VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 143 



certain changes in the differentiates went on in the intra- 

 telluric stage of the crystallisation of the basaltic magma, 

 and ceased on completion of the activity, which is expressed 

 by the basalt plugs and dykes, after the consolidation of 

 which, changes in the minerals of the inclusions took place, 

 which are referable to katamorphic agencies. 



The granophyric intergrowth between pleonaste and 

 monoclinic pyroxene is characteristic of both localities and 

 occurs in gabbroic rocks which are olivine-bearing. The 

 intergrowth of augite and picotite is peculiar to the Basin. 



Fart III. Summary and Acknowledgements. 



At the Basin, Nepean River, there occurs a volcanic 

 neck which evidences at least two stages in its past 

 activity. The first epoch was one of explosive violence 

 when a fine-grained breccia consisting mainly of fragments 

 of sedimentary rock was formed. Three dykes and an 

 irregular plug of basalt, which intrude this breccia, form 

 the second phase of vulcanicity. The formation of the 

 neck, with the production of a long narrow vent, has been 

 due to explosive action concentrated upon a weak fissure 

 structure, lying more or less perpendicular to the Eastern 

 Cordillera. There is a genetic relation between the igneous 

 activity and the earth movements of late Tertiary times. 



The Basin neck has formed an important unit in the 

 physiographic evolution of the Nepean-Warragamba system 

 during Oainozoic times. 



The breccia consists of fragments of quartz, plagioclase, 

 chert, quartzite cemented by chloritic and kaolinic material. 

 It is hard to say whether the two dykes and plug of basalt, 

 which do not contain xenoliths, on the one hand, and the 

 inclusion-bearing dyke on the other hand, were contem- 

 poraneous or not, because the former may contain inclusions 

 at lower depths, and also since a similar association is to 



