120 VOLCANIC ROCKS OP SOUTH-EASTERN QUEENSLAND 



area, but in a general way the order can be established. 

 Volcanic activity seems to have been ushered in with the 

 extrusion from fissures of a normal basalt; this was suc- 

 ceeded by olivine-basalt. The latter portion of the period 

 was characterised by explosive action in certain isolated 

 localities, and may have been due to the choking of the 

 fissures. This ended the first period of activity. After 

 some time the intensely explosive action marking the 

 second period developed and large deposits of rhyolitic 

 agglomerate containing fragments of rhyolite and pitch- 

 stone were hurled out, to be succeeded in certain centres by 

 lava flows of an acid nature, these being succeeded by the 

 volcanic agglomerate again. The ashy material in some few 

 localities was deposited in water and tuff resulted, but the 

 occurrence of bedded ashy material is very limited. While 

 the acid material Avas being extruded in the southern 

 portion of the area, sub-acid ashes and lavas were being 

 extruded in the south-west and generally along a north- 

 east and south-west line to JMount Flinders. Dr. Jensen,^^ 

 in dealing with the Mount Flinders and Fassifern areas, 

 pointed out this probability. Most of the material during 

 this period was extruded through vents of the central type, 

 though the trachytes along the Main Range were probably 

 the result of fissure eruptions. 



Succeeding this 'period of activity, which was charac- 

 terised by such prolonged and intense explosive action, 

 there was the third period during w^hich a vast accumula- 

 tion of basic and sub-basic rocks was poured out. Generally 

 speaking, the operations of this period seem to have been 

 of a tranquil nature, and the material to have been ex- 

 truded through a large number of fissures. Olivine-rich 

 basalts are the predominant lavas of this period, but there 

 is in the south-eastern portions of the main area, and also 

 in the south-eastern coastal area, a development of ande- 

 sitic rocks. These seem to have been extruded at an early 

 time in this third period, but in almost all cases basalts 

 or olivine basalts were first poured out. The final extru- 

 sions were almost all of a rather basic nature, and olivine 

 basalt is characteristic of the uppermost flows. The 



"Proc. Linn. Soc, N.S.W., xxxiv. (]909), p. 72. 



