230 REV. J. MILNE CURRAN. 



referred to, is plentiful, a section being visible on every field of 

 the slide. It is seen in the ground mass, and in one instance a* 

 an inclusion in an augite crystal. Magnetite is abundant, evenly- 

 distributed over the slide, and showing a disposition to group itself 

 around the augites and olivines. The clear soda mineral is seen 

 as a primary constituent, in many cases felspar laths penetrating 

 it. It alters to a cloudy grey sub-granular substance, but not in 

 the least resembling the devitrification of a glassy base. 



Slide No. 209. — The augites here attract attention by their 

 reddish-brown tint, which increases in depth towards the faces of 

 the crystals. The more strongly coloured augites are somewhat 

 dichroic. Both augites and olivines are well preserved and fresh 

 looking. Just along the cracks of the latter a light green serpen- 

 tinous product may be noted, but only in one instance is a small 

 olivine altered into serpentine. The larger olivines are shattered 

 and bent, while very many of the smaller crystals are idiomorphic. 

 Magnetite is more plentiful in this slide. 



As to the age of this basalt there are no data to go on to form 

 an opinion. It is of course newer than the Hawkesbury Sand- 

 stone, into which it is intruded. It may be as old as the newer 

 Mesozoic rocks, or as recent as the Eocene basalts of the Colony, 

 and was probably in situ before the shaping of the present coast- 

 line. 



Conclusion. 



The basalt occurring as a dyke at Bondi contains a high per- 

 centage of soda. The soda bearing mineral of the basalt is isotropic 

 and gelatinizes with cold hydrochloric acid. As there is chlorine 

 in the rock and no sulphuric acid, the mineral is most probably 

 sodalite. The hydrochloric acid solution of the rock gives abun- 

 dant cubes of sodium chloride on slowly evaporating. Based on 

 the chlorine present, it is found that there is a surplus of soda, 

 after allowing for sodalite and soda felspars. It is probable that 

 some soda exists as a constituent of the residual basis. 



There are two igneous dykes at Bondi, one including the fissures 

 running north and south, a second along a line coming seawards 



