182 C. W. MANN. 



Heliolites sp. 



Syringopora and the characteristic Silurian Coral 



Tryplasma liliiformis 



Trilobites and Brachiopods are fairly numerous. 



The Igneous Rocks. 



This paper is concerned mainly with the Bango, Douro 

 and Laidlaw porphyries. These alternate with the sedi- 

 ments and were formerly thought, probably on account of 

 their conformity with the bedding planes of the sediments, 

 to be flows with associated tuffs (refer to map in Plate X). 

 The evidence to hand would suggest that these are intrus- 

 ions. The rocks of the intrusions are typical quartz por- 

 pyries, large phenocrysts of quartz being embedded in a 

 crystalline mass. The rock in places is coarse, approaching 

 a granitoid texture, and is columnar in a few occurrences. 

 It is rich in ferromagnesian minerals and in all places is 

 holocrystalline, many crystals being well developed. 



The Douro porphyry is best examined in the cliffs in the 

 Yass River a few chains above the Yass Bridge. The 

 occurrence here is columnar, large columns measuring 

 several feet along the column face. 



The approach to the cliffs from the town is over the 

 porphyry which is much weathered, the decomposition of 

 the felspar being very advanced. The porphyry here is 

 rich in biotite. On the Laidlaw Trig. Station is a good 

 occurrence of Laidlaw porphyry. The porphyry here forms 

 a hill some 150 feet high at the top of which the porphyry 

 outcrops in large columns. 



Evidences of Intrusion. 

 In order to establish the intrusive nature of the por- 

 phyries, it is necessary to examine the contact on the top 

 surface of the intrusions. Thus the work detailed in this 

 paper, is almost entirely confined to the western edge of 



