150 S. SËKlYÂ AND Y. KIKUCHI 



the different nicies of the same magma that supplied the materials 

 for their formation. 



The microscopical examination of these rocks shows that the 

 ground-mass is microcrystalline, with a very little of colourless glass- 

 basis in the lighter coloured variety between the microlithic plagio- 

 clase, while in the darker variety a brown coloured glass is found 

 more abundantly. Numerous magnetite crystals and grains are in 

 both cases always scattered within the general mass, and as enclosures. 



The micro- porphyritic mineral components are Plagioclase, and 

 Pyroxenes, which are represented by the monoclinic and the rhombic. 

 The most frequent accessory component is Apatite. Among second- 

 ary minerals of \es;i frequent occurrence may be mentioned Tridymite 

 and Iron-pyrites. 



The principal characteristics of the porphyritic components are 

 here given : — ♦ 



Play ioclase.— The porphyritic crystals are found generally in lathe-shaped out- 

 line, having characteristic twin-lamellae of the albite-type. The extinction-angle 

 of the plagioclase examined in the sections exhibiting these twin-lamellae varies 

 to a considerable extent ; the range being from 2Ü 3 to 30°. In general character 

 the plagioclase is quite fresh and transparent, often with numerous glass-enclo- 

 sures, which sometimes till up the entire space of the crystal and are arranged in 

 distinct zones. In polarized light, the phenomenon of zonal structure is often very 

 typical ; the extinction angles differing in the inner and the outer zones, thus show- 

 ing a difference in the chemical composition of these layers. 



The specific gravity of the plagioclase as determined by Thonlet's solution gave 

 as a mean 2-ÖS6. These characteristics indicate that the chemical composition 

 should approximate to that of Labradorite. 



Sanidine. — Although this mineral under the microscope is so difficult of detect- 

 ion, we are justified in claiming its existence, since we observed cases in which the 

 basal cleavage face of a glassy felspar devoid of twiu-lamellaa, exhibited straight 

 extinction. 



The following chemical analysis of the 'feldspathic' components isolated from a 



