186 C. W. MANN. 



formed as the result of contact metamorphism. Heated 

 waters were injected into the sandstone of the Yass-Hume 

 series at this point, resulting in the siliciflcation of the 

 sandstone into a quartzite. The rock is very compact and 

 exceedingly hard. It is brownish-pink in colour and has 

 many annelid burrows. The quartzite dips to the west 

 and grades into a sandstone along the line of strike. 



In addition to the above evidences there is the subord- 

 inate, but confirmatory evidence of faults. Numerous 

 small faults occur in the Yass-Hume Beds. At Button 

 Street (O'Oonneltown) is a small trough fault, the fault 

 plane of which strikes in a direction W. 18° S. The throw 

 of the fault, about 3 feet and the heave 1 foot 6 inches* 

 The faulted portion measures 30 feet (faults marked in 

 Plate X). 



The Yass Beds in the bank of the Yass River, near the 

 train bridge, and opposite the Yass District School are 

 faulted in at least two places. The faults occur in the 

 sandstone-shale beds. 



The collated evidence would indicate that the Yass 

 porphyries are certainly intrusive. The dip of the junction 

 as far as can be ascertained, from a number of observations 

 conforms with the bedding plane of the accompanying 

 sediments. Hence the occurrences are sills. In support 

 of the intrusive nature of the porphyry is the holocrystal- 

 line texture of the rock. The high degree of crystallinity 

 would point to the period of slow cooling and hence a sub- 

 surface position. This would not be possible if the por- 

 phyritic masses were lava flows contemporaneous with the 

 sedimentation. Instead of a slaggy upper surface, the 

 contact with sediments has produced a brecciated material. 



Age of the Intrusions. 

 The fossils found in the sediments point undoubtedly to a 

 Silurian age. The presence of the characteristic Silurian 



