Speight. — Soda Amphibole Trachyte. 183 



A. Trachytoid phonolite, Lvttelton-Sumner Road ; analysed 

 by P. Marshall ; Trans. N.Z. Inst., vol. xxvi (1893). 



B. Trachytoid phonolite, Heathcofce ; analysis by T. Bute- 

 ment ; quoted by H. F. Ulrich, Trans. Aus. Assoc. Adv. Sci., 

 vol. iii (1891). 



C. Vesicular trachyte from agglomerate bed ; analysis made 

 in laboratory of the Geological Survey ; quoted in Haast's 

 " Geology of Canterbury and Westland." 



D. Dyke (side) cut by tunnel, No. 29b, same dyke ; analysis 

 made in laboratory of the Geological Survey ; quoted in Haast's 

 " Geology of Canterbury and Westland." 



E. Dyke (centre) cut by tunnel, No. 29a, same dyke ; analysis 

 made in laboratory of the Geological Survey ; quoted in Haast's 

 " Geology of Canterbury and Westland." 



F. Dyke (centre) cut by tunnel ; analysis made in Paris by 

 Dr. H. Filhol ; " Mission de File Campbell." 



G. Tridymite-trachyte, Lyttelton-Sumner Road ; analysis 

 by P. Marshall ; Trans. N.Z. Inst., vol. xxvi (1893). 



H. Soda amphibole trachyte, Cass's Peak ; analysis made in 

 chemical laboratory, 'Canterbury College ; inserted again for 

 convenience of comparison. 



This list includes nearly all the published analyses of trachytic 

 and allied rocks of this series. There seems to be one or two 

 striking features about some of them. Assuming that they are 

 tolerably correct, those marked D, B, F show an abnormal per- 

 centage of soda, and also a very small percentage of potash ; 

 also A, B, C show an excess of soda over potash. The high 

 percentage of MnO in C is also remarkable ; this apparently ex- 

 plains the presence of frequent thin coatings of a black mineral 

 resembling pyrolusite, which occurs on the fracture surfaces of 

 the rock. Analyses A and B undoubtedly show the characters 

 of a trachytoid phonolite, and C, D, E, and F those of a soda 

 trachyte. These last rocks have anorthoclase as a common 

 phenocryst, but the practical absence of potash in the analysis 

 is rather peculiar. The two analyses G and H afford in- 

 teresting comparisons. The marked agreement of the silica, 

 alumina, magnesia, and the soda are very noteworthy. The 

 only differences appear to be the greater proportion of iron- 

 oxides and the practical absence of lime in H. These peculiarities 

 are explained by the microscopical analysis. There is a fair 

 proportion of plagioclase (andesine) and a very small amount 

 of iron-bearing mineral in the tridymite-trachyte. In his de- 

 scription of the rock Dr. Marshall noted the percentage of mag- 

 nesia without being able to account for it. On looking over a 

 section of it I found in the groundmass a considerable quantity 



