GEOLOGY OF ERUPTIVE AND ASSOCIATED ROCKS, POKOLBIN. 401 



scattered throughout the base. The larger ones are con- 

 siderably dusted with kaolin. Patches of the rock are 

 stained with iron oxide. Magnetite is the only other 

 mineral visible. 



Trachyte, top of Matthews' Gap Road. 

 Light brown coloured, vesicular rock. The steam holes 

 are flattened and have their longer axes all lying in one 

 direction. Pink crystals of felspar can be seen in hand 

 specimen, making the rock slightly porphyritic. The base 

 is fine-grained, aphanitic. Under the microscope the rock 

 is hemicrystalline, hypocrystalline, slightly porphyritic and 

 with a slightly fluidal fabric. The minerals are : — Ortho- 

 clase, plagioclase, quartz, and magnetite, and a small 

 amount of glass is present. The orthoclase is most abundant 

 and is also considerably decomposed. There is a second 

 generation of felspar in the form of minute microlites in 

 the base. Quartz is present in somewhat rounded grains. 

 Magnetite is not abundant. The base consists mostly of a 

 brownish-coloured glass. 



O. Volcanic, (ii) intermediate. 



Melanocratic trachyte, New Matthews' Gap Road. 



Bluish-black in colour and very solid. Fracture subcon- 

 choidal; fine-grained, aphanitic. Very small felspars can 

 be recognised in hand specimen, and the rock resembles a 

 light-coloured basalt. Under the microscope it is hypo- 

 crystalline; the fabric is trachytic and somewhat fluidal. 

 Both orthoclase and plagioclase are present, the former 

 being the more abundant. It is in rather decomposed 

 tabular crystals. The plagioclase is an acid labradorite. 

 There are two generations of the felspar, the larger ones 

 averaging about '75 by '25 mm., and the smaller ones are 

 only fine needles. Magnetite is abundant in small grains. 

 There is a very dark mineral present in small grains which 



Z-Dec. 6,1911 



