318 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



basalts, olivine-basalts and porphyritic andesites resting 

 on one another, or at Lamington Plateau, a series of about 

 twenty diiferent flows of basalt. The individual flows vary 

 greatly in thickness, in vesicular nature and in texture ; 

 in addition to this, they do not seem to be very persistent, 

 but rather to overlap one another. The maximum thick- 

 ness of the upper division is approximately 2,000 feet. 



Pyroelastic material does not occur in this upper 

 division throughout the southern plateaux, but on the Main 

 Range one finds it on top of Mount Spicer in the form of 

 agglomerate, and at Toowoomba there is a most extensive 

 development of tuff. Basalt is much more widespread 

 tluui andesite. and the greatest development of the andesitic 

 ty]je occurs in the east and south-eastern portions of the 

 area, while the southern and western portions are almost 

 entirely basaltic and usually rich in olivine; this is parti- 

 cularly the case at Toowoomba. 



Xaturc of Extrusions and Sequence of Flows. 

 All three divisions are widespread, and there is a 

 good deal of variation in the thickness in different locali- 

 ties. The acid and sub-acid members occur in the middle 

 division; the lower and middle divisions are characterised 

 by abundant pyroelastic material ; while the upper division, 

 except in one or two isolated and widely distant localities, 

 has no development of pyroelastic material, but is built 

 up of a large series of flows none of which attain any very 

 great thickness. It is clear that the upper division at least 

 M'as characterised by a tranquil welling out, such as 

 characterises fissure-eruptions, and preceding this period, 

 during which such a tremendous amount of volcanic 

 material was quietly effused, there was one of violent out- 

 bnrst giving the large accumulations of acid volcanic 

 agglomerate and tuff. This period of explosive violence 

 must have been a fairly extensive one or else it was most 

 prolific in the production of material. There is a 

 remarkable similarity throughout the whole of the 

 southern area, where the acid agglomerate is particularly 

 developed, in this pyroelastic material ; and the uniformity 

 of nature and the widespread distribution incline one to 

 the view that there must have been a large number of 



