ON THE DEPENDENT ISLES OF TAIWAN. 51 



Plagioclase is long- rectangular along the zonal axis at right- 

 angles to 010, and tabular when parallel to that face {Fig. 5). 

 It varies in size so that between the phenocrystic felsj)ar and that 

 of the ground-mass we could find a series of dimensions. 

 Zonary structure is typically developed in almost every indivi- 

 dual, especially on the tabular section of 010. It contains, as 

 usual, glass arranged in zones ; sometimes encloses crystals of 

 augite and hornblende, parallel to base and the positive dome ; 

 it extinguishes light in symmetrically opposite directions with the 

 maximum angle of 30°-34°. The extinction observed on 010 

 amounts to 15° with reference to P/M, the trace of the peric- 

 line twins making 1.5° with P/M on the same face. These rough 

 observations all point to the labradorite-nature of the felspar. 

 Hornblende occurs only as the phenocryst and small in quantity. 

 It is a brownish-green variety of optically normal character. 

 The crystals are all corroded and enclosed by the opacitic 

 margin {Fig. 5) which is composed of confused aggregate of crys- 

 talloids and grains of monoclinic pyroxene, and clumps of mag- 

 netite. The pyroxene appears in tolerably large size that it could 

 be opticall}^ ascertained. Sometimes the substance of the margin 

 has been replaced by broiniish, double-refractive fibres. In one 

 slide the body of the hornblende is impregnated with countless 

 swarms of black dots which lend to the crystal a darker shade. 

 With high powers, they resolve into glass-enclosures with bubbles. 



Augite occurs sporadically as a phenocryst. Its coarse dis- 

 tinct cleavage, 2:)ale colour, and small angle of extinction (less 

 than 32°) prominently characterise this pyroxene, and contrast 

 pronouncedly with the brown, Andésite augite. That it is diop- 

 side is highly probable, but not proven. In one slide, I observed 

 porphyritic aggregate of needles, producing the glomeroporphy- 



