Maeshall. — On Tridymite-Trachytc of Lyttelton. 379 



100-06 100-69 101-08 100-10 99-75 



The large percentage of silica combined with such a con- 

 siderable percentage of lime is a very unusual feature, but 

 would be expected when the large amount of plagioclase is 

 taken into consideration. The excess of silica over and above 

 that required for combination wdth the bases is of course 

 present in the mineral tridymite. The analyses tend to show 

 that the feldspar present is chiefly a species of plagioclase — 

 i.e., a soda-lime variety — and this is confirmed by optical 

 examination. 



The presence of manganese was indicated by the bluish- 

 green colour of the fusion mass, but this was not strong 

 enough to warrant a special quantitative determination. 



The percentage of magnesia shows that some ferro-mag- 

 nesian mineral is present in small quantity, but microscopical 

 exainination shows that they are practically absent. Some of 

 the larger accumulations of iroii- oxide may possibly indicate 

 the former presence of some ferro-magnesian mineral which 

 has been re-fused owing to the relief of pressure, or some 

 other changed condition subsequent to its original crystal- 

 lization. 



Phosphoric acid is of course accounted for by the presence 

 of apatite crystals. No test w^as made for this substance in 

 the last three analyses. 



In the first two analyses the iron was all converted to the 

 ferric condition before precipitation, and no determination of 

 ferrous oxide was made. 



From these analyses a rough approximation may be made 

 as to the amount of free silica present in the form of tridy- 

 mite, for, assuming that all the potash is in combination with 

 silica in sanidine, and all the soda goes to make up tibe- ^ 

 plagioclase (andesine), the excess of silica over that rp^i^ab/i/ 



Of/ 



