184 W. N. BENSON. 



pebbles in Bullawa Creek in the Nandewar Mountains, over 

 one hundred miles N.N. W. of Nundle and Goonoo Goonoo. 



With the exception of Mr. Wilkinson's important dis- 

 covery that these rocks overlie the normal Tertiary olivine 

 basalt, and the writer's discovery of them at Nundle cap- 

 ping a high hill, we have little direct evidence of their mode 

 of occurrence. Nevertheless an observation made by Dr. 

 Woolnough is of importance in this question. He ascended 

 Mount Warrawalong thirty miles W.S.W. of Newcastle and 

 over a hundred miles south of the occurrences described 

 above. He there found the Tertiary basalt cap penetrated 

 by dykes of coarse-grained olivine dolerite. He has gener- 

 ously handed specimens of this rock to me for description. 



Microscopically the rock is seen to contain large elongated 

 ophitic crystals of dark purple augite up to two centimetres 

 in length, with felspar tabulae and interstitial white zeolite. 

 The microscope shows that the augite is strongly titanifer- 

 ous, and has the hour-glass structure developed to some 

 extent. A peripheral zone of a greenish colour is some- 

 times developed, the colour-change being usually gradual, 

 though in some instances quite sharp, as though the crystal 

 had grown by secondary enlargement during a later epoch 

 of pyroxene crystallisation. The felspar is chiefly ortho- 

 clase, in large or smaller prismatic crystals, idiomorphic 

 against a yellowish zeolitic ground mass. It is mostly 

 glassy and untwinned, but decomposition is commencing 

 along the cracks. Plagioclase tabulae are also present. 

 The refractive index is distinctly above that of the Canada 

 balsam. There are large crystals of olivine, irregular plates 

 of ilmenite, slightly leucoxenised, and numerous large 

 apatite prisms. Interstitially there is a considerable 

 amount of an almost isotropic yellowish zeolitic material, 

 (analcite ?), like that in the northern rocks. Imbedded in 

 this are minute graphic fragments of green augite, similar 



