OF THE DEPENDENT ISLES OF TAIWAN. 27 



traces of cleavage, but with uumerous fissures ; and has an ap- 

 pearance exactly like olivine. 



The hypersthene possesses a brown colour, and its pleochroism 

 is scarcely discernible. In favourable cases, the ray vibrating 

 parallel to the c-axis is slightly green. Sections present a rough 

 surface, owing to its having a high index, approaching to that 

 of olivine; its polarisation- colour is grey. 



From the brief diagnosis, given above, of the hypersthene, 

 its cleavage, colour, non-pleochroism or very weak if present, high 

 index, but low magnitude of refraction, extinction-direction, and 

 similar chemical composition, — these several physical properties 

 afford no means of discriminating it from a fresh olivine. Olivine 

 has, however, a lighter colour, and has usually but one trace of 

 cleavage in a section. The hy23ersthene on the other hand pos- 

 sesses the characteristic traces of prismatic cleavage, which in 

 a random section gives scarcely a clue to distinguish it conosco- 

 pically from monoclinic pyroxene. A basal section, once observed, 

 presented a square outline, truncated little at the four corners. 



From the combined evidence of more slender section, of the 

 want of decomposition-products, of indifferent behaviour towards 

 common acids, of the presence of comparatively numerous 

 traces of cleavage, I infer, in the Basalts, the presence of a hypers- 

 thene. It is to be remembered that the prismatic sections of 

 olivine show also a low colour of polarisation, exactly like that 

 of a hypersthene. It seems to me that the hypersthene in 

 the Hoko Basalts stands in its chemical composition near to 

 that of bronzite. The want of a distinct pleochroism may be 

 attributed to the same cause. Axial angles, therefore, become 

 large, and the axial poles were not observed in any of the 

 pinacoids by ordinary methods. 



