VOLCANIC NECK AT THE BASIN, NEPEAN RIVES. 123* 



under the microscope this structure is due to the orienta- 

 tion of the constituents, still there is a suggestion of grano- 

 blastic structure. This point was noted by Prof. Benson 

 also. The rock may therefore have been derived from a 

 mass in which metamorphism had not progressed very far. 

 The occurrence of granitic rocks in these Tertiary necks is 

 very interesting, in that, as it is improbable that they are 

 derived fragments, they must have been brought from a 

 great depth. Recently Mr. H. Yates, B.Sc Iras shown the 

 writer au inclusion of granitic nature from the Hornsby 

 vent, and the possible occurrence of granite in the Mount 

 Giiead neck is to be noted. 1 



3. Occurrence of Basalt. — Other than the three dykes 

 there is an irregular outcrop of basalt near the eastern end 

 of the neck, (see map). It is almost identical in petrological 

 features with the rock in dyke No. 2, slight textural differ- 

 ences being observed. 



The Dyke Rocks. — The general structural features of the 

 dykes are given in a tabular statement in Part I. They 

 are all decomposed and unfit for chemical analysis. 



(a) Dyke No. 2. — In hand specimen this dyke rock is 

 somewhat amygdaloidal but the body of the rock is ex- 

 tremely dense and compact. Microscopically it appears 

 as a very fine grained groundmass in which are set altered 

 phenocrysts. The phenocrysts are all decomposed and the 

 criteria of form and nature of pseudomorph are the only 

 guides to their original nature. Some of the individuals 

 have been olivine showing the characteristic macropina- 

 coidal sections, and poorly developed cross-sections of 

 augite can be made out. Oalcite and possibly dolomitic 

 material have taken the place of most of the phenocrysts 

 although some altered olivine shows pseudomorphism by 



1 M. Morrison, Kec. Geol. Surv. N.S.W., Vol. vir, 1904,, p. 19. 



