' BY H. C. RICHARDS. ' 173 



olivine which are usually rounded, but occasionally sub- 

 idiomorphic. These range from about 4 mm. down to very 

 small granules, although occasionally individual plieno- 

 crysts may be nearly half an inch in length. The ground- 

 mass is pilotaxitic and is composed of augite, plagioclas3, 

 olivine, magnetite and ilmenite. 



Plagioclase is much less abundant, and olivine much 

 more abundant in this rock than in any other rock 

 examined. The plagioclase is difficult of determination, 

 but is possibly an acid andesine ; it occurs as lath-shaped 

 crystals averaging 0-4 mm. in length. The specific gravity 

 is 2-88. Name: Olivine-hasalt. 



The olivine-basalt at Picnic Point, Toowoomba, is very 

 similar to this rock. 



Specimen 218. — Locality : Summit of Hip Roof Range, 

 Tsouth of Laidley. This rock in many respects is similar to 

 the Toowoomba basalt, and has an abundance of olivine, 

 both as phenocrysts and granules in the groundmass. It is 

 coarser-grained than the Toowoomba rock, and the olivine 

 is seen decomposing into iddingsite. 



Titaniferous augite is also fairly abundant through 

 the groundmass. This rock differs considerably from one 

 at a lower level in the same locality at Deep Creek. The 

 Deep Creek rock is mineralogically the same, but has well- 

 .developed ophitic structure. Name : Olivine-hasalt. 



Specimen 11. — Locality: Point Danger. This occurs 

 as the top tiow% which is about 60 feet thick. It has been 

 extensively quarried and used for forming the river walls 

 along the Tweed River. 



In the hand-specimen, large phenocrysts of a dark 

 coloured plagioclase showing both earlsbad and lamellar 

 twinning are abundant. 



Microscopically the rock is seen to have a holocrys- 

 talline groundmass of plagioclase, augite and magnetite 

 in which are set the phenocrysts of acid labradorite, 

 Ab^gAn^g. {See Plate XIV., fig. 1.) The phenocrysts 

 range up to 10 mm. in length and have somewhat broken 

 edges. In some of them, regularly arrayed inclusions of 



