235 



and Mitchani (two miles apart), appears to be tlie same bed, 

 and is about 100 feet thick. 



The petrological characteristics of these quartzites are re- 

 markably uniform and constant throughout the series. Mac- 

 roscopically the stone is highly vitreous, and has a piebald 

 appearance. This effect is produced by the presence of clastic 

 felspar, of a white colour, distributed through the siliceous 

 cement, in company with grains of quartz which are often 

 confluent. The proportion of granular felspar to quartz 

 grains ranges from 30 to 40 per cent. This constituent is 

 sometimes excessively fine, and can only be clearly distin- 

 guished after the stone has been immersed in water. The 

 stone may be regarded as a fine-grained, arkose sandstone or 

 grit, derived from the waste of granitic rocks, and subsequently 

 metamorpbr>sed by the infiltration of silica from solutions. The 

 proportion of siliceoas cement present in the stone determines 

 the economic use. Where the proportion of silica is rela- 

 tively low the stone is used as a freestone for buildingf ; but 

 where high it is best adapted for road metal. In many 

 pla^^es fault breccias occur, and when these carry vughs, very 

 fine nests of quartz crystals and tabular crystals of barite are 

 sometimes found. An interesting series of petrological studies 

 in South Australian quartzites was carried out by my late 

 colleague. Dr. Woolnough, and published in the Transactions 

 of this Society."^ 



IV.— The Thick Slate (Glen Osmond Slate). 



The quartzites of the Glen Osmond and Mitcham dis- 

 tricts are interstratilied with slates which have an aggregate 

 thickness exceeding that of the associated quartzites. In 



structure they vary from laminated shales, with slight evi- 

 dences of cleavage, to slates in which the cleavage is com- 

 plex, and obscures the bedding. The metamorphic effects on 

 the slates increase with the relative depth, so that the lower 

 beds differ mucn in structure and lithological aspects from 

 the higher. 



For purposes of identification the shales and slates of this 

 series present equal difficulties as those connected with the 

 quartzites. Beds widely, removed in their vertical order are 

 at times faulted against each other with no superficial evi- 

 dences of such displacements. In the great fold movements to 

 which this series has been subjected it has been the weak 

 and yielding slates which have suffered the greatest defor- 

 mation. The pressure which caused the earth-folds was 



* Petrological Notes on some South Aiis. Quartzites, &c , 

 Trans. Roy. S<w. South Aus., vol. xxviii., p. 193. 



