902 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



or very pale greenish variet)- (salite). The felspar of the base is 

 essentially albite. The base contains no olivine or basic felspar, 

 and it consists essentially of albite, aiigite, magnetite, and a 

 little apatite. 



This rock is formed either by a mixture of reliquefied basic 

 rock with alkaline lava, or by the extrusion of the last basic 

 residuum of au alkaline magma. It is typical of a large number 

 of varieties of very felspathic and almost olivine-free basalt and 

 augite andesite, which occur capping the older trachytes and 

 phonolites south of Eulah Creek. The basalt from the Sandilands 

 Ranges, Xew England, is very like this rock in section and 

 handspecimen. 



I have tried in the foregoing notes to give the main features 

 of all the rock-varieties met with in the Nandewars, rather than 

 tedious detailed petrological descriptions of a few types, with the 

 special object in view of inviting comparison. 



All these rocks have certain features in common, from the 

 most acid to the most basic; most striking correspondence is 

 exhibited in 



(1) The predominance of felspars rich in soda. 



(2) The abundance of zirconium and titanium minerals, such 

 as arfvedsonite, katophorite and zircon in the trachytes; wohler- 

 ite (?), ilmenite, cossyrite (?), etc, in the phonolites; rutile and 

 sphene in the more basic rocks. 



(3) The rarity of magnesian minerals, such as olivine, even in 

 the most basic rocks. 



(•4) The prevailing tendency in all to verj' marked porphyritic 

 structure, and in very many to vesicular structure, even amongst 

 sill rocks; the porphyritic structure points to a period of cooling 

 in a deep-seated reservoir during which the minerals of the first 

 generation formed; the vesicular structure suggests that masses 

 of water or water-vapour (charged with mineralisers as shown 

 by the rare minerals) gained access to the cooling niass and gave 

 it renewed mobility, enabling it to force its way along all weak 

 points between sedimentary beds to form sills, and to force 

 openings to the surface, whence it flowed as lava streams. 



