122 G. D. OSBORNE. 



Petrographically the former specimen is very felsitic 

 with a little flow structure and a few phenocrysts of quartz* 

 The Survey collection slide is of greater interest. Micro- 

 scopically it consists of quartz and felspar in a crypto- 

 crystalline groundmass which is fluidal in places. This 

 criterion coupled with its tuffaceous nature is significant 

 as regards origin. Many small devitrified cuspate bodies 

 and examples of spherulitic aggregates are present. 



The quartz phenocrysts are clear and free from strain 

 effects. Partial corrosion has occurred in some cases. A 

 few of the felspar grains have properties suggestive of 

 anorthoclase, but the grains in question are very altered, 

 and hence the point is in doubt, Intergrowth of quartz, 

 and felspar occurs in one or two very small patches in the 

 base. Radial chlorite is also present. As a whole the 

 rock appears very similar to some of the Pokolbin and 

 Ourrabubula types. It is strongly suggestive of the New 

 South Wales Carboniferous facies. If derived from a Car- 

 boniferous terrane thousands of feet below, a possible 

 southward extension of the Kuttung Series as far as Sydney 

 is indicated. 



(c) Ultrabasic Plutonic Rocks. — Xenoliths of these 

 rocks are less numerous in the breccia than in the basalt. 

 However the types in both are identical and a detailed 

 description of the specimens in the basalt is given below- 

 The types recognised in the breccia are harzburgite and 

 lherzolite. 



(d) Granitic Rocks. — A short description of a granite 

 inclusion found at the Basin is given by Prof. Benson. 1 A 

 similar inclusion has been examined by the present writer, 

 and a comparison of the two slides shows that they are 

 almost identical. In hand specimen the rock in the writer's 

 collection shows a gneissic appearance, and while generally 



1 Proc. Linn. Soc, N.S.W., 1914, p. 453. 



