GEOLOGY OF YASS DISTRICT. 119 



breccia, which are found both above and below it. When I first 

 had my attention drawn to this No. 3 porphyry, I looked upon it as 

 decidedly intrusive, the beds above and below looking like altered 

 sedimentaries, between which the igneous rocks had been forced, 

 similar to the case mentioned by Mr. P. T. Hammond at Melrose. ^ 

 Later investigations, however, have made me decide otherwise, 

 and I have been further strengthened in my o])inion by the help 

 given me by Dr. Woolnough's examination of the rocks in the 

 vicinity. 



That Nos. 1 and 2 porphyry are due to a similar origin is also 

 probable, as tufts and fossihferous beds are found associated with 

 them. Some parts certainly have the appearance of intrusions, and 

 may be the result of the action of the post-Devonian porphyries, 

 which Harper points out have caused such havoc amongst the 

 Silurian sediments at Boambolo. 



The quartz porphyry between the conglomerate and the felspar 

 porphyry of Bowning Hills might be termed for the present No. 

 4 porphyry. It forms the country to the south and south-west of 

 Bowning Hill, and the lower parts of Derrengullen and Bowning 

 Creeks run through it. It crosses the Yass River, and continues 

 on to the Murrumbidgee, through the Parishes of Warroo and 

 Boambolo, thus joining up with the Yass porphyries of Harper. 

 This No. 4 porphyry is therefore directly in touch with it, and I look 

 upon it as part and parcel of Harper's post-Devonian intrusion. 



Another reason why I consider some of the porphyries are much 

 earlier than the Devonian, is that the Bowning conglomerates are 

 mainly composed of porphyry, alike in every respect to that ob- 

 tained in Nos. 1, 2, and 3 belts. Not only is the uppermost bed of 

 porphyritic origin, but the lowest, which are only thin beds, contain 

 the rounded porphyry pebbles. The interbcdded shales, being of 

 decided Silurian age, show that the conglomerates were laid down 

 during Silurian times, and from their composition prove the existence 

 at that time of beds of porphyiy. 



Other references to the Yass Porphyries iind Granites : — 



Clarke, Rev. W. B. : " Parliamentary Blue Book," 1853, p. 84 ; "Southein Gokiaeld? of X.S.W.,'* 



1860, pp. 103, 104, 105, 236. 

 Wilkinson, C. S. : Ann. Rep. Dept. Mines and Ag., iV.S.If., 1876, p. U9, 155. 

 Leigh, W. S., and Etheridge, R., |unr. : Ann. Rep. Dept. Mines and Agr. A'..S.H''., 189:J, p. 1 ::4. 

 Carne, ]. E. : Ann. Rep. Dept. .Mines and Ag. N.S.W., ISiH. 

 Watt, A. J. : Ann. Rep. Dept. Mines and Ag. N.S.W., 1897, p. 186, 188. 

 Baker, R. T. : " Building and Ornamental Stones of N.S.W.," 1909, p. 26, 70, 72, 74.:; 



EXPLANATION OF MAP AND PLATES. 



Plates IIL^ — \"[\. — Photographs of Silurian Formations and Porphyry 

 Outcrops Yass District. 



Plate VIIL — Geological Sketch Map of the Y'ass District. 



IlHmraond, P. T. : " Note on Intrusive l^orpbyry at Melrose," Rec. Geo. Surv. N.S.\V., Vol. III., p. 33. 



