158 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



Chemical Composition. 



The Glass House Mountains dacite is seen to be a 

 normal dacite, and certainly sub-alkaline in nature. This 

 is interesting, as it is found in the closest association with 

 the very alkaline rocks such as the comendite of Mount 

 Conowrin. The partial analysis of the Mount Alford rock 

 shows that in a general way it is similar chemically to the 

 Glass House Mountains dacite. 



(vi.) Andesites. 



These have been recognised in several parts of the 

 area, but in no place is there any very extensive develop- 

 ment. It is a somewhat difficult matter to distinguish 

 between the andesites and the andesitic basalts, as there 

 are all gradations from andesites to olivine-basalts. The 

 presence or otherwise of olivine, the relative amounts of 

 augite and plagioclase, and the nature and habit of these 

 have been taken as the main considerations in determining 

 whether a flow is an andesite or not. Andesites were 

 poured out during the first period of aetivit.y, but appa- 

 rently towards the end of it. They are much more abundant 

 in the last period of eruption, however, and in some places 

 seem to have followed upon the trachytes; this is particu- 

 larly the case in the most typical ones, as at the Main 

 Range. On the other hand, andesite occurs at Mount 

 Meerschaum in the upper division, forming one of the 

 most recent flows and resting on considerably more basic 

 flows. 



The Esk andesites really stand apart from the others 

 as they are definitely of a porphyritic type and seem to 

 be more basic representatives of the material which pro- 

 duced the rhj'olites and trachytes occurring there ; all of 

 the other andesites, on the other hand, seem to be rather 

 the acid representatives of the material which resulted in 

 the extensive basaltic flows in their respective areas. The 

 ^sk andesites differ somewhat in their mode of occurrence 

 also, for they occur as large intrusions through the 

 ? Walloon coal-measures ; the other occurrences are either 

 as definite lava floM^s or sills. Apart from the Esk district, 

 one finds andesites at the Main Range in the neighbourhood 



