406 GEOLOGY AND PETROLOGY OF THK GREAT SERPESTIXF. BELT OF X.S.W., 



be established, except iiulirt'ctly tliiougli tlie fact ot tlic Martin's Cieek type of 

 andesite (which presumably is closely connected with the pyroxene andesite) being 

 found at Werris Creek Gap among the conglomerates. 



The earlier and more acid portions of the series appear to have been ejected 

 largely in fragmental form, and are now represented by the tuffs which occur so 

 abundantly at the base of the Kuttuug Series, and resemble very closely those 

 found in the Hunter \'aliey. This tendency of certain of the acid rocks to form 

 pyroclastic material has been noted and commented on by Flett (47) and othei-s. 

 It is exhibited in this State at the base of the Devonian at Taemas, near Vass, 

 and it appears as a constant feature of our Carboniferous vuleanicity. Probal)iy 

 it is in some way connected with the viscosity of the highly alkaline magmas. 



At intervals during the deposition of the tuffs (which continued during most 

 of the Kuttung epoch) small Hows of basalt were poured out, and there were also 

 local eruptions of soda rhyolite and associated tuffs. 



The most important and definite manifestation of volcanic activity took place 

 subsequent to the deposition of the Kuttung Series, when there were extensive 

 outpourings of l)aisalt, with tuffs and agglomerates, succeeded l)v wides]iread intru- 

 sions, and probably extrusions, of magma. Of these we have evidence in the sills 

 and dykes of dolerite. Iioridilende and pyroxene andesite. and. lastly, of trachytoid 

 keratophyres. 



It js here assumed that the sills, dykes, and other intrusions which are found 

 so extensively about the district were closely connected if not actually synchronous 

 with the igneous activity of which tlie Warragundi volcano was one expression; 

 the field evidence, as well as the petrological study of the rocks, tends to .justify 

 this assumption. 



The petrological examination of the rocks has been carried out on material, 

 the Inilk of which wa.s collected by Benson during the progress of the field-work, 

 and the field-relations as determined by Iiim have been accepted . 



EXTRI'SIVK K0CK.S. 



Tuffs of the Burindi Beds. 



A comprehensive study of these lias not been nuide, but specimens have been 

 collected and examined from the railway cutting in Portion 34, about 1} miles 

 north-east of the railway bridge over Currabubula Creek. Here the mudstones 

 and tuff's have been invaded and indurated by a sill of glassy andesite, and an 

 interesting though complicated section is exposed. 



The tuft' itself is an extremely hai'd, compact, siliceous-looking rock with a 

 short and subconchoidal fracture. Under the microscope it is seen to be a 

 keratophyric tuff, consisting of a microfelsitic or cryptocrystalline matrix thickly 

 set with angular chips of felspar. These are of pure all)ite and vary nuu'h in 

 size, the bulk of the fragments being very minute iiuleeil. while the largest are 

 not more than .5 mm. long. This felspar is a good deal kaolinized and the rock 

 as a whole is slightly stained with limonite. Quartz could not be detected with 

 certainty, and ferromagnesian minerals are absent. Tliere has l)een some silicifica- 

 tion, indicated by little aggregates of chalcedonic (luartz-granules. Just where 

 the andesite has invade<l it, the tuff giades into a type crowded with angular 

 fragments of a hard butf or cream-coloured porpliyritic rock. Tliese grade from 

 pieces about l.J inches in diameter down to the nunutest chips. It is evident, on 

 examination with the microscope, tliat the rock has undergone a cei-tain amount 



