892 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



this mountain there is probably a large laccolitic mass, fiom 

 different levels of which the sills are offshoots. All these rock& 

 are undoubtedly differentiation-products of the one mass, present- 

 ing numerous features in common, chief of which are (1) the 

 occurrence of nepheline and pseudomorphs after nepheline, (2) 

 the occurrence sparingly of apatite phenocrysts of a greenish 

 colour capped with pyramids at both ends and exhibiting a fine 

 longitudinal striation, (3) the occurrence of soda-bearing pyroxene 

 in the base. 



The chemical analyses correspond excellently to the petro- 

 logical compositions. Thus N.ll, though the darker in colour, 

 is much more acid than N.12, and contains much less of the iron 

 oxides and lime. .The dark colour of X.ll is due to the pheno- 

 crysts consisting of dark cloudy microj^erthile felspar, while the 

 more basic phenocrysts of N.12 consist of white labradorite. 



The alteration-products in these rocks are hard to determine. 

 I have spent much time in trying to work out the exact nature 

 of the fine micaceous pseudomorphs without avail. There is, 

 however, no doubt that they sometimes approximate very closely 

 to liebnerite and sometimes to geisikite. though usually 

 analcite, calcite and zeolites occur with them. These minerals 

 are secondary after nepheline and are associated with others 

 such as cancrinite and katapleiite of similar origin. Sometimes 

 the aggregates show definitely the outlines of the original 

 nepheline crystals. 



I have slides of nepheline tingiiaite from the range between 

 Spencer's Creek and the Snowy Eiver (Guthrie Range), Mt. 

 Kosciusko, which show exactly the same decomposition-products. 



Associated with these minerals in the Nandewar nepheline 

 rocks are certain chloritic, sericitic, and pinite alteration-products 

 after regiiine-augite, felspar and soda-hornblende. This class of 

 alteration-products resembles the other so closely that it has not 

 been found possible to distinguish betw^een them except where 

 they form definitely pseudomorphs of regular outline. 



All the alkaline rocks so far described contain an abundance 

 of minerals which gelatinise with dilute HCl and stain with 

 malachite-green. 



