410 geology an'l) i'etkol.ogv ok tiik gkkat skkl'kntine itklt of x.s.'.v., 



Intrusive Kock.*<. 

 Trachytoid Quartz Keratoplnjres. 



Tlie locks (lescribed iii the account of the field-geology as felsites, trachytes, 

 etc., forming sills, dykes and other intrusions, fall, with few exceptions, into the 

 keratophyre class, although varying among themsehes in details of texture and 

 mineral constitution. The variations are fairly reg^Uar, so that subdivision is 

 rather difficult, and it will be l)est perhaps to mention tlie general mineralogical 

 features of the series before proceeding to more detailed description. 



The series as a whole has l)een called the trachytoid quartz keratophyres to 

 distiuguisli them from the soda rhyolites, from which they differ in texture, mode 

 of occurrence and most probably also chemical composition. 



As no essential differences were found between the rocks composing the dykes 

 directly emanating from the Warragundi volcano and those forming the sills and 

 dykes cutting througli the Kuttung Series elsewhere in tlie area, all liave l)eeu 

 grouped together. 



Megascopically the rocks are fairly conijiact. occasionally slightly \esicular, 

 and varying in colour through buff, pink and pinkish-grey to dark greenish-grey. 

 The majority of the specimens have phenocrysts of pink or white felspar ranging 

 up to 6 mm. in length; in other cases the, plienocrysts are small, while in certain 

 types they are entirely absent. 



Microscopically the rocks are all holocrystalline; the grain-size is tine to very 

 fine, and the fabric may be trachytie or orthophyric, with or witliout lluxion struc- 

 ture, or again no definite fabric may be recognisable. Of the minerals present 

 acid plagioelase is by far the most abundant and iui])ortant. Accurate deter- 

 minations of composition were generally most difficult and sometimes (|uite im- 

 possible owing to alteration and fineness of grain, but there appears to be a varia- 

 tion from pure or almost pure albite in certain members of the series to oligocla,se 

 in others. Tlie felspar may be zoned, with apparently an oligoclase kernel and 

 an albite rim. It is possible that the variation in habit of the felspai-s is to 

 some extent a function of their chemical ccmiposition, the slender, almost aeicular, 

 crystals of oligoclase being conti'asted with tlic stumjjv prisms of tiie more albitic 

 rocks. The extent to which orthoclase enters into the rocks is doubtful. Tn some 

 it is certainly present, in others, especially those ccmtaining the more acid plagio- 

 elase, it is undoulitedly absent, while in otiiers again its presence cannot be de- 

 'finitely determined. It is probable, of coui-se, that orthoclase molecules exist 

 to a small extent in solid solution in the jilagioclase. 



Quartz is i)erhaps the mineral next in importance after felspar; it is almost 

 always interstitial, but in two cases its occurrence is i>eculiar and gives the imjires- 

 sion of early crystallisation. Rarely it encloses the felspar in uiicropoikilitic 

 fashion. Tlic ])roportion of quartz varies very much in the different rocks, but 

 does not ai)])iNir tn be con-elated with the fluctuation of any other constituent 

 save possibly tlic iron ore. The latter varies consideral)ly in amount, both mag- 

 netite and ilmenite being present. 



Primary ferro-magnesian minerals are comparatively scarce; certain of the 

 rocks must have been entirely devoid of them, while in others the occurrence of 

 chlorite and other secondary material indicates that some dark minerals were once 

 present. In a few rocks the original l)iotite, hornblende or pyroxene still re- 

 mains. The Jiresence of tlie last two iioints to a genetic connection with the an- 

 desites. Apatite in small amounts is a fairly constant constituent. 



