302 GEOLOGY ANLi PETROLOGY OF THE GREAT SERPEXTIXE BELT OF N.S.Vl . 



row of hills which lie east of the railway line. Possibly this continuous ciyke-zono 

 marks the site of fissures from which the Werrie basalts were eject etl. 



The consideration of the age of these rocks is deferred to a later section 

 (p. 308), but the most probalik; conclusion is that they were formed in late Car- 

 boniferous or earlv Pernid-Carlinniferous times. 



Tlie Iiitrnsive Igneous Itnckn. 



We have just remarked that the Werrie Series and we may now say the 

 underlying sedimentary formations also, have been invaded by numerous, more or 

 less concordant intrusions and dykes composed of a variety of igneous rocks. 

 These intrusions fall into several gi'oups: , 



(a) Sills and sheets. The most striking of the intrusive rocks ai'e sills of 

 glassy or lithoidal pyroxene andesite. Of these there are three zones, a com- 

 paratively short ard intermittent eastern zone, the main continuous zone, and the 

 western zone of intermittent Init large intrusions which rise t<i form two of the 

 highest peaks in the district. The resistance to erosion offered b.y these sills is 

 very great, and as they are inclined at a considerable angle they usually form 

 high, sharp-ridgeil cuestas. 



The eastern i'one commences in a low hill in the south-western corner of the 

 Parish of Warral and thence continues intermittently southwards, rising into a 

 well marked ridge. In portions 2(56 and 197 of the Parish of Currabubula, ex- 

 posures of the upjier surface of the sill seen in a creek, sliow that the andesite 

 has invaded the mudstone, and included fragments of it, wliich have been con- 

 verted into a dense flinty hornstone. The sill is not seen from about a mile south 

 of the Duri road until it appears again in portion 83. after which it continues 

 intermittently, forming a low ridge extendino for a mile and a half further to the 

 south-east. The railway cutting through tliis ridge reveals a complex of shat- 

 tered and indurated mudstones, conglomerate, and tuffs. 



The main zone of pyroxene andesite extends from the Parish of Wiuton on 

 the north into the area mapped, crosses the low divide at the head of China- 

 man's Gully, and rises int(j the strongly asymmetric hill east ot Duri Peak which 

 has a precipitous westerly-facing scarp and a long, smooth, but steep dip-slope 

 to the east, rising until the surface of the andesite is half a mile wide. The 

 andesite is truncated by a fault, the same which forms the soutliern boundary of 

 the igneous rocks of Duii Peak, and by this fault the outcrop is displaced a 

 quarter of a mile to the west, the down-tlirow being to the north. A basic dyke 

 occupies this fault-fissure. Tlicnce the andesite continues in a succession of dip- 

 ridges or cuestaii. broken by minor dip-faults and occasionally crossed by nari'nw 

 dykes of l)asalt, which weather so rapidly that they have determined in several in- 

 stances the positions of the valleys crossing the andesite (see p. 310) . The most 

 marked instance is that between Mts. Cobla and Sugarloaf. Fault-fissures also 

 have determined transverse valleys, as in the ease of Currabubida Creek. South 

 of the latter the andesite liand wedges out and is replaced by a second zone com- 

 mencing to the east of this termination of the main l)and. The nari'ow strip of 

 sediments between these two masses of pyi'oxene andesite consists of fragnaentnl 

 jaspery rock. The zone now continues across the upper part of Currabubuhi 

 Creek, and rises into the higli ridges forming the western boundary of the Parisli 

 of Goonoo Gdonoo. It aiipcars to cont'nue for several miles furtlier tn tlie 

 south-east . 



