448 PETROLOGICAL XOTES ON N. S. "WALES ROCKS, 



pyroxene is a normal grey augite, forming small prisms more or 

 less altered to chlorite and epidote, magnetite and ilmenite in 

 small grains and plates. Xenocrysts are present, that seem to have 

 been derived from a dolerite : they may occur aggregated or singly, 

 and are rather corroded. The basic plagioclase is being replaced 

 by irregular patches of albite; the pyroxene is a grey augite, with a 

 basal striation and varying optic axial angle, sometimes large, but 

 in two instances almost 0°. This indicates that it is an enstatite- 

 augite. Large plates of ilmenite occur also. The vesicles of the 

 rock are filled with quartz and chlorite. 



The majority of the dykes on the north face of the mountain are 

 traehyandesites, related to the last rock. They are fine-grained 

 green rocks, showing phenocrysts of plagioclase and orthoclase in 

 a matrix of laths of the same minerals. The coloured constituents 

 have been largely decomposed, but are sometimes seen to be minute 

 prisms of grey augite, with rarely large chlorite-pseudomorphs 

 after the same mineral: the extinction-angle of these pyroxenes is 

 quite high ; alkaline pyroxenes or amphiboles are not recognisable. 

 ^Magnetite occurs in large and in very minute grains. Considerable 

 variation is seen in the grainsize of the ground-mass, and in the 

 proportion between the potash and lime-soda felspar, the decrease 

 in the former indicating a passage towards the andesites. 



A quite different type of dyke occurs in a road-cutting near the 

 foot of the mountain, adjacent to one of the more basic green 

 dykes. It consists entirely of colourless minerals, being a very 

 fine-grained mixture of andesine, quartz and a minor amount of 

 orthoclase, with a few small phenocrysts of quartz and andesine. 

 The rock is much obscured by sericite. 



One cannot be certain yet of the affinities of these rocks. From 

 a maeroscopical examination of the writer's collection. Dr. Jensen* 

 considered that they might be riebeckite-traehytes, and he himself 

 found dykes of alkaline trachyte in the neighbourhood of Murwil- 

 lumbah; the microscopical study, however, has not confirmed the 

 presence of riebeckite. 



* Report Aust. Assoc. Advt. of Science, 1911, p. 193. 

 [Printed oflF, 16th September, 1914.] 



