MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 211 



not known in rocks older than the Tertiary. Australia furnishes 

 no exception to this rule. I had an opportunity of examining 

 these interesting rocks in Central New South Wales, and there 

 is no doubt but that they represent fragments of comparatively 

 recent lava flows. The Harden basalt is also a Tertiary lava now. 

 The slices of this rock are seen studded with magnetite in clearly 

 and sharply defined grains, ranging from one-thousandth to one- 

 two-hundredth part of an inch in diameter. The largest leucite 

 is not more than the one-fiftieth part of an inch in diameter. A 

 few good examples of the peculiar leucite inclusions are present 

 on this slice. One will be found about the one-hundredth part of 

 an inch from the ink dot on the cover glass. There is a secondary 

 mineral of a rich reddish-brown colour scattered through the slice. 

 It is not dichroic and is soluble in weak hydrochloric acid. There 

 is a crystal of olivine, that has been broken along its principal 

 axis ; this, with other evidence of a similar kind, shows that these 

 micro-porphyritic olivines floated as fully developed crystals, in 

 the liquid or pasty magma before the consolidation of the rock. 

 They are probably allogenic in origin. 



Slices 41 to 46, inclusive, are all examples of JSTew South 

 Wales leucite basalts. I collected them from localities where the 

 rocks were developed to most advantage. A considerable likeness 

 will be found to exist between parts of these slides and some of 

 the typical leucite basalts figured by Fouque and Levy* in their 

 splendid work, and by Zirkel in his work on the Petrography of 

 the Fortieth Parallel (Plate 5, fig. 4.) 



Prof. T. W. E. David late of the Geological Survey of New South 

 Wales was the first to describe tachylytes or basalt glasses from 

 this Colony, f Basalt glasses are by no means uncommon as 

 fragments and lapilli in basaltic country. But it is rare to find 

 this interesting rock in situ. The best occurrence with which I 



* Mineralogie Microgaphique, Pis. 48, 49, 50, and 51. 



f Geology of the Vegetable Creek Tin-mining. Field, Government 

 Printing Office, Sydney 1887; and Proc. Linn. Soc. N. S. Wales, Vol. n., 

 (New Series) p. 1078. 



