VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 119 



lying to the west of Norton's Basin, trending meridionally 

 and representing the southward continuation 1 of the sen- 

 kungsfeld structure whicli exists west of Kurrajong. The 

 location of the Springwood, Euroka Creek, and Mountain 

 Lagoon 2 volcanic necks also points to a local genetic con- 

 nection with some such zone of weakness. 



Speaking of the intimate relation between the epeirogeny 

 and the vnlcanicity mentioned above, Dr. Jensen says, "The 

 extrusion (and intrusion, G.D.O.) of basalt, may be here, as 

 elsewhere, a phase, function or effect of a senkungsfeld 

 formation." 3 To the author the last suggestion seems the 

 correct one. 



Physiography. 



(a) Local. — The purely local physiographic detail can be 

 described in a few words. The neck has been considerably 

 eroded leaving a large area of excavation, accomplished 

 mostly by the Nepean River. This stream flows within 

 the neck for about half a mile of its course, and part of its 

 northern bank is formed by a precipitous scarp which 

 represents approximately the junction of sandstone and 

 breccia. 



Norton's Basin is a deep pool of water, with an average 

 depth of fifty feet, and has been produced by the scooping 

 action of the Nepean on the soft breccia. 



(b) Regional* — Briefly a few points may be mentioned 

 with regard to the relation of the position of the neck to 

 broader problems of physiography in the surrounding dis- 

 trict. On the heights adjacent to the Basin fairly extensive 



1 A preliminary examination by the writer of the section from Emu 

 Plains to the Basin resulted in the observation of westerly dips on the 

 sandstone over a considerable distance, suggesting a continuance of the 

 western limb of the Olenbrook anticline. 



2 Prof. David, Ann. Add. Roy. Soc. N.S.W.. 1896, p. 59. 



3 Journ. Roy. Soc. N.S.W., Vol. xlt, 1911, p. 257. 



