226 ON THE GEOLOGY AND PETROGRAPHY OF BATHURST, N.S.W., 



measure 20° in the zone OP coPgo, the felspars were not more 

 basic than andesine. For the present it will be sufficient to 

 describe the felspar, whether andesine or labradorite, as plagio- 

 clase. Twin crystals are very common in every slice. Sometimes 

 broad cruciform twins are seen, one good example of which may 

 be noted on slide 41. 



Magnetite. — T have never isolated the black crystalline 

 bodies which I have provisionally named magnetite. On being 

 analysed they may prove to be ilmenite or titaniferous magnetite. 

 By drawing a magnet through detrital matter, near the basalt, 

 large quantities of a magnetic iron can be collected. This gives a 

 strong reaction for titanium. I have not been able to decide 

 whether this may not be derived from the adjoining granite. 



The magnetite in the basalt I take to be a primary constituent. 

 It is invariably sound and undecomposed. It can be noticed 

 enclosed in clear augites and olivines. I have noticed secondary 

 magnetite in other basalts, but in that case the olivine and some 

 of the augite had disappeared, and the iron of the ferro-magnesian 

 minerals was represented by the magnetite. The augites in our 

 rock are beautifully clear, and no olivines are wholly decomposed. 



I have selected three slices as fairly representing the microscopic 

 character of the whole basalt. I will describe their general 

 structure. 



1. (Slide 45). The micro-porphyritic structure of this slide is 

 just visible to the unaided eye. Under the microscope large 

 olivine crystals are seen, set in a paste or granular base of magne- 

 tite, augite and felspar microlites. The olivine crystals are beauti- 

 fully clear, magnetite and blebs of glass being the only inclusions. 

 The olivines are better preserved than it most slices, showing 

 very little signs of serpentinization. The streaming of the felspars 

 is very characteristic. One large olivine has evidently moved when 

 the paste was partially set, as it is seen to have pushed on either 

 side a collection of felspars. Besides the lath-shaped felspars, 

 broad rectangular plagioclases of another species probably are 

 represented. The magnetite crystals seem disposed to gather 

 a round the edges of the augites and olivines. 



