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



gathered into quite large flakes of very irregular outline, besides 

 occurring in a finely granular aggregate through the rock. 

 Hornblende has decreased, and pyroxene has relatively increased 

 so as to be the dominant ferromagnesian mineral present, 

 Chlorite has disappeared, but sphene is almost more abundant 

 than in No. 9; and, in addition, there is a notable amount of 

 ilmenite present. 



This brief sketch of the contact-phenomena is sufficient to 

 indicate the great interest which attaches to this particular 

 section; and I hope to present a more complete and detailed 

 account of the occurrence in the near future. 



Contact-metamo7'2}kisJ7i of the Ordovician slates by the grano- 



diorite. 



A very good exposure illustrating this phenomenon exists 2000 

 yards W. from Glenrock Falls. The whole of the rocks forming 

 this slate-area are considerably indurated, but, at the spot men- 

 tioned, the contact can be located to within a few inches. Close 

 to the boundary the sedimentary rock has become very dense and 

 heavy, and dark in colour. On slightly weathered surfaces it 

 has a somewhat silky lustre, owing to the occurrence of abundant 

 submicroscopic flakes or needles of magnetite and micaceous 

 haematite. 



Microscopically it shows a distinct " hornfels " structure. The 

 grain-size is even, about 0045 mm. The rock consists essentially 

 of a quartz-mosaic of completely interlocking grains. There is a 

 small amount of mica (muscovite), which, though slightly yellow, 

 is nonpleochroic. This occurs in ragged flakes of the same order 

 of size as the quartz-grains. The colourless minerals are crowded 

 with minute flakes and grains of haematite and magnetite. At 

 intervals, these minerals are aggregated into dense masses up to 

 04 mm. by 0'04 mm. in size, generally, but not always, associated 

 with muscovite. At first sight these aggregates remind one of 

 the pseudomorphs in magnetite after hornblende, often met with 

 in hornblendeandesites. It is possible that, in the rock under 

 discussion, some mineral has suffered complete alteration, but 



