OF THE DEPENDENT ISLES OF TAIWAN. 19 



often being reduced to a mere grain, and is also traversed with 

 fracturai lines, from which the mineral begins to form a ser- 

 pen tinous substance. The olivines are undoubtedly the intra- 

 telluric products, being sometimes enclosed by an automorphic 

 augite, and large individuals are habitually surrounded by 

 heaps of the crystals of augite (in Andésites, instead of it often 

 hypersthene). Inclusions of gas and liquid are not rare, and 

 the octohedra of magnetite are also found in the olivines. 



Zonal structure of olivine is, as is well known, of rare 

 occurrence, and if it really exist, this could only be discerned 

 either by measuring the optical angles at different portions, or 

 by finding the altered zones in a crystal in consequence of the 

 formation of the minercd rouge. The zone of the red mineral is 

 not constant in position, for, it makes its appearance sometimes 

 on the periphery, at other times in the interior; but, so far as 

 my experience goes, the recurrent zones are never found. Tlie 

 condition under which the isomorphic shells of different chemical 

 compounds are formed in the olivine, seems to depend, as 

 Lagorio^^ and Morozewics'-^ say, mainly on the Blasse moirkiing, 

 that is, the degree of saturation of magma in certain temj)erature 

 and 2^1'essure. In my slide, in which olivine has a red central 

 zone (the Kippai Island specimen), magnetite is scarce, and large 

 in its size and rod-shaped ; while the magnetite-rich rock (the Hoko 

 specimen) has an olivine with an external red zone. Here the 

 magnetite occurs in small isometric crystals and grains. 



The red mineral, that forms the periphery (PL I. Figs. 1, 

 4 and 5) and the kernel {PL I. Ing. 6), differs in habit. The 



1) 'Leber die Natur der Glasbasis, sowie der Krystallisationsvorgänge im eruptiven 

 Magma.' T. M. M. Bd. VIII, ISST. 



2) Jbld. Bd. XVIII, 1898. 



