866 THE GEOLOGY OF THE NANDEWAR MOUNTAIN 



The western side was thrown down, so that lavas ving from 

 the main fissure poured chiefly westwards. The ptive and 

 intrusive (sill) materials from the main fissure anc-om inter- 

 secting secondary cracks gave rise to the lava dome. 



Mr. Andrews has shown that the New England a adjoining 

 areas underwent rapid elevation in the late Cretacec and early 

 Tertiar}^ periods, and were at the same time sufing rapid 

 denudation. At the same time the west of New ^jth Wales 

 was being loaded with sediments. The Nandewarlie on the 

 border of these two zones. Probably strain caed by the 

 differential movement on either side was an impornt cause of 

 faulting. 



The downthrow of the western and elevation ofhe eastern 

 side of the fault have tended to preserve relics of riassic and 

 upper Permo-Carboniferous rocks west of the faulted to expose 

 Carboniferous and Devonian rocks east of it. 



Tertiary denudation has dissected the conoplain :tablished at 

 the end of the volcanic cycle, and the products of>,rosion have 

 been deposited (aggradation) in a depression formecby faulting, 

 so that a thickness of over a thousand feet of soft ertiary rock 

 and alluvium has been formed at Narrabri. The-^amoi Kiver 

 has also helped to aggrade the depression. 



(c) Volcanic Action. — The order of eruption, ju'aing by field 

 observations, seems to have been as follows : — 



(1) Pre-Tertiary, probably earl}^ Cretaceous, basi»intrusions in 

 Permo-Carboniferous strata. 



(2) Tertiary : (a) Sill-like and laccolitic intrusi<ns of syenite 

 accompanied by flows of phonolite, trachyte, and JlUed alkaline 

 lavas. (6) Alkaline andesites and more porphyry siHs. (c) Basic 

 porphyrite dykes and basalt flows. 



The basaltic eruptions probj^^^ lasted well into tbe Miocene 

 and Pliocene periods. The vo^^^^^-^ period was an era of great 

 uplifts caused by the injectr"^^ ^^ g-^^^ ^^^ laccolites. The uplifts 

 led to changes of dip in th^ sedimentary rocks and disturbed the 

 uniformity of the ^!L^^ levVin the region, considerably increas- 

 ing it towards the^ ^ c ^-i.^ rvi^r^nr. 



^ centre or the group. 



