272 
Geology op Sydney. 
obstruction, meeting again beyond, giving rise to an 
appearance that may be compared to “ logs in a mill- 
stream, the olivines representing small islands.” 
In like manner, when a slice of Bondi basalt is 
prepared in this way, the olivine crystals show the 
character of the rock before the slide becomes generally 
transparent. 
At Bondi, as shown on the map (Fig. 74), we have 
fissures left by the decomposition of previously exist- 
ing dykes, masses of decomposed basalt, and some of 
the unaltered basalt in situ. The main fissure runs 
north and south, in the direction of the dyke at Long 
Reef, some ten miles further north. This is at present 
a line of weakness, and no doubt has been one of the 
factors in determining the present coast-line. 
The Bondi basalt occurs as part of a dyke intru- 
sive in the Hawkesbury Sandstone, on the coast to the 
east of the city of Sydney. As a land-mark, the venti- 
lating shaft of the Bondi main sewer is easily found. 
Close by is a quarry of columnar sandstone. From the 
floor of this quarry a wiuding 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 sand- 
stone away from the mainland. There is no doubt but that 
this and similar fissures in the neighbourhood are left by 
the decomposition of previously existing dykes of basalt. 
