174 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



a brown colour are seen. The plagioclases in the ground- 

 mass are lath-shaped, average 04 mm. in length, and have 

 an extinction angle which indicates acid andesine. 



The augite occurs in ophitic patches about 1 mm. in 

 diameter. The iron-ore occurs in granules and in rods^ 

 the latter averaging -4 mm. in length. Olivine is not 

 present. 



Underlying this rock there is another flow, which rests 

 on Paleozoic rocks. Between the two flows there are 

 lenticular patches of common black opal ** which represents- 

 altered diatomaceous earth. Also, in the lower part of the 

 flow, casts of trees several feet in length and up to a foot in 

 diameter are seen; these are always horizontal. The lower 

 flow is somewhat similar in the hand-specimen, but it has 

 fewer phenocrysts of felspar and microscopically it is very 

 different, as it is hyalopilitic and much finer-grained. This 

 latter basalt is in many respects similar to the basalt at 

 Fingal Point, which is about a mile away and at the same 

 level. 



Specimen 4. — Locality: Burleigh Heads. This forms 

 a capping on the Palaeozoic schists, and supports a sub- 

 tropical vegetation. It is typical of the basalt rocks in this 

 locality and is characterised by magnificent columnar 

 structure. The columns are about 4 feet in diameter, and 

 in many cases are 40 feet long. 



In the hand-specimen occasional phenocrysts, about 

 half an inch long, of very clear plagioelase are seen. Under 

 the microscope the rock is seen to be hyalopilitic with a 

 fluxional arrangement of the felspars. 



The plagioelase laths are about -05 mm. in length, and 

 are of two varieties, acid labradorite which is most abun- 

 dant, and acid andesine which occurs less frequently and 

 in somewhat square prisms. 



Augite occurs in sub-ophitie rounded patches but 

 occasionally in long patches 1 mm. in length. Iron-ore is 

 abundant, and occurs mainly as irregular granules. An 

 analysis of this rock is given. Specific gravity 2.87. 

 Name : Andesitic Basalt. 



^'B. W. SkeatB, Proc. Eoy. Soc, Q'land, xxvi. (1914). 



