804 GENERAL GEOLOGY OF MARULAN AND TALLONG, N.S.W., 



Under the microscope it is a dark green, granoblastic rock, with 

 uniform grain-size about 0-27 mm. It consists ahnost entirely 

 of dark green pyroxene, which, in thick section (0-07 mm. thick) 

 is decidedly pleochroic. The pleochroism suggests an amphibole, 

 but cleavage, and optic orientation and sign are those of a pyrox- 

 ene. The pleochroism is peculiar — a = dark green; b =- yellowish- 

 green; t = dark green. 



There is a little sphene of two distinct types : i. dark yellow, 

 semiopaque and probabl}^ primary; ii. greyish and transparent, 

 probably secondary. A small amount of quartz, interstitial in 

 character, appears to be secondary. There is a very small 

 amount of a greyish aggregate, very fine-grained and nearly 

 opaque; this is probably completel}'- saussuritized (or sericitized) 

 felspar. One striking feature is the relative abundance of 

 apatite in the form of large, clear crystals, up to 0-8 mm. by 

 075 mm. 



The line of demarcation between the eruptive and sedimentary 

 rocks probably lies between Nos. 7 and 8 of this series; hence it 

 is best to consider the remaining three specimens in reverse 

 order. 



No. 10 is the normal grano-diorite. Macroscopically it differs 

 from the Glenrock type in being a little more leucocratic in 

 aspect. While this is so, the individual dark minerals are some- 

 what larger than in the other rock. Microscopicall}' it is very 

 similar to the type-rock from Glenrock Falls (p. 795). One point 

 of difference is the form and character of the felspar. There is 

 no orthoclase at all, and the plagioclase is relatively basic, and is 

 \^ery strikingly idiomorphic. It is strongly zoned, the kernel 

 varying from labradorite at the centre, to andesine on the outer 

 part; and this kernel is surrounded by an untvvinned, but 

 optically continuous zone of felspar, whose refractive index is a 

 little higher than that of quartz, and, therefore, classed as oligo- 

 clase. The felspars are considerably sericitized, the alteration 

 often being confined to the outer portion of the kernel. 



There is no recognisable pyroxene, as there is in the Glenrock 

 type, but hornblende and biotite are quite similar. There is a 



