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



0-09 mm. Phenociysts of plagioclase, h3^persthene and quartz 

 and nests of uralite. Groundmass consists of orthoclase, quartz, 

 and biotite. 



Plagioclase : idiomorphic crystals 1 mm. to 3 mm. in length, 

 with sharply defined outlines. These are very much decomposed, 

 and it is extremely difficult to recognise internal structures. 

 External form shows that the mineral is twinned on the Carlsbad 

 law, and traces of albite twinning can be detected also. Rather 

 unsatisfactory measurements of extinction-angle and refractive 

 index indicate that this felspar is a little more acid than that 

 described for the grano-diorite. One would expect the two to 

 be the same, if the conclusions arrived at above (p. 795) are 

 correct. 



The occurrence of hypersthene is very remarkable. It is fairly 

 abundant in the slides in irregular masses up to 3 mm. in 

 diameter, forming a perfect network filled with quartz and ortho- 

 clase grains identical with those of the base. The whole network 

 is optically continuous; in other words, the hypersthene pheno- 

 cry.sts are pcecilitic towards the groundmai^s (Plate Ixix., fig 3). 

 This is somewhat remarkable considering the order of crystalli- 

 zation determined for the grano-diorite. Smaller individual 

 crystals of hypersthene up to 1 mm. in length also occur. The 

 pleochroism, if present, is very slight indeed. 



No diopside is seen in this rock; it is probably represented by 

 -aggregates of uralite. It is extremely difficult to distinguish 

 between primary and secondary hornblende in this rock. It is 

 probable that a good deal of the araphibole seen is primary. 



Quartz is present in the form of occasional grains up to 1*5 mm. 

 diameter. It is very irregular in outline, and contains bays of 

 the groundmass. 



Biotite appears to be rather one of the minerals of the ground- 

 mass. It occurs in individual crystals, from 1 mm. to 3 mm. 

 in length, sometimes forming larger aggregates. The little 

 crystals are sharply defined. 



In addition to biotite, the groundmass consists of a granular 

 mixture of quartz and orthoclase. Both minerals show traces of 

 idiomorphic boundaries. 



