OF THE DEENDENT ISLES OF TAIWAN. 21 



chemical composition of the two alteration-products, yet on the 

 whole they must be practically identical. The lamellœ are 

 oriented parallel to one of the pinacoids, as may be deduced from 

 the position of the optic plane (in the fresh substance of olivine), 

 which stands at right-angles to the easy cleavage {PL I. Figs. 1 

 and Ö). Pleochroism is distinct; it is brownish-green in the 

 direction of facile cleavage, but greenish-brown when at right- 

 angles to it. Hence, oa or b. MiiggV, however, sa^-s that 

 the absorption is stronger m the direction perpendicular to the 

 ' Längsrichtung ' than in that parallel to it. ZirkeP and Kosen- 

 buch^^ interpret the above statement in the terms, that the rays 

 vibrating parallel to c absorb far less than those parallel to a 

 and b. The observers, however, seem to have examined the 

 mineral rouge. My observation, therefore, accords well with that 

 made by Lawson for iddingsite ; but it is not known to which 

 pinacoid, 010 or 100, the lamelke are parallel, though it is 

 probable that the brachypinacoid is the lamellar plane, as may 

 be inferred from the fact that the elasticity perpendicular to 

 the lamellae is greater {^i = b) than that parallel to the c-axis, the 

 latter corresponding to the mean axis of elasticity. 



With HCl, the iddingsitic mineral becomes bleached, and 

 then acquires a greenish-yellow colour, with corresponding decrease 

 of pleochroism. Bearing in mind the fact of the brachypina- 

 coidal lamellar cleavage, of the colour, and of the chemical com- 

 position which is a hydrous non-aluminous silicate of iron, lime, 

 magnesia, and soda, / am rather inclined to consider the iddingsite 

 to be a mineral approaching to basite. Prof. E.osenbusch^^, 



1) Neues Jahrbuch, 1883, II, S. 205. 



2) 'Pétrographie/ Bd. I, 1893, S. 353. 



3) ' Phjsiographie,' Bd. I., 1892, S. 4G9. 



4) ' Physiogmphie,' 1892, Ed. I., S. 461. 



