142 VOLCANIC AREA OF EAST MORETON, ETC , DISTRICTS, Q., 



The coarsely porphyritic trachyte from the S.E. side of Mt. 

 Beerburrum is better suited for exact investigation. The 

 felspar phenocrysts have a refractive index of 1-525, and some 

 crystals are distinctly seen under the microscope to consist of a 

 fine intergrowth of orthoclase with albite or microcline. The 

 usual faces are developed, and in general appearance the 

 phenocrysts resemble orthoclase. However, the optical sign is + , 

 and the axial plane is nearly perpendicular to b, c = a\ b = b\ 

 Bx o nearly coincides with normal to b. Between crossed nicols a 

 fine crosshatching is seen in some crystals, due to twinning on 

 the albite and pericline laws. Probably microcline-albite-perthite 

 is the best name for this felspar. The structure of these pheno- 

 crysts serves to cast light on the shadowy extinction observed in 

 some phenocrysts in other trachytes, unaccompanied by multiple 

 twinning. It is possible that some of the sanidine anorthoclase 

 is, in reality, a cryptoperthite which the high power cannot 

 resolve, but the ultramicroscopic twinning results in shadowy 

 extinction. While the commonest twinning observed is Carlsbad, 

 Baveno doublets and quartets are not rare, and occasionally we 

 meet with twinning on the Manebach law associated with 

 Carlsbad twinning. Shadowy extinction is very abundant. 

 Often, as in the Round Mountain trachyte, it is due to strain in 

 consolidation, the laths being frequently bent and broken; but 

 often it may be due to ultramicroscopic twinning. 



As inclusions in the felspar we met with zircon in minute 

 idiomorphic, sometimes geniculate needles, apatite in needles, 

 dusty magnetite (rare), arfvedsonite, riebeckite, segirine. In the 

 Ngun-Ngun, Conowrin and Beerwah trachytes, a yellow mineral, 

 probably Guarinite, occurs very sparsely included both in the 

 hornblende and felspar. A few minute grains of a peach blossom- 

 red colour, perhaps eucolite, have been observed in the Ngun- 

 Ngun felspars. 



The hornblendes of the trachytes vary somewhat; many kinds 

 have been recognised, and they may occur either singly or all 

 together. The species found present are arfvedsonite, riebeckite, 

 cossyrite, barkevicite, and a species intermediate between arfved- 

 .sonite and cossyrite. 



