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Transactions. 



many cases it is moulded on the larger crystals of sanidine in 

 the base, and has evidently separated out at a late period in 

 the consolidation. Small irregular flakes of a greenish augite 

 also occur, but it is very difficult to differentiate them from 

 the blue amphibole, their colour and faint pleochroism being the 

 special criteria for discrimination. 



This description of the rock shows that it belongs to the 

 phonolitic variety of trachyte, using that term in its general 

 acceptation — viz., a trachyte which contains sanidine (and anor- 

 thoclase), with alkali iron pyroxenes or alkali iron amphiboles. 



The microscopical examination thus explains the peculiarities 

 in the chemical analysis. The fairly large percentage of soda is 

 due to the presence in large quantities of the soda amphibole 

 and the green augite. The practical absence of lime shows the 

 absence of all plagioclase feldspars except albite, and, taken in 

 conjunction with the poorness in magnesia and the absence of 

 any other mineral explaining the percentage of iron-oxide, it 

 shows that amphibole is most probably an almost pure soda- 

 iron variety. This may contain a small proportion of mag- 

 nesia, although the presence of this oxide may be due to the 

 fragments of augite in the groundmass. The high percentage 

 of silica, and its presence in the free state in the groundmass, 

 though rare in trachytes, seems undoubtedly to occur in those 

 of orthophyric character (vide Rosenbusch s " Elemente der 

 Gesteinslehre "). The tridymite trachyte dyke from the Lyttel- 

 ton-Sumner Road described by Marshall also shows a high per- 

 centage of free silica ; but he came to the conclusion that this was 

 of secondary origin, whereas it appears to me to be a primary 

 constituent in the groundmass of this rock. I include the 

 analysis of the tridymite trachyte made by Marshall for the pur- 

 poses of comparison, as well as all other analyses which I have 

 come across of the trachytes and related rocks belonging to the 

 Mount Pleasant series. 



