106 voLCA>;ic ROCKS or springsure. 



The basaltic rocks both above and })eIow may repre- 

 sent outpourings of a comparatively undifferentiated 

 primary basaltic magma and on chemical evidence this 

 view^ is supported. Gravitative differentiation may have 

 gone on to some extent as the lower series is olivine free, 

 while the upper series is rich in olivine (fayalite). This 

 arrangement is the reverse of that cited by Daly*, as found 

 by Lewis and Du Toit in certain areas in New Jersey and 

 East Griqualand respectively. If the magma had been 

 affected by the olivine crystals separating out and owing 

 to their higher gravity settling at the bottom the material 

 in the upper part of the magma if extruded first would tend 

 to be olivine free, Avhile the material at the bottom of the 

 reservoir and extruded last would be olivine rich. How 

 has the phonolitic material resulted ? Has it been formed 

 irom the basic calcic magma by fractional gravitative 

 differentiation as advocated by Bowen, by the absorption 

 of foreign material as advocated by Daly, by the action 

 of gases or mineralisers as advocated by Smyth, or by any 

 other means ? In answering this, a consideration of the 

 nature of the intruded terrane is of importance. The under- 

 lying rocks are sandstones, shales and limestones as far as 

 can be ascertained. These sediments f outcrop just to the 

 north of the area covered by the volcanic rocks. 



The intruded terrane almost certainly contains 

 limestone, and the solution of this material to a small extent 

 would be regarded by Daly as sufficient to result in the 

 production of the phonolitic material from the calcic 

 basaltic magma. 



The writer, in dealing with the origin of the volcanic 

 rocks of south-eastern Queensland, regarded them as 

 being differentiates of a single original magma and owing to 

 the almost entire absence of any known limestone in the 

 intruded rocks rejected the sediment-syntectic hypothesis 

 of Daly. 



Daly J in the " Genesis of Alkaline Rocks " points out 

 that he does not regard limestone as necessarily a partner 

 in a syntectic from which trachytes, etc. are differentiates, 



*Daly, R. A., Igneous Rocks and their Origin, p. 316. 

 tAnn. Prog. Rept., Q'ld. Geol. Survey. 1894, Pub. 103, p. 9. 

 +Jour. Geol., xxvi, 1918, p. 108. 



