GEOLOGY OF YASS DISTRICT.. 107 



the two were a continuous sheet. The Bango Beds, however, 

 separate them. Yass Junction (189 miles) is on porphyry, but this 

 gives way to sedimentaries a quarter of a mile to the west. These 

 are a continuation of the Yass Beds of Jenkins.^ The railway cuts 

 these at right angles and enters porphyry again near the 190 mile 

 peg. The width of this strip of porphyry is only about a mile. 



Silurian sediments are again encountered to Bowning (196 

 miles). These beds form the Hume Beds of Jenkins,- and the 

 Bowning Beds of Mitchell.^ They are replaced by porphyry about 

 two miles beyond Bowning. 



Gunning Shales. — These extend in a southerly direction 

 through Gundaroo, on the Yass River, towards Queanbeyan. To 

 the north they extend beyond Dalton. They are pierced here and 

 there by granite and porphyry hills. This part of the section has 

 not been thoroughly examined, but no fossils have been found, 

 although some areas were examined minutely. At Nelanglo Creek 

 near Gundaroo, peculiar markings were found, but their organic 

 origin is doubtful. That the Gunning shales differ from the Jerrawa 

 shales in mineral constituents is evident from the fact that they 

 weather into a much more fertile soil than is met with in the latter. 



Intrusions. — The shales have in many instances been metam- 

 orphosed into slates by intrusions of quartz porphyry and granite. 

 Appearances point to the porphyry being the earlier instrusion, as 

 in places the granite has forced its way through both porphyry and 

 slates, the result being that the porphyry has been crushed and 

 altered. An interesting example of this double intrusion of the 

 shales is to be seen in a creek to the north of the Gunning railway 

 station. In the bed of this creek, about half a mile above where 

 the Dalton road crosses it, the grey shales are noticed to abruptly 

 give place to a peculiar white rock. The transition is very sharp. 

 Closer examination reveals the intrusive nature of the white rock, 

 little tongues of which penetrate the slate, the junction of the two 

 showing a distinct contact selvage. This intrusive rock is without 

 doubt a quartz porphyry crushed and metamorphosed by the more 

 recent grey hornblendic granite, which is found at the same spot 

 intruding both the porphyry and the slates. It is very extensive, 

 outcrops of large area being found many miles from Gunning in all 

 directions. 



Tertiary Deposits. — At Dalton, about four miles north of Oolong, 

 an interesting Tertiary deposit is found lying on the almost vertical 

 edges of the Gunning shales. (PI. III. Fig. 1). It occurs in the village of 

 Dalton, on Wm. Brown's portion 35, and consists of a very compact 

 conglomerate of quartz pebbles and sand, highly silicified. The 

 bedding planes are highly fossiliferous, well-preserved specimens of 

 Miocene leaves occuiTing in abundance. Unfortunately, the deposit, 

 which is apparently only a few acres in extent, has suffered greatly 



1 C. Jenkins : " Geology of Yass Plains," Proc. Linn. Soc. N.S.W., Vol. HI., 1878. 



2 C. Jenkins : Op. cit. 



3 J. Mitchell: "Notes on Geology of Bowning," Proc. Linn. Soc. N.S.W., Vol. I, 1887; "The 

 Geological Sequence of the Bowning Beds, N.S.W.," Kept. Austral. Assoc. Adv. Science, 

 Vol. I., 1888 (1889). 



