116 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



the volcanic activity extended over a lengthy period, and 

 also that there has been considerable time since the last 

 volcanic outburst. 



The extrusions appear to have been poured out under 

 sub-aerial conditions, for nowhere in the area is there any 

 evidence of marine deposits, but on the other hand, one 

 has the occurrence between the flows, of soils in situ, of river 

 gravels, of deposits of diatomaceous earth, and of the 

 lacustrine Oxley beds which directly underlie the basalts 

 at Cooper's Plains, and the trachytic rocks at Redbank 

 Plains. The volcanic rocks show great variation in types, 

 and they range from rhyolites to basalts. Rhyolites, 

 trachytes, andesites and basalts are all abundantly repre- 

 sented though rhyolites and basalts seem to have the 

 greatest development. Pitchstones and obsidians are 

 associated with the rhyolites. In addition to the normal 

 lavas, there are tremendous accumulations of pyroclastic 

 material. 



In considering the alkaline or sub-alkaline nature of 

 the rocks it is found that there is a definite series having 

 alkaline characteristics, but the great majority are defi- 

 nitely sub-alkaline. The terms alkaline and sub-alkaline are 

 used all through as the equivalents of the terms "atlantic" 

 and "pacific" as used by Harker. In addition to these 

 two series, a third series which has characteristics inter- 

 mediate between those of the alkaline and sub-alkaline 

 series is developed to a minor extent. 



Deposits of fragmental material are common in many 

 parts of the area, but the greatest development of these is 

 in association with the acid members. In the Lamington 

 Plateau area in particular, basaltic agglomerate is associated 

 with the lower basic rocks, while on the Main Range at 

 Spicer's Peak, and in particular at Toowoomba, there are 

 fragmental deposits associated with the basic rocks. At 

 the latter place the development of basic tuff exceeds that 

 of any other part of the area. While fragmental rocks 

 among the lower and upper basaltic series are localised, 

 there is a widespread distribution of acid and sub-acid 

 fragmental material, especially of the former, over an 

 area of 300 square miles right in the south. 



