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



(2) Constituents (in order of decreasing abundance) — essential 

 (a) plagioclase, (6) orthoclase; notable (c) green amphibole, 

 (d) pyroxene; minor (e) magnetite, (J") quartz, (g) biotite, and 

 (h) some sphene, apatite and zircon in very minute amount. 



(3) Remarks.— The plagioclase is essentially albite and an 

 oligoclase-andesine showing Carlsbad, albite, and occasionally 

 also pericline twinning. Frequently crystals are zoned and the 

 peripheral layers consist of albite and orthoclase. The orthoclase 

 is decomposing, giving kaolin. It occurs both as phenoci'ysts 

 which are highly corroded and contain inclusions of hornblende, 

 apatite, &c, and in the base forming a micrographic intergrowth 

 with quartz. Both felspars are of two generations. The horn- 

 blende is olive-coloured and pleochroic from brownish to bluish- 

 green. It occurs in idiomorphic, often highly corroded fragments. 

 Double refraction about 022. Pleochroism same as in Sp.No.8y. 



A light greenish, very faintly pleochroic diopside is present in 

 the same proportions as hornblende. An equal amount of 

 enstatite is also present. The biotite occurs in corroded crystals 

 with ragged ends. The magnetite is mainly primary, idiomorphic 

 and titaniferous, but a little is secondary and in dust}' grains. 

 The zircon occurs in minute acicular rods, while apatite also 

 occurs in needle-shaped crystals. 



(4) The. order of consolidation is normal A eutectic mixture 

 of quartz and orthoclase was the last to consolidate. 



(5) The microscopic structure and mineral composition of this 

 rock place it amongst the monzonites or monzonitic quartz- 

 diorites. Its granophyric base, however, shows that it is of a 

 hypabyssal nature. The chemical analysis ( Anal.vi. p. 168) reveals 

 that much of the felspar is albite. The alkalies are high. The 

 greenish diopside may be soda-bearing and allied to acmite. The 

 norm agrees fairty well with the mode. In handspecimen 

 the rock is like a trachyte, and the chemico-mineralogical examina- 

 tion places it on the border-line between the alkaline trachytic 

 rocks of the district and the alkali-calcic andoses, like the Noosa 

 and Pt.Arkwright rocks. The magmatic name is " adamellose." 

 Perhaps Monzonitic Granophyre is the term which best expresses 

 its constitution. 



