OCCURRENCE OF PORPHYRITIC INTRUSIONS AT YASS, N.S.W. 18S 



the Douro and Laidlaw porphyries, the dip of the junction 

 and the shale being to the west. 



A few chains west of the junction of the Yass Beds and 

 the Douro porphyry is a margin of contact breccia (Plate X). 

 This contains quartz and felspar, which has included large 

 fragments of shale in its intrusion towards the surface. As 

 the molten magma was injected between the bedding plane 

 of the Hume- Yass Beds, it tore from the upper contact 

 surface portions of the sediments, and these undigested 

 fragments became enclosed in the magma, forming a 

 breccia. The actual thickness of the breccia is about 15 

 feet and the breccia strikes parallel to the Yass-Douro 

 junction. 



A similar occurrence of breccia to the west of the Laid- 

 law-Hume junction forms an important physiographic form 

 on the way from Yass to Hatton's Corner. Here a ridge 

 of resistant material on the crest of a steep slope marks 

 the position of the breccia. The breccia on the Laidlaw- 

 Hume junction is about 20 feet in thickness and strikes 

 parallel to the upper junction. 



Associated with each of these breccia are thin sills of 

 fine grained porphyries, a felsitic phase of the main mass. 

 These occur to the west of the large masses, and are thus 

 on the upper contact surface. 



The felsite is a fine grained rock with well marked joint 

 planes, splitting in large hexagonal and rectangular prisms. 

 The hexagonal prisms would certainly suggest an igneous 

 origin for the felsite. The outcrop in the Booroo Ponds 

 Creek, a few chains from its junction with the Yass River, 

 is very compact arid contains phenocrysts of quartz. It 

 conforms with the bedding plane of the overlying shales 

 and has included in its intrusion fragments of the basal 

 beds of the Hume phase of sedimentation, many fragments 

 being fossiliferous. 



