BY H. I. JENSEN. 573 



in activity, basaltic eruptions. This seems to have occurred in 

 the Glass House Mountain region, where the melanocratic facies 

 of alkaline lava is practically absent. 



Fig. 8. — Diagram showing the effect of sedimentation in squeezing the 

 alkaline rocks into Continental areas. 



Broken line = spheroid of earth's curvature — a Zone of sedimentary rock — 

 6. Zone of alkaline rocks and Archaean schists — c. Zone of acid igneous 

 rock — d. Zone of basic igneous rock. 



If in the process of lateral squeezing a mixture with basic rock 

 has occurred, the initial eruptions in the fracture-period will be 

 of the leucocratic trachyte- type, and melanocratic trachytes will 

 follow Trachy-dolerites will also come in before the basalts are 

 extruded. This type of sequence obtains in the Warrumbungles 

 and Canoblas. 



Now in the Nandewars we have an indication that the first 

 intrusions consisted of an essexitic magma. In such a case if we 

 imagine a part of a plateau (i.) drawn down by secular contraction 

 (ii.), folding will take place in the outer zone which cannot 

 accommodate itself to the lessened space (iii.); and the alkaline 

 magma will be forced into the anticlines as laccolites by the sub- 

 siding segment being squeezed between the stable segments M 

 and N (fig. 9). 



But before the compression of the subsiding segment commences 

 it will have fractured, and basic magmas may rise along the 

 fractures and intrude themselves into the alkaline zone. Here 

 they will assimilate alkali and also help to bring about the 

 refusion of the whole alkaline zone. The formation of a vent 

 may lead to the expulsion of this mixed magma first, the alkaline 

 magma next, then mixed magma again and lastly calcic basic 

 magma. 



It will be remembered that the Nandewars appear to form a 

 gentle anticline of Post-Triassic age which was subsequently 



