STRUCTUEE AND COMPOSITION OF A BASALT FROM BONDI. 221 



At the date of writing, undecomposed basalt could be found in 

 two places only, in the vicinity of these dykes and fissures — the 

 projecting knob of basalt already referred to, on the path down 

 the cliff, and on a point of rock standing above the sea in calm 

 weather, a few chains from the mainland, and marked G. on the 

 sketch map. The small amount of undecomposed basalt remain- 

 ing is noteworthy, as continued operations at the quarry may hide 

 the little that there is completely from view. But even should 

 the last traces of the basalt proper be lost, the fissures, the 

 decomposed rock filling them in part, and the prismatic sandstone 

 will point unmistakably to the proximity of masses of basalt. 



The Bondi basalt dykes have, it is evident, been one of the 

 factors in the shaping of the present coast line. In my opinion 

 there are two separate intrusions, but not necessarily of different 

 ages. The fissures at A. B. C. and E. (see sketch map) are all 

 belonging to a dyke which cut through the sandstones in a north 

 and southerly direction. Besides the dyke running north and 

 south there exists a dyke as described above by the Rev. W. B. 

 Clarke, coming from the direction of Greenwich, and of which the 

 basalts at D. and G. are branches. 



The most notable feature of the locality is the columnar sand- 

 stone as shown in Plate 9, fig. 2. This structure was no doubt 

 induced by the proximity of the igneous rock. Some of the 

 sandstones, that were in contact with the molten basalt, show very 

 little trace of metamorphism, while in other parts the same sand- 

 stones are rendered quartzose in texture and prismatic in structure. 

 No very clear relation can now be made out between the more 

 extreme examples of metamorphism and any special massing of 

 the igneous rock. But denudation has so affected the locality 

 that, at this date it is practically impossible to say whether the 

 basalt may not have almost surrounded the masses that show the 

 columnar structure so perfectly. 



Macroscopic Examination. 

 In hand specimens, the Bondi rock is a blue-black dense rock, 

 that rings under the hammer. The texture is compact, and to 



