Smith. — Alkaline and Nepheline Rocks, Westlaud. 133 



product of one differentiated magma, of which the granites 

 represented the first phase. The differentiation would then 

 appear to have been one of increasing alkalinity, followed by one 

 of increasing basicity. The granitic magma was originally 

 slightly alkaline, as microcline and microperthite are common 

 accessory minerals throughout the intrusion, and become more 

 important in the earlier variations represented by the aplites and 

 the pegmatites. The next differentiation of which we have any 

 evidence is that of a granite porphyry, a hypabyssal rock in which 

 the alkalinity reached its highest point before it was masked by 

 the increasing basicity, supposing the whole series to have the 

 same genetic origin. Between the granite porphyry and any 

 of the rocks already described there is a petrological gap both 

 structural and mineralogical which may possibly be represented 

 by the porphyries and porphyrites mentioned as occurring in 

 the detrital rocks, but which are weathered too much for in- 

 vestigation. Chemically, the gap may not be so great as it 

 appears. The author regrets that he had neither the time 

 nor the means at his disposal to make a chemical analysis of the 

 different rocks described, by which method alone could the 

 extent of the apparent gap be determined. Whether further 

 explorations in the field will throw more light on the subject 

 remains to be seen. The granite porphyry is interesting in that 

 it contains riebeckite, an amphibole the occurrence of which 

 has never hitherto been recorded in New Zealand. 



A description of one of the aplites and the porphyry is here- 

 with appended. 



115. Aplite. Megascopic-ally, a fine-grained white rock re- 

 semb ing Carrara marble. Microscopically, that which for con- 

 venience of description may be called the groundmass exhibits 

 granulitic structure, and consists essentially of roughly equi- 

 dimensional grains of quartz, orthoclase, and microcline. In 

 this mosaic, with a tendency to hypidiomorphic structure, 

 occur somewhat larger grains of micrographic intergrowths of 

 quartz and feldspar, microperthite, and microcline microper- 

 thite. A yet stronger development of microperthite is present 

 in roughly rectangular plates without well-defined outlines, in 

 which sometimes the mineral is twinned in accordance with the 

 Baveno law. As accessory minerals, a brown biotite is sparingly 

 distributed in small shapeless plates, and the iron-ores are 

 represented bv a little scattered magnetite dust and a few scales 

 of haematite. ' (Plate XIX, fig. 10.) 



19. Riebeckite granite porphyry. A rock with a cream- 

 coloured ground, with brown spots. A small lens reveals the 

 spherulitic nature of the groundmass sufficiently to enable the 

 rock to be classified as a granite porphyry in the field. Under 



