BY H. I. JENSEN. 569 



Dykes of bostonite and monchiquitic lamprophyre were also 

 formed. 



The last eruptions consisted of alkaline basalt, trachy-dolerite, 

 and normal olivine basalt. Although I have no fossil evidence 

 to fix the age of the eruptions with certainty, the lavas being 

 later than beds known by the Glossopteris, Gangamopteris, and 

 Nceggerathiopsis leaves in them to be Upper Coal Measures, also 

 later than beds at Deriah Mountain classified by the Mines 

 Department on direct evidence as Triassic, and overlying the 

 remnants of a peneplain carved in the Cretaceous (The Mole 

 Peneplain), must be Post-Cretaceous, that is probably Eocene or 

 Miocene. The basalt crater-rings are often so fresh that the 

 basic eruptions can hardly be older than Middle Tertiary. Of 

 course the Post-Tertiary arid cycle has tended to preserve the 

 mountains here and in the Warrumbungles, but I have shown 

 that there is strong evidence of a wet era preceding the arid one. 

 Erosion must necessarily have been more rapid then. 



The centre of the Nandewar group probably occupies the posi- 

 tion of intersection of two important fault-lines, one running 

 N.N.W.-S.S.E., along which most of the alkaline lavas have 

 been expelled, and one running N.N.E.-S.S.W., the Dubbo- 

 Narrabri line of weakness. Between these two fissures a great 

 area of country capped with Coal Measure and Triassic sediments 

 has subsided in Tertiary times. The downthrow cannot be as 

 old as Cretaceous, for no part of the area is affected by the Cre- 

 taceous transgression. It is probably contemporaneous with the 

 Tertiary subsidence of New England. Certain parts of the 

 basin subsided much more than others, probably by the agency 

 of transverse fractures. Thus at Narrabri about 1500 feet of 

 fluviatile material of Tertiary age overlie igneous rocks of 

 Cretaceo-Tertiary age similar to the dolerites and essexites of the 

 Nandewar Mountains. This portion may have subsided 3000 

 feet. 



We have thus observed that all the alkaline rocks of New 

 South Wales lie along a continuous curve surrounding the Permo- 

 Carboniferous basins. Probably a number of faults join so as to 

 form a continuous line of weakness. 

 42 



