274 



B. KOTO. 



encircled by fibres of chrysotile wbich separate individual grains 

 from each other ; and as several grains behave oi)tically in exactly 

 similar manner, they must have been formerly parts of a larger in- 

 dividual which by serpentinization has now become detached and 

 isolated grains. The olivine-core is filled with the dust of chromite, and 

 also with round or rectangular grains of the spinel group. The spinels 

 are isotropic, and highly refractive, and have theref(3i-e dark margins. 

 Some of the opaque aggregates which might easily be confounded 

 with chromite are surely octahedra of spinels. Examined with high 

 powers the minerals are found coloured either dark-brown or green ; 

 the former may be jjicotite, the latter iileonaste or hercynite. 



The colourless tremolite is bacillar in form, some rods attaining 2 

 mm. in length ; their terminations are never perfect, being always 

 resolved into tufts of fibres which, in turn, eventually pass into a fila- 

 mentous aggregate of serpentine. The substance of the tremolite is not 

 compact, and seems to be extremely thin; it is also traversed through its 

 whole stretch l^y fine divisional lines parallel to the princi])al axis, in 

 consequence of which the mineral displays only aggregate polarization- 

 colours. That the tremolite is the primary component and not a 

 secondary one is proved by the fact that it penetrates the remnant of 

 the olivine, and also cuts sharply the edges of thedismemljered grains of 

 the same mineral. 



Tlie final product of degeneration of both the tremolite and 

 olivine is the well-known serpentinous mass ; the one originated from 

 tremolite appears in the form of splinters of wood, which are thrown 

 too-ether in such a manner as to form an imperfect " bladed " structure 

 (Balkenstructny) of antujorite ; while that resulting from olivine is 

 the fibrous chrysotile wldch produces a characteristic mesh- work, with 

 au-oreu-ates of chromite along the original fissures of the olivine. 



A rock fi'om Saimaru in the collection of the Geological Survey is 



