186 VOLCANIC ROCKS OP SOUTH-EASTERN QUEENSLAND 



All the analyses available were used with the exception 

 of that of the Glass House ^Mountains dacite and the 

 Cainbable Creek trachj^te. These are both of very minor 

 importance, and would not really aifect the determinations 

 carried out above. 



Magmatic Relationships. 



In order to see whether the volcanic rocks were magma- 

 tically related, the recalculated analyses, in percentages by 

 weight, were plotted with the different oxides against the 

 silica in the usual way in the constitution of variation dia- 

 grams. It is quite obvious that the rocks as a whole do 

 not conform to one particular curve for each oxide, and 

 investigation shows that it is necessary to have at least 

 three curves. The rocks which are definitely alkaline con- 

 form closely to one series of curves having all the charac- 

 teristics of curves for alkaline rocks, while the rocks 

 definitely sub-alkaline conform closely to one series of 

 curves which have all the characteristics of curves for 

 sub-alkaline rocks. There are, however, nine rocks whose 

 somewhat intermediate character had been detected already 

 microscopically, and these are all found to conform fairly 

 w^ell with a series of curves which take up, as a rule, an 

 intermediate position between the alkaline and sub-alkaline 

 curves. 



An examination of all the diagrams {see Plates VI.- 

 IX.) shows that generally speaking there is a close con- 

 formity of the three series of rocks to their respective 

 curves in each diagram. 



While in general the term "alkaline" as applied to 

 rocks may ))e satisfactory the author felt that he wanted 

 some specific information for the correct determination of 

 the character of certain rocks, particularly the acid ones. 



A fine series of generalised variation diagrams which 

 Harker shows on pages 150 and 151 of "The Natural 

 History of Igneous Rocks" proved of very great value in 

 this direction and the author adapted them for the purpose 

 mentioned above. By drawing up the variation diagrams, 

 each with its three curves, and also plotting Harker 's 

 adapted generalised variation diagrams showing the curves 

 for the alkaline (atlantic) and sub-alkaline (pacific) series, 



