262 
Geology op Sydney. 
afforded at Blakehurst, where a quarry was opened in 
hardened sandstone, without any visible trace of the 
Igneous rock. In the course of working the quarry, as 
was expected, the dyke was found, but altered to a 
condition seen in the Bondi fissures. Another instance 
is the occurrence of incipient columnar structure in 
the sandstone on the North Head, south of Manly. 
No Igneous rocks are at present known as being- 
exposed here ; but it may be noted that the place is 
right in a line between Bondi and Long Reef, so it is 
more than probable that the Igneous rock is not far 
away, and may yet be found in this neighbourhood. 
The basalt quarry at Pennant Hills has been 
described by Mr. C. S. Wilkinson. He states: — “It 
is an immense excavation, from which road metal is 
said to have been taken for over fifty years. The 
rock consists of a dense but jointed basalt, containing 
small fragments of other rocks, and some huge masses 
of coaly shale, from which it would appear that this 
spot is the site of an ancient volcanic point of erup- 
tion.” Most geologists agree with this view ; but it 
was left to Professor David and his assistants to note 
the interesting fact that there is chromite and a chrome- 
bearing mineral associated with the basalt. L 
Still more interesting is the great mass of intru- 
sive rock at Prospect. The country rock is Hawkes- 
bury Sandstone and shales. “ Penetrating these 
formations are dykes and masses of Igneous rock, 
1 In the opinion of the present writer, the chromite occurs in erupted blocks 
of a much altered serpentine, or a closely related ultra-basic rock. 
