BY H. C. RICHARDS. 195 



There is a marked difference in the modes of the two 

 rocks although they are so close chemically and that is with 

 respect to the mineral fayalite. The basalt from Scott's 

 Well does not contain any fayalite or olivine as seen in the 

 microsection, while the Mt. Boorambool basalt is very rich 

 indeed in the mineral. 



Relationships and Origin of the Volcanic* Rocks. 



The close association in this field of normal sub-alkaline 

 or calcic basalts and alkaline trachyte rocks causes one to 

 speculate as to the cause of the variation. The phonolitic 

 magma has been extruded after calcic basaltic magma was 

 effused and in its turn it was followed by a succession of 

 outpourings of basaltic flows. 



The analyses of typical samples of both the upper and 

 lower series of basaltic flows show them to be very similar 

 and to be closely comparable with the average composition 

 of the basalts of the world. 



The several outpourings seem to have followed one 

 another at frequent intervals as there is no evidence of any 

 period of time having elapsed between successive outpour- 

 ings. The lower series of basalts were extruded in the 

 immediate vicinity of Springsure, as evidenced by the 

 agglomeratic character of much of this series ; also the 

 middle series was effused from a focus in the same vicinity, 

 as shown by the occurrence of trachytic tuffs and agglomer- 

 ate and by the character of the main mass of the trachytic 

 rock. 



There is no evidence as to whether the upper series was 

 effused from the same centre of activity or not. The fact 

 that the flows on the disconnected mesas " line up " rather 

 suggests a different centre of effusion as the upper series 

 is of greater thickness and on a far more extensive scale of 

 development than the lower and middle series. 



There are, however, many points of resemblance between 

 the lithological and chemical characters of the two basaltic 

 series and a somewhat common origin may be assumed for 

 them. 



The phonolitic material is interbedded between these 

 two series of basic calcic rocks and the interesting question 

 of accounting for this association arises. 



