192 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



There is little doubt as to the inagniatic relationship 

 of the rocks of this series, for in practically all the curves 

 all the rocks conform closely. 



Considering the extent of the area from which these 

 rocks have been collected, and the fact that at least four 

 different analysts carried out the chemical work, the general 

 conformity to the curves might be considered good. The 

 magmatic relationship existing between acid and basic 

 rocks in this series is of note, because the alkaline series 

 seems restricted to acid and sub-acid representatives. 



The mean average composition of the sub-alkaline 

 series has been determined, and in doing so the results of 

 the eighteen analyses of the series were combined equally, 

 for it was considered that the rocks analysed in the general 

 way represented the rocks of the series in the proper pro- 

 portions. Analyses used were those of rocks numbered 

 1, 3, 4, 6, ]3, 15, 18, 20, 23, 24, 25, 26, 29, 30, 31, 32, 33, 34. 



Intermediate Series. 



This series embraces rocks from the upper, middle, and 

 lower divisions, and their positions in these divisions are 

 worthy of note. The basalt from Mount Lindsay is the 

 uppermost tlow of the lower division, and the basalts from 

 the summit of Mount Spicer, Purga, and Fingal Point, 

 Tweed Heads, are some of the most recent flows of the 

 upper division, while the five remaining rocks of the series 

 from the Esk district are believed to belong to the middle 

 division. Whether the occurrence of these rocks at the 

 termination of two periods of activity during which basic 

 rocks have been poured out is a mere coincidence or not, is 

 a question. It is true that following upon the Mount Lind- 

 say basalt there were acid rocks made up almost entirely 

 of quartz and alkali felspar, but whether this has any 

 bearing or not on the somewhat intermediate character of 

 this basalt is at present indefinite. 



One may explain this series of rocks as resulting from 

 a magma formed by an admixture of the alkaline and sub- 

 alkaline magmas, or else from a separate partial magma 

 which had been split off from the parent magma in the 

 same way that one assumes other partial magmas to have 



