BY H. I. JENSEN. 867 



Considering all the igneous rocks of the Nandewar Mountains, 

 we have to add the rhyolites and quartz-porphyries lying south- 

 west and west of the group. These were of Devonian and 

 Carboniferous age, and closely resemble lithologically the Snowy 

 River porphyries. 



The relative ages of the various igneous rocks have been 

 determined as follows : — 



(1) Hhyolitic tuffs occur in the Carboniferous sediments, and 

 rhyolitic pebbles occur in Permo- Carboniferous sandstone con- 

 glomerates. The rhyolite therefore antedates the Permo- 

 Carboniferous. 



(2) The basic laccolites intrude the Permo-Carboniferous, but 

 are cut by trachyte dykes. They have formed at a considerable 

 deptli, hence before the dissection of the Mole peneplain. They 

 must therefore be Triassic or early Cretaceous; for reasons 

 advanced above, probably the latter. They may even be of the 

 same age as the basic granites and intermediate rocks of New 

 England. 



(3) The trachytes intrude Permo-Carboniferous rocks and the 

 later basic laccolites. Lithologically they are identical with the 

 trachytes of the Warrumbungle Mountains and Glass House 

 Mountains. In the absence of evidence to the contrary and in 

 the presence of indirect evidence (drawn from plateau-erosion) 

 in favour of the supposition, the same age, namely Eocene, must 

 be assigned. They may be divided into two series : — 



(1) Light grey trachytes, arfvedsonite trachytes, and rhyolitic 

 trachytes, and syenite of a pepper-and-salt colour in sills and 

 laccolites. 



(2) Dark green segirine trachytes (phonolitic) and sills of 

 segirine-augite syenite and of alkaline plagioclase porphyrites 

 often containing fine-grained inclusions of phonolite of allied 

 composition. 



The second series is later than the first. 



(3) The andesites form a connecting link between the aegirine 

 trachytes and the basalts and grade into both. Many of the 

 andesites are alkaline allied to phonolites. 



