126 VOLGA UC AREA OF EAST MORETON, ETC., DISTRICTS, Q., 



(3) Remarks. — The orthoclase and quartz are intergrown in a 

 graphic manner, and between the larger groups lies a base con- 

 sisting of micropegmatitic and cryptographic intergrowths of the 

 same minerals. This prevailing eutectic structure shows that 

 the magma consolidated slowly under pressure and was in motion 

 right up to the last phase of consolidation. 



(4) The hornblende crystallised first. The quartz and felspar 

 consolidated together. 



(5) Nomenclature. — This rock is a granophyric aplite. It 

 contains 69 - 22 % of silica. It has probably differentiated from 

 the same magma which gave rise to the other Pt. Arkwright 

 rocks and represents the last portions of liquid magma — the 

 mother liquor— which were squeezed into fissures in the already 

 consolidated portions. It is wonderfully like Sp. No. 36, from the 

 Toolburra Range (p. 121). 



Inclusions in typical Point Arkwright Poiyhyrite. — In some 

 portions of the Pt. Arkwright mass there is an extraordinary 

 number of inclusions of a darker colour than the main rock. 

 Some are round, some subangular. The constituents of these 

 are seen under the microscope to be the same as of the main 

 mass, though the relative proportions vary. Usually the 

 inclusions are more fine-grained, and hornblende is a more 

 plentiful constituent but occurs in smaller crystals. Quartz may 

 be present in corroded phenocrysts or wholly absent The 

 inclusions contain usually only one generation of felspar. They 

 appear to represent a portion of the magma which cooled quickly 

 on the outskirts of a magmatic reservoir at a period of rest, prior 

 to the intrusion; and fragments of this consolidated rock were 

 torn off by the magma when activity was renewed, and included 

 in it. The corroded quartz phenocrysts which occur in the 

 inclusions as well as in the main mass are probably allogenic, 

 derived from the sandstones which the igneous rock has intruded. 



A specimen closely allied to the typical Pt. Arkwright rock 

 was obtained at Mr. Story's well, near the base of Coolum Mt. 

 The outcrop is surrounded by trachyte on all sides, and is there- 

 fore either an included mass, or a portion which was covered over 



