GEOLOGY OF THE GLOUCESTER DISTRICT, N.S.W. 



259 



stated whether this rock is really intrusive or whether it 

 is a contemporaneous flow, as the field evidence was quite 

 unsatisfactory so far as it was investigated, but it appeared 

 to be an intrusive sill occurring about 500 feet below the top 

 of the Gloucester Buckets rhyolite flows. 



In the hand specimens this rock is a fairly dark-coloured 

 compact aphanitic rock, showing under the pocket lens 

 occasional small phenocrysts. Under the microscope the 

 groundmass is seen to be crystalline to microcystalline, 

 and to consist of a felted mass of very small laths of felspar; 

 these felspar laths were too small for optical determination 

 but appeared to be for the most part a plagioclase felspar. 

 A few small phenocrysts of a plagioclase felspar are em- 

 bedded in this groundmass, but were too much altered for 

 specific determination. These felspar phenocrysts are 

 strikingly marked by zonally arranged iron-oxide inclusions 

 at or near the outer margin of the crystals. There also 

 appears to be a considerable quantity of minute grains of 

 iron oxide scattered through the groundmass. 



Si0 2 



55-64 



A1 2 3 



17-70 



Fe 2 O s 



7-85 



FeO 



1-75 



MgO 



1-46 



CaO 



1-31 



Na 2 



8-51 



K 2 0... 



1-20 



H 9 O(100°C.) 



0-72 



H 2 O(100°C. + ) 2-24 

 CO, 0-04 



Chemical Composition. 



S0 3 ... 

 CI ... 

 S (FeS 2 ) 

 Cr 2 3 



NiO and CoO 

 MnO... 

 BaO ... 

 SrO ... 

 Li0 2 . . 



v 2 o 5 



Ti0 2 

 £r0 2 



PoO, 



1-45 



0-11 



Total 



trace 



absent 



absent 



001 

 absent 

 0-25 

 0-01 

 trace* 

 absent 

 trace 



100-25 



Specific gravity 2-721 



