SE. EE ee ee eee eee 
*450 DR. BUNDJIRO KOTO ON SOME JAPANESE ROCKS. 
in small quantity and in obscure forms. Taken as a whole, basalt 
is confined to the region near the crater of extinct volcanoes, and 
consequently appears younger than augite-andesite. These basalts 
may for the most part be regarded as dolerites, save a few glassy 
modifications—true basalt or basaltite. 
Olivine is not found in great abundance in these rocks. It is 
always the first to decompose, passing into hydrous iron-oxide. 
Neither picotite nor chromite could be detected in these olivines. 
Typical basalt occurs in Funabara. It is a uniformly crystalline, 
apparently homogeneous rock of a dark grey colour, in which the 
only macroscopically discernible ingredient is the olivine, in yellow 
patches. The microscope shows us a multiplicity of other ingre- 
dients. Plagioclase occurs in the small columnar form of simple 
crystals, and rarely in twins. In the twin crystals the direction 
of maximum extinction makes an angle of 15°-20° with the twin 
lamelle. Broad tabular sections are rare. ‘The felspar is partly 
soluble in HCl, the central portion being most easily attacked. 
Augite is most abundant and occurs in grains, possessing a yellowish 
brown colour, and showing no perceptible dichroism; it is free from 
enclosures except magnetite grains and felspar fragments. Its 
sections are usually six-sided, or it occurs in irregular grains, and 
all have suffered partial decomposition, being traversed by a brown 
rectilinear hair-like substance, which runs strictly parallel to the 
chief axis. The olivine contains many liquid-enclosures with 
movable bubbles. Magnetite octahedra are always enclosed in it, 
and when the olivine is fresh serve to distinguish it from augite. 
The magnetite occurs in well-developed crystals, which are larger 
than those usually found in augites. The globulitically-devitrified 
glass-basis fills up the interstitial spaces. The following is the 
analysis of this rock, the iron having been determined as Fe,O, :— 
SiGe ase ane 5112 
ieee aus 10-91 
BICIOE 5S Mees seoa 21°89 
CaO cc ee ek 10°32 
Mi oo Chee: 2°41 
Na iO ana Le se o74 
Ooch so 1°64 
102°03 
Specific gravity 2°555. 
The dark-grey dolerite from Amagi-San (Omigut8i) and from 
Isibe resembles those already described. The felspar is mostly 
decomposed in the central portion into a grey kaolin-like mass, 
while the surrounding portion remains quite intact. The augite 
presents no special characteristics. It shows a feeble indication 
of the variegated zonal structure. Olivine is found in crystals of 
various sizes, the largest being 3 millim; but these are rare. Small 
altered grains of it are widely distributed. Apatite is tolerably 
