160 GREAT SEKPENTIKE BKLT OF NEW SOUTH WALES, iv., 



adinole where tliey are in contact with diabases of the Harz 

 Mountains. 



It is clear that the cherts from the Nundle district do not 

 contain any noteworthy amount of albite. On the other hand, 

 the presence of the albite-aplite above the Munro's Creek sill, 

 shows that locally there was some slight albitic extrusion from 

 the magma. 



Termier has explained(35) the albitisation of some Alpine 

 diabases, by the supposition that soda-bearing soil- waters drain- 

 ing off gneissic areas on to diabases may bring about a replace- 

 ment of lime by soda, concurrently with the decomposition of 

 the ferromagnesian minerals, and he has brought forward an 

 interesting series of analyses in illustration of this. From the 

 nature of the case, this hypothesis is quite inapplicable to the 

 explanation of the Nundle rocks. 



It seems permissible to suggest that albite in spilitic rocks 

 may be either a direct magmatic crystallisation, or may have 

 been concentrated into the magmatic aqueous solutions, and have 

 then replaced the first-formed basic felspar. The albite of the 

 Nundle dolerites and spilites, like that of the British minverites, 

 seems to be chiefly primary. This does not preclude the possi- 

 bility that it may be largely secondary in spilitic rocks in 

 adjacent areas. The conditions, that would determine the one 

 mode of crystallisation or the other, probably depend on the 

 amount of water in the spilite-magma, its source, and its mode 

 and time of escape. 



In the keratophyres, there can be no doubt that, at the end of 

 the sequence of differentiations, the magma was very hydrous, 

 and post-volcanic processes were very active. The felspar of 

 these rocks is almost pure albite, but even here there is no evi- 

 dence of the secondary nature of the felspar. Neithammer, from 

 a study of some Javanese rocks, concludes that the keratophyres 

 maybe albitised porphyrites(23). Sundius states that the felspar 



