198 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



The lower division has a maximum thickness in one 

 place of 1.500 feet, though 100 feet is the average thickness, 

 and it is made up mainly of basic lavas with occasional 

 Hows of andesitic material. 



The age of the activity which produced all the volcanic 

 material of this area, with the exception of the Brisbane 

 tuff, is believed to be Cainozoic, and that of the lower, 

 middle and upper divisions to be of approximately lower, 

 middle and upper Cainozoic age. 



The evidence of previous observers, notably E. 0. 

 Marks and R. A. Wearne, in favour of an upper Trias- 

 Jura age for portion of the extruded material has been 

 critically examined and rejected in favour of a post-Trias- 

 Jura age for all the material. 



The field occurrence of the volcanic rocks is recorded 

 and they are shown in all cases to rest on top of the 

 uppermost Trias-Jura deposits. 



Extrusions botli from fissures and central vents have 

 taken place, though the greater portion of the basic and 

 sub-basic material has been effused quietly from fissures. 



■ Rhy elites, trachytes, dacites, andesites and basalts 

 have been recognised and petrographic descriptions re- 

 corded. 



The distribution of trachytes as indicated by Dr. 

 Jensen is criticised, also a general sketch-section through 

 the area which he has published. 



The chemical charaeters of the rocks are illustrated 

 by thirty-five complete analyses, twenty-five of which have 

 been recently made in the laboratory of the State Agricul- 

 tural Chemist. 



By means of variation diagrams, the genetic relation- 

 ship existing between all the rocks is shown and three 

 definite series have been recognised. 



The three series are — (a) An alkaline series, the 

 members of which range from acid to sub-acid, and belong 

 to the middle division ; {h) a sub-alkaline series, embracing 



