DR. BUNDJIRO KOTO ON SOME JAPANESE ROCKS. 433 
S10, ou ‘38 Oxygen quotient 30°69 
Al,O, 27-80 2:95 
Cad- Ai? . peat 
Na,O 386 ,, aaa 00) 
100-00 
} 4-49 
The oxygen ratios are 7:10 :3: 1:02, or approximately, 
Te corre 
The oxygen ratios would correspond to the following theoretical 
chemical composition :— 
4Na,0+2Ca0 +4 2A1,0, + 7810, = Na,O +3Ca0 + 4AL,0, + 14810, 
Calculating this in percentages, we have the following :— 
SION eee eke et 56°83 
PA OR TREAT Se Pit 27°61 
CAO eit dete ors 11°36 
INaOM 4°20 
100-00 
On comparing this analytically obtained result with Tschermak’s 
theoretical composition of the plagioclase group, it is clear that our 
felspar approximates to a mixture of three molecules of albite | 
with four molecules of anorthite, which gives the following per- 
centages :— 
SiOn 53°6 
EWR ee 29°8 
CHORE a ieee 
INGO) Sk, 4-9 
The felspar in the Japanese augite-andesites is therefore labra- 
dorite. This labradorite is characterized by its large: amount of 
silica and small quantity of alumina. The soda and lime approxi- 
mately agree with the theoretical numbers. 
Experiments in accordance with Boricky’s method by treating 
‘thin sections with a few drops of hydrosilicofluoric acid, afford a 
large number of monoclinic or spindle-shaped crystals of calcium- 
silico-fluoride (CaSiF,,+2H,0); but a comparatively small quantity 
of short hexagonal prisms of sodium-silico-fluoride (Na,SiF’,) were 
likewise formed ; so at all events this plagioclase 
belongs to the 
soda-lime felspars. It often exhibits the “ progressive” zonal 
structure described by Hopfner *, and this is especially well marked 
in the brachypinacoid «© Po. The optical behaviour of the inner 
and outer zones is not thesame. Inreference tothe zonal structure 
the direction of extinction makes a greater angle by from 6° to 10° 
* “Ueber das Gestein des Monte Tajumbina in Peru.” 
Band. 
N. Jahrb. 1881, ii. 
—— 
> = — = — y — = i » 
