VOLCANIC NECK AT THE BASIN, NEPEAN KIVEH. 113 



THE VOLCANIC NEOK AT THE BASIN, NEPEAN 



RIVER. 



By G. D. Osborne, 



Deas-TJiomson Scholar in Geology (1919) University of Sydney. 



(Communicated by Professor T. W. Edgeworth David.) 



With Plate VII. 



[Read before the Royal Society of N.S. Wales, September 1, 1920.'] 



CONTENTS. 

 Part I. General. 



Introduction. 



General Geology — Relation of the Neck to the Tectonics of 



the Area. 

 Physiography — (a) Local, (b) Regional. 

 Part IT. Petrology. 



1. The Breccia. 



2. Inclusions within the Breccia. 



3. The Intrusive Basalt. Dykes and Plugs. 



4. Inclusions in the Dyke No. 1. 



5. Relation between the Basalt and Xenoliths. 



6. Comparison with the Dundas Rocks. 

 Part III. 



Summary and Acknowledgements. 

 Explanation of Plate. 



Part I. General. 

 "The Basin" is the name given to the locality at the 

 junction of the Warragamba and Nepean Rivers, twelve 

 miles south of Penrith. In this area there occurs a vol- 

 canic neck which in the following pages, will be designated 

 the "Basin" neck. Similar in general features to many of 

 the Post-Triassic pipes in the Sydney-Blue Mountain dis- 



H— September 1, 1920. 



