VOLCANIC ACTIVITY IN SOUTHERN QUEENSLAND. 117 
BrRECCIAS AND TUFFS. 
These vary in size from quite coarse breccias, with 
fragments the size of walnuts dewn to very fine tuffs. Not 
uncommonly one finds interbedded layers of material of 
widely different coarseness. Some of the breccias have 
been much silicified and consist of angular fragments of 
eryptocrystalline rhyolitic material cemented together by 
secondary quartz and chaleedony. The breccias all appear 
to be rhyolitic in character, which is, perhaps what one 
would expect, while the tuffs are rhyolitic, dacitic, and 
andesitie in nature. 
CHEMICAL CHARACTERS. 
The accompanying five analyses and their norms serve 
to indicate the general chemical characters of the volcanic 
rocks. The rhyolite and the basalt are both in a fresh 
condition, but the rhyolite tuff and the two andesites show 
by the carbon dioxide and water contents that they have 
undergone a considerable amount of alteration. 
The lavas are all very much richer in soda than potash, 
and their most marked character is their deficiency in potash 
without any corresponding increase in soda content. For 
example, the world’s average rhyolite has about 34 per cent. 
of soda and 4 per cent. of potash, so in comparison the 
rhyolite in question has a normal soda content but is 
deficient to the extent of 2 per cent. of potash. The world’s 
average basalt has about 3 per cent. soda and half that 
amount of potash, while the basalt here shows considerable 
deficiencies in both, more especially the potash. 
If one compares the lime and magnesia contents it is 
found that they are about normal. 
In comparison with the Cainozoic lavas of Southern 
Queensland, the Permo-Carboniferous rhyolite has higher 
alumina, lower lime, slightly higher soda, and much lower 
potash. The Permo-Carboniferous basalt, in comparison 
with the Cainozoie basalts, has higher alumina, lower total 
iron oxides, higher lime, and lower alkalies. 
A comparison with the analyses of the voleanic rocks 
of the Southern Coalfield of New South Wales is made later 
in the paper, 
