62 T. W. E. DAVID. 
north-easterly from the preceding is the much smaller volcanic 
pipe (?) which has been worked for road-metal at the Pennant 
Hills quarty. This quarry has been described by Mr. C. 8S. 
Wilkinson. It has also been described by Messrs. W. F. Smeeth, 
J. A. Watt, and myself.” 
It is an oval mass of basalt about one hundred yards in greatest 
diameter, and has obviously intruded both the Hawkesbury Sand- 
stone and the Wianamatta Shale. A remarkable feature in con- 
nexion with this pipe is the occurrence of blocks of a rare chromite 
rock adhering to the sides of the basaltic pipe and having a 
diameter of from six inches to twenty inches. They are irregularly 
rounded, probably through partial fusion in the basaltic magma, 
and consist of chrome-diallage, chromite, and a felspar of the lime- 
soda series. 
In addition to the bosses mentioned above, the rocks of the 
Blue Mountains and the coastal strip are traversed by a network 
of basic dykes. These have been described by the Rev. W. B. 
Clarke,’ by myself,* by Mr. E. F. Pittman,’ and by the Rev. J. 
Milne Curran. The last mentioned is the only detailed petro- 
logical account of these dykes that has yet been published, and is 
well illustrated. ‘The mineral sodalite is stated to be present in 
this basalt, and the analyses shew that the total soda = 7:34%. 
These basalt dykes at Bondi, at La Perouse, at Lane Cove, at 
Five Dock etc. have rendered the Hawkesbury Sandstone distinctly 
prismatic (v. Plate 9, op. cit.) As regards the age of the eruptive 
rocks described above, while it is clear that they are all Post- 
1 Annual Report Department of Mines for 1879, p. 218, Appendix A. 
2 Journ. Roy. Soc. N. 8S. Wales, Vol. xxvit., 1893, pp. 401 — 406. 
3 Transmutation . sie in Australasia—Trans. Phil. Soc. N.S. Wales 
1862 — 1865, p. 294 
4 “ Notes on some eek of basalt eruption in N. 8. Wales ”’—Geol. Soc. 
Aust., Vol. 1., part i., p. 25, Melbourne, 1886. 
5 Report on Site for a New Bore at Cremorne.”—Annual Report 
Department of ‘iio for 1892, pp. 109-111. 
6 “Structure and Composition of a Basalt from Bondi.”—Journ. Roy. 
Soc. N. 8S. Wales, Vol. xxvir1., 1894, pp. 217 - 231, pls. 9-12. 
