168 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



Olivine is very abundant in many of the rocks and occurs 

 as rounded granules ; this mineral occurs in two generations 

 quite frequently. Augite has not been found in two 

 generations in the basalts, nor has it been found in them 

 showing twinning. Olivine may occur sparingly in the 

 andesitic basalts, but augite is extremely abundant. 



Both magnetite and ilmenite occur in great abundance, 

 as small granules or crystals in the groundmass, and 

 frequently as inclusions in all the other minerals. In some 

 of the finer-grained basalts there is a great abundance of 

 small granules of either ilmenite or magnetite, but it is 

 not easy to determine which of them. The Mount Lindsay 

 basalt is particularly rich in iron ore, and in all probability 

 it is mostly ilmenite as it contains 12-15 per cent. FeO, 

 1-78 per cent. FcoO,, and 3-08 per cent. TiOg. The iron ore 

 occurs frequently as inclusions in olivine, augite and plagio- 

 clase, and while many of the crystals are undoubtedly 

 magnetite from their shape, it is usual to find them quite 

 allotriomorphic, and frequently in long thin rods sometimes 

 •75 mm. long and cut through by felspar laths. Apatite is 

 not often seen although it is undoubtedly present in all the 

 rocks to a small extent. The order of crystallisation is 

 always the same : iron-ore. olivine if present, felspar, augite. 



A very large number of micro-sections have been 

 examined, and descriptions of a few of the more typical 

 ones are given below. 



Specimen 86.^ — Lamington Plateau. This is a compact 

 porphyritic basalt in which phenocrysts of plagioclase and 

 olivine may be seen. Microscopically it is perpatic with 

 phenocrysts of acid labradorite showing carlsbad and broad 

 lamellar twinning. 



These phenocrysts are idiomorphic, do not appear to 

 have been corroded at all, and range up to 2 mm. in length. 

 Olivine phenocrysts which are roughly idiomorphic in 

 outline and up to 1 mm. in length are abundant. They are 

 altering into the red lamellar mineral iddingsite, and in 

 all cases have a thin red band around them where the 

 alteration has gone on. (See Plate XIV., fig. 2.) The 

 groundmass is holocrystalline and shows good fluxion 

 structure ; the augite is distributed abundantly through 



