know that melanite-garnet is also an ingregient. From 

 the few specimens which we have seen, it is likely 

 that^ as in the Port Cygnet series, these also are rich in 

 varieties. 



On the whole they exhibit a tendency to vary in the 

 direction of the andesites, the dominant porphyritic felspar 

 being plagioclastic, and an increase of iron ore in the 

 groundmass showing itself. The small felspars of the 

 groundmass are often minutely granular or allotriomorjihic, 

 but where prismatic they show straight extinction, which 



