118 O. D. OSBORNE. 



One of the striking facts which the geologist realises, 

 when investigating the post-Mesozoic history of the Sydney 

 area (part of a larger geological unit), is the genetic rela- 

 tionship existing between the Glenbrook fold, with its 

 accompanying geosynclinal development, and the Tertiary 

 vulcanicity of the area. 



The volcanic neck at the Basin, in common with many 

 other necks in the district, was one of the manifestations 

 of this volcanic activity which followed or perhaps in part 

 accompanied the epeirogenic movements. 



The elongated nature of the vent was mentioned in the 

 early portion of the paper, and a careful consideration of 

 the facts observed leads the writer to conclude that the 

 east-west orientation of the vent has in general been 

 determined by the existence of a dominant fissure which 

 formed a weak structure and along which explosive action 

 was concentrated. 



Such fissures do exist in folded regions, running at right 

 angles to the fold axes, and along them in areas where the 

 folding has been intense dislocation and faulting often 

 occur. In most cases there is a transverse drag between 

 the two portions of the folded block with the production of 

 horizontal slickensides. Suess 1 has described such move- 

 ments in detail. 



In the case in question, the monoclinal folding, due to 

 differential epeirogenic uplift, has been gentle and the zone 

 of Assuring represents the expression of only the initial 

 stages of the development which is exhibited in more 

 intensely folded regions. That the postulated fissure 

 crosses the fold is shown by the occurrence of breccia to 

 the east of the monoclinal flexure. 



Another contributing factor in the location of the vent 

 has been the probable existence of a zone of weakness 



1 Suess, " Das Anlitz der Erde," Sollas, Trans. Vol. i, pp. 115-118. 



