188 VOLCANIC ROCKS. OF SPRINGSWRE. 



A microscopic examination {see Plate xii. Fig. 6) shows 

 the rock to be holocrystalline and to be made up of lath- 

 shaped plagioclase crystals set in allotriomorphic, drawn-out 

 crystals of deeply violet-tinted augite. Granules and rods 

 of the iron ores (magnetite and ilmenite) occur very abund- 

 antly throughout the section. The felspar crystals have an 

 average length of about 0.5 mm. while the drawn-out 

 augite crystals which are optically continuous but much 

 dissected bj' the plagioclase crystals, range up to 1.0 mm. in 

 length and 0.5 mm. in width. The chemical analysis of 

 the rock shows the high percentage of 4.04 for tita,nia 

 which corresponds with the deep violet tint of the augite. 

 Patches of a dark-green chloritic product occur occasionally 

 through the .section and this material is probably derived 

 from the augite. The plagioclase obviously ceased crystal- 

 lising before the augite, and the allotriomorphic drawn-out- 

 crystals of the latter with idiomorphic plagioclase crystals 

 sticking through them indicate movement in the flow 

 after the plagioclase had crystalli.sed .and before the augite 

 has ceased crystallising. 



The plagioclase could not be determined optically. 

 There is a marked absence of olivine in this rock which is- 

 in strong contrast with the oliviue-rich basalt of the flows 

 forming the uppermost .series of volcanic rocks in the area. 



A chemical analysis carried out by Mr. G. R. Patten^ 

 of the Agricultural Chemist's Laboratory, Brisbane, is shown 

 on Table I. A comparison of the analysis with that of the 

 olivine basalt from Mt. Boorambool shows how closely allied 

 the two rocks are chemically ; this is still further empha.sised 

 by comparing the norms. 



Specimen 279, from the Opal Ridge about three- 

 quarters of a mile south-west of Springsure, is a very finely 

 scoriaceous, glassy basalt. It forms the uppermost portion 

 of the lowest division of volcanic rocks and immediately 

 underlies a trachytic tuff which in its turn is succeeded by" 

 trachytic pitchstone and trachyte. The material is some-^ 

 what agglomeratic and in colour it is a dark bluish-grey. 

 The material has been met with at the bottom of the shaft 

 which was sunk on the opal patch through the trachyte 

 and trachyte tuff. It outcrops over the lower portion». 



