172 VOLCANIC ROCKS OF SOUTH-EASTERX QUEENSLAND 



Plate XIV.. fig. 5.) Iron-ore is abimdant and occurs 

 both as allotriomorphic granules and as minute oetahedra. 

 Olivine occurs as phenocrysts and is largely altered to 

 serpentine. These phenocrysts range in size up to 1 mm. 

 in length. This rock is somewhat peculiar chemically, and 

 although its felspar is indeterminable, it is probably 

 oligoclase. 



Another micro-section of the same flow shows some- 

 what more crystalline characters, and the microlites can 

 be determined as oligoclase. Dr. Woolnough*- described 

 this rock and determined it as a porphyritie olivine-basalt. 

 The author has seen sections of the same material that he 

 dealt with, and while agreeing with his description, would 

 prefer to call this rock oligoclase-basalt. The rock has a 

 few phenocrj'sts of acid labradorite up to 2 mm. long, and 

 olivine largely altered to serpentine set in a pilotaxitie 

 groundmass of oligoclase. granules of augite, granules of 

 iron-ore and small patches of olivine and serpentine- 

 Small needles of apatite are abundant. 



This rock is rather different from any of the others 

 in the area, and it has certain similarities to the mugearites 

 described by Harker.*^ In its mode of weathering, fissile 

 character and specific gravity of 2-74. it is similar. Chemi- 

 cally it is characterised by low magnesia and low lime, 

 by high alkalies, both soda and potash ; also it has the 

 characteristic high value for phosphoric pentoxide. 



This rock may be regarded as much allied to the 

 mugearites described by Harker and probably it is best 

 described as oligoclase-basalt. 



Specimen 139. — Municipal Quarries, Toowoomba. 

 This rock shows magnificent columnar structure, and it is 

 very fine-grained, breaking with a conchoidal fracture. 

 Phenocrysts of olivine are frequently seen in the hand- 

 specimen, and in some cases "pockets" are found, several 

 inches in diameter. These are accumulated masses of 

 small olivines which are eagerly sought for gem purposes. 

 ^Microscopically the rock shows abundant phenocrysts of 



*== Proe. Eoy. Soc, N.S.W., xlv., p. 158. 

 " Skye Memoir. 



