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



181 



assisted me on every possible occasion by his advice and 

 kindly criticisms. 



General Geology. 

 From Jerrawa in a westerly direction, a section which 

 has been described by Shearsby, cuts through the following 

 structures: — 



Beds. Thickness. 



1. Jerrawa Beds ... ... ? 



2. Bango Porphyry 



3. Bango Beds 



4. Douro Porphyry ... 



5. Yass Beds 



6. Laidlaw Porphyry 



7. Hume Beds 



8. Bowning Porphyritic Series .. 



1200 feet. 



800 

 1500 

 1200 



650 

 2000 



9 



The Sediments, 



The Jerrawa Beds consist of shale and limestone and 

 contain many characteristic Silurian Corals. 



The Bango Beds, Yass Beds and Hume Beds contain 

 limestone, shale and sandstone. These are all rich in fossils. 

 The close resemblances of these three series suggest a com- 

 mon origin and also that they were laid down during 

 uniform conditions. The only reason for giving charac- 

 teristic names is that of convenience in working on the 

 beds; they were possibly laid down during the same great 

 phase of sedimentation. The beds dip at from 10° to 15° in 

 a direction W. 10° S. 



Fossils are very abundant in each bed; the Hume Beds 

 at Hatton's Corner and Derrengellen Creek being extremely 

 interesting to the palaeontologist. The most abundant are 

 corals : — 



Favosites sp, 



Cyathophyllum sp, 



Heliophyllum Yassense 



