THE GREAT EEUrTION OF SAKUEA-JIMA IN 1914. 13 



§ II. On the Formation of the Trench Bay of Kagoshima. 



A) General Geology of the Environs of Kagoshima Bay.— 

 As to the outstanding question about the foundation, on which 

 the volcano Sakura-jima stands, it is first of all necessary to 

 know the geology of the border land of the bay. As has just been 

 stated, the extensive (1,650 sq. km.) and thick (100 ??2.), ash-grey 

 and loose bed of lapilli and ash is a characteristic formation of 

 the present area, forming a high flat of more than 230 m. Tliis 

 formation is underlaid by a complex of sandy slate of problematic 

 Mesozoic age, and the Tertiary Operculma' and shell-bearing sand- 

 stones with coaly shale. Both are intruded at several places by 

 the ash-stone which plays a great rôle in the geology of the region. 

 Ash-stone, a) The jish-stoue^^ or hcd-ishi — îi hypersthene-tracliy- 



Laya andésite (PL XYI. Fig. 1) is an ash-gray, friable and porons 



rock, so named on account of its having the appearance of hardened 

 volcanic ash. It is extensively quarried for building materials, due chiefly 

 to its being easily worked rather than to its intrinsic value. 



The a variety is dopatic, with phenocrysts of idiomorphic oligoclase 

 and corroded sanidine. Femic minerals, such as hypei*stliene, augite, 

 hornblende and biotite, are found only sparingly and often not discovered 

 in a shde, the first component being, however, comparatively abundant. 

 The main bulk of the rock is devitrified glass of various degrees of 

 alteration, often spherulitic (PL XVI. Fig. 2) and sometimes silicified. 

 The ß variety is a black porphyiitic obsidian, and sempatic with 

 macrophenocrysts of feldspars, the black base being colorless glass 

 swarming with streams of feldspar-microlites. It seems to correspond in 

 a strict sense to the so-caUed Aso lava. (PL XVI. Fig. 3) 



The light-grey y (PL XVI. Fig. 2) modification makes shreds in the 



1) It is a pipemo-like eutaxitic rock, striped with black glass in j)umiceous white mass. 

 The stripes may be either schlieren-like patches detached by flows, or doughs of lava drojDped 

 in white ash mass. It is habitually called the ' Aso lava ' by our geologists. 



