166 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



or microscopically to a sufificient extent to enable one to 

 do so. Also macroseopical investigations fail to distinguish 

 between the andesites and basalts, except possibly by 

 specific gravity determinations. The basalts range from 

 2-74 to 2-92 in specific gravity, while the andesites range 

 from 2-57 to 2-79 ; so that in many cases it would be possible 

 to jvidge whether the rock was an andesite or a basalt. 



It has been, however, found impossible to distinguish 

 them in the field, and the whole series of andesites and 

 basalts have been coloured the same on the sketch-map ; 

 also in many cases it is found that successive flows pass 

 from one to the other. 



At Spicer's Peak there occurs an oligoclase-basalt 

 which is very different from any other known basaltic rock 

 in the area. It is similar chemically, mineralogically and 

 in its appearance and weathering, to the mugearite described 

 by Harker^^ from Druim na Criche in Sl<ye. 



While one finds in certain areas great developments 

 of olivine-basalts and basalts, there is also an extensive 

 development of andesitic-basalts. The andesitic-basalts 

 have the texture of andesites with a certain percentage 

 of glassy base while the olivine-basalts are holocrystalline. 



These rocks are distributed over the whole of the 

 area, and they are found at all levels from 4,100 feet above 

 sea-level down to sea-level. 



Many of the lavas are amygdaloidal, and in the case 

 of several flows on the Main Range in particular, they have 

 abundant zeolites filling the cavities. The zeolite chaba- 

 zite is the most abundant ; several other minerals occur in 

 the cavities, but are of minor importance when compared 

 with chabazite. The rock containing these cavities in 

 most cases appears perfectly fresh, and these minerals have 

 apparently been formed during the cooling period of the 

 rock, and they do not represent ordinary secondary 

 minerals. Analcite also has been noticed. 



Generally speaking, the flows are not porphyritic, 

 although very porphyritic flows do occasionally occur, and 



" Op. cit., pp. 264, 265, 



