Maeshall. — On Tridymite- Trachyte of Lyttelton. 383 



The name tridymite-dacite would give the idea of presence 

 of free sihca, as well as the preponderance of plagioclase over 

 sanidine, but would imply the presence of augite or horn- 

 blende. On the other hand, the feel of the rock, its specific 

 gravity, the absence of a glassy base, and its very light colour, 

 as well as, to a certain extent, its highly acid composition, all 

 tell largely in favour of its being pronounced a trachyte ; and 

 I have therefore preferred to call it an andesine-tridymite- 

 trachyte, the two prouomens being used to indicate its excep- 

 tional mineralogical characters. 



In order to demonstrate the relations of the neighbouring 

 rocks to this trachyte, several sections have been cut and 

 some analyses made of the lavas that appear to have pro- 

 ceeded from the same orifice, as well as of the intrusive dykes 

 and other neighbouring rocks. 



Immediately underlying the trachyte, and undoubtedly 

 proceeding from the same vent, is a black lava that resembles 

 pitchstone except for large porphyritic crystals of feldspar. 

 Cavities are fairly numerous, but not so abundant as in the 

 overlying rock. Sp. gr. 2-4:6. 



Under the microscope there are large idiomorphic crystals 

 of sanidine, and occasionally plagioclase surromided by sani- 

 dine, as in the overlying rock. The sanidine is generally 

 twinned on the Carlsbad type, some splendid crystals of this 

 twin being obtained. The plagioclase is twinned after both 

 the albite and Carlsbad laws, but does not present such in- 

 teresting or marked combinations as in the overlying rock. 



Augite of a bright-green colour occurs in idiomorphic crys- 

 tals, generally of small size, as well as in microlites. It is 

 slightly dichroic, but calls for no special mention. 



Magnetite and a^^atite are present in fair abundance. 

 The ground - mass consists of microlites of augite and 

 plagioclase and abundant interstitial brown glass. A quan- 

 titative analysis gave the following result : — 



SiO^ 



AUOs 



Fe^Os 



CaO 



MgO 



K,0 



Na^O 



102-44 103-44 



The rock appears to be a dark trachyte, with a large per- 

 centage of plagioclase, and, from the close resemblance that 

 the analysis bears to that of the overlying rock, there can be 

 no doubt that they both have a common origin. The geologi- 



