STRUCTURE AND COMPOSITION OF A BASALT FROM BONDI. 219 



11 transmuting " the sandstones, and so giving rise to the very 

 remarkable columnar sandstone seen hereabouts. (Plate 9, fig. 2.) 



I can find only one other reference to the rock I am dealing 

 with. Professor David writes, that " The basalt here forms an 

 irregular mass surrounded and intermixed with shattered frag- 

 ments of Hawkesbury Sandstone, and of an intercalated bed of 

 carbonaceous shale. The mass is in places about a chain wide 

 and several chains in length, and sends off horizontal sheets and 

 vertical dykes into the surrounding sandstone." There is a draw- 

 ing illustrating the intrusive nature of the basalt with Professor 

 David's paper.* 



Locality and Occurrence. 



The Bondi basalt occurs as part of a dyke intrusive in the 

 Hawkesbury Sandstone on the coast to the east of the city of 

 Sydney. As a land mark the ventilating shaft of the Bondi main 

 sewer is easily found. Close by is a quarry of columnar sand- 

 stone. From the floor of this quarry a winding path leads down 

 to the base of the cliffs and to sea level. To the right of this path, 

 some fifty feet from the top, undecomposed basalt may be found. 

 Part of the path is cut through the same basalt, decomposed and 

 altered to a grey, soft, and when wet, pasty rock. A few chains 

 south, a deep and narrow fissure can be seen cutting a mass of 

 sandstone away from the mainland. There is no doubt but this 

 and similar fissures here about are left as the walls of previously 

 existing dykes of basalt. 



A second fissure is noted (marked D. on map), running in a 

 north-westerly direction, and still retaining some decomposed 

 basalt in situ. Further south is a third fissure, (C. on the map 

 attached). Plate 9, fig. 1, is a photograph of this fissure, taken 

 from a point at sea level, which may be reached through the 

 Meriveri Pass. The photograph shows basalt filling the lower 

 part of the fissure, but now soft and decomposed. Still further 



* " Notes on some Points of Basalt Eruption in New South Wales/' 

 by T. W. Edgeworth. David, b.a., f.g.s. — Geological Society of Australasia 

 Vol. 1., part i., page 25, Melbourne 1886. 



