BY W. N. BENSON. 585 



chalcedony may be obtained, together with botryoidal masses of 

 magnesite. Here, too, the rocks may be thoroughly leached, and 

 left as a siliceous sinter, more or less filled with haematite. This 

 was described by Wilkinson, in 1885, as due to geyser-action(25). 

 While this is by no means probable in the strict sense, it is cer- 

 tainly the effect of ascending, hot, siliceous solutions. 



The date at which the great silicification of the Woolomin Series 

 took place, is as yet uncertain. Probably it was during the oro- 

 genie period. Huge quartz-veins were formed through the Woolo- 

 min rocks, and to a much less extent in the Bowling Alley Series, 

 while they are absent from the Nundle beds. While these veins 

 are found in the dolerite (with epidote), the granites and por- 

 phyries are quite subsequent to them. 



6. Granodiorites, Granites, and Porphyries. — There is no 

 direct evidence as to the relative age of these rocks and the 

 Permo-Carboniferous, but owing to the want of metamorphism in 

 the latter, and the slight resemblance between the granites, etc., 

 and the oldest types described by Mr. Andrews as Carboniferous, 

 the Nundle rocks are tentatively classed as of that age. As will be 

 seen, the granite probably underlies the greater portion of the area 

 at no very great depth. The largest exposure is on Duncan's 

 Creek, and is eight or ten square miles in area. Its composition 

 has not been thoroughly investigated. Some variation was noted 

 in the field; the specimen collected proved to be granodiorite, 

 orthoclase being subordinate to plagioclase. It was noted that 

 the upper surface of the mass had only a slight inclination, as 

 along the stream-scared, western slopes, the boundary of the gran- 

 ite swung back and forth in sympathy with the contour-lines. All 

 around the boundary is an immense number of intrusions of por- 

 phyry of several types, a dark blue fine-grained rock with white 

 felspar and black hornblende-crystals, being most abundant. 

 About fifty of these have been mapped, but there are many more, 

 particularly just above the granite-boundary on the steep slope 

 above-mentioned. The long, northern point of the massif passes 

 into this rock, and intrusions of the porphyry, into the grano- 

 diorite itself, have been noted further to the south, below Yerro- 



43 



