200 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



In conclusion my best thanks are due to Mr. R. A. 

 Wearne, B.A., of Ipswich, for invaluable aid in the field, 

 to Mr. J. C. Briinnich the Agricultural Chemist and his 

 officers, particularly Mr. G. Patten, for the many valuable 

 rock-analyses which they have carried out, and to Professor 

 E. W. Skeats, D.Sc, of the University of Melbourne, for 

 very kindly criticisms. 



X. DESCRIPTION OF PLATES. 

 Plate V. 

 Brogger Diagrams. 



This shows Brogger diagrams for the thirty-five rocks 

 whose analyses have been used to illustrate the chemical 

 nature of the rocks. The diagrams are arranged in three 

 divisions and show (a) the 18 rocks belonging to the sub- 

 alkaline series, ( h ) the 9 rocks belonging to the intermediate 

 series, and (c) the 8 rocks belonging to the alkaline series. 



The diagrams have been draw^n up in the usual way 

 with the lengths laid of? according to the molecular pro- 

 portions of the ditferent oxides; the iron-oxides have been 

 calculated together as ferrous oxide. 



An interesting comparison between the sub-alkaline 

 and alkaline series is shown, and the differences between 

 the lime, magnesia and alkalies are brought out in a 

 striking manner. 



The intermediate character of the third series is 

 clearly indicated. 



The numbers on the diagrams correspond to those of 

 the rocks on Table VII. (page 183). 



Plates VI.-IX. 



Variation Diagrams. 



These have been drawn up by plotting the silica 

 percentages against the other oxides in percentages. In 

 addition to the three curves for the three series, there are 

 two curves as adapted by the author from curves drawn 



