Iwasaki 



The petrification may often be seen distinctly in recent lavas. 

 As an example, let me explain here the occurrence of tridymite in 

 Ishigami-yama, a hillock in the environs of the city of Kumamoto. 

 This hill is a part of a lava flow extruded from Kibo-san, a now 

 extinct volcano, and is composed of amphibole-pyroxene andésite. 

 The rock is fresh in composition, whitish in color, with conspicuous 

 phenocrysts of amphibole converted into the pseudomorphs 

 of magnetite and augite grains by the resorption of the 

 original crystals. In the miarolitic fissures and cavities formed 

 during the consolidation of the lava, several minerals are found, 

 such as tridymite, phlogopite, breislakite,^^ specular iron, and 

 calcite, all forming very fine crystals. The tridymite is in 

 hexagonal plates, sometimes attaining 5 mm. in diameter. The 

 mineral, when picked out of the rock cavities, is transparent, but 

 very soon becomes whitish and translucent on exposure to the air. 

 The change of color may be clearly explained by microscopic study. 

 The trid3aiiite occurs in the form of a pile of thin larainse, and has, 

 when it is picked out of the rock cavities, a light-brownish liquid 

 in the interspaces between the plates. When exposed to the air, 

 the liquid immediately evaporates and the tridymite becomes 

 whitish by total reflection of light. The liquid contained in the 

 tridymite is supposed to be what is left of juvenile water extruded 

 from the cooling lava. 



The presence of calcite as an emanation-product in recent 

 lava is, so far as known, extremely rare. The other minerals in 

 the miarolitic cavities are also supposed to be all of juvenile origin, 

 and not of the vadose formation ; in other words, all belong to the 

 so-called fossil emanations of Lincoln."^ To prove it, I shall give 

 here the following three data: (1) the side-wall of the miarolitic 

 cavities and fissures presents the slaggy aspect usually seen on 

 the surface of lava, (2) the cavities and fissures are perfectly closed 

 as if to prevent the infiltration of vadose water, (3) the andésite 

 in which the cavities and fissures are found is quite fresh, showing 

 that the minerals in question are not decomposition-products. 



1) It was so determiaed by B. Koto. 



2) Liacoln : " Economic Geology," Vol II.. No. 3, p. 253,1907. 



