BY H. C. RICHARDS. 165 



Specimen 125. — Uppermost flow, Mount Meerschaum. 

 This is a very fine-grained dark-grey rock which is almost 

 free from phenoerysts. Microscopically it shows a typical 

 hyalopilitic groundmass with very occasional corroded 

 phenoerysts of plagioclase containing glass inclusions. The 

 groundmass is extremely fine-grained, and contains a 

 rather larger amount of small granules of the iron ores 

 than usual. 



Chemical Composition. 



In the case of the three normal andesites there is a 

 lack of alumina, but a corresponding richness in the iron- 

 oxides, particularly ferrous oxide. The total of alumina 

 and iron-oxides gives a normal value. The only other 

 respect in which they are at all abnormal is in the slightly 

 higher value for potash in the Tamborine Plateau and 

 Observation Hill rocks than is usual for the andesites ; the 

 Springbrook rock seems to be more normal in that respect. 



In the augite-andesite from Ottaba the alumina value 

 is normal, but this rock is characterised by a relatively 

 high soda value. 



(vn.) Basalts. 



These are the most abundant rocks and they form the 

 bulk of the material extruded in both the first and last 

 periods of eruption. 



The accumulated thickness of the flows of basalt in 

 some cases reaches 1,500 feet and it is made up of a number 

 of small flows one on top of the other. The flows vary a 

 great deal in thickness, and 120 feet is the maximum thick- 

 ness noted for any one flow, but 20 feet or 30 feet is a very 

 common thickness. 



The bulk of the material is believed to have been 

 poured out as surface flows, not intruded as sills; close 

 observations were made in the field, and only sills of a 

 very minor nature were encountered. Microscopic evidence 

 does not help one in this direction, as there is great variation 

 in the texture, and ophitic structure is developed to a 

 large extent in undoubted lava flows. It is impossible 

 to distinguish between the basalts of the lower and upper 

 divisions, as they do not difl'er chemicall3^ macroscopieally 



