OCCURRENCE OE POKPHYKITIC INTRUSIONS AT YASS, N.S.W. 187 



coral, Tryplasma liliiformis; the sponge, Carposponyia, 

 and the Annelid, Arenocolites is evidence of the age. 



That the sediments were marginal upon the Silurian sea 

 is adduced by the presence of rain prints, ripplemarks and 

 suncracks in the three sediments. These occur in the 

 paddocks adjoining the School Grounds. Thus the marginal 

 deposition of such a large thickness would point to an area 

 of slow subsidence. Subsequent to the deposition and con- 

 solidation the quartz porphyry sills were intruded between 

 the bedding planes probably not reaching the surface. 



It is therefore necessary to assign a post-Silurian age to 

 the intrusions. This conforms with the general character- 

 istics of Silurian deposition in New South Wales. The 

 Silurian Period in this country was one of quiet deposition, 

 volcanic activity and lava flows being the exception rather 

 than the rule. In the south, the Murrumbidgee Area 

 received large intrusions and volcanic flows during the 

 Devonian Period and this suggests that the intrusion of the 

 Yass province occurred during Devonian times. Concurrent 

 with the intrusion, the contact breccia, and brecciated 

 limestones were formed, the sandstone silicified and the 

 faults produced. Subsequent erosion has denuded the por- 

 phyries which still form the resistant residuals of the 

 locality. 



Bibliography-. 



i. A. J. Shearsby, "The Geology of the Yass District," Aust. 

 Ass. Adv. Sci., Vol. xin, 1911, pp. 106 - 119. 



ii. C. A. Sussmii.ch, "Geology of New South Wales," pp. 34-37. 



