Marshall. — On Tridymite-Trachyte of Lyttclton. 385 



No augite was seen, but a highly-dichroic mica with 

 parallel extinction, probably belonging to the biotite series, 

 was present in a few crystals. 



The ground-mass consists of crowds of interlacing crystals 

 showing parallel extinction, their length being many times as 

 great as their width. Their polarisation colours are greys and 

 bluish-greys, not so high as those of the feldspars. They 

 would appear to be orthorhombic zeolites, though no fibrous 

 aggregates have been detected. 



Microscopically, the rock is of a greenish-grey colour, with 

 shining lustre in places. The specific gravity is 2-65. A 

 quantitative analysis resulted as follows : — 



102-31 



These percentages bear a certain resemblance to those in 

 phonolites, the high percentage of alkalies and small percent- 

 age of lime being particularly characteristic. As, however, 

 the rock is evidently considerably decomposed, and no speci- 

 mens of the unaltered rock could be obtained, little reliance 

 could be placed on the analysis. 



On digestion in cold hydrochloric acid a large quantity of 

 gelatinous silica separated out, a reaction eminently charac- 

 teristic of phonolites. It is highly probable, however, that the 

 gelatinous silica is in this instance a product of the decom- 

 position of the zeolites. 



Judging from its chemical composition, it would appear 

 highly improbable that this rock has a common origin with the 

 tridymite-trachyte. No other rocks were found in the neigh- 

 bourhood of the tridymite-trachyte except a few small dykes 

 Gin. to 18in. wide, and, as they obviously have no bearing upon 

 the origin of "the lava-flows, they were not submitted to a 

 critical examination. 



In considering this small subsidiary eruption of the Lyt- 

 telton system as a whole, it may be said that the order of the 

 extrusion of the rocks is wholly in accordance with Durocher's 

 law of succession of igneous rocks. First we have an inter- 

 mediate lava represented by the black trachyte ; next comes 

 the tridymite-trachyte, as a representative of the acid group ; 

 and finally the basic rocks, as represented by the basalt. 

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