432 DR. -BUNDJIRO KOTO ON SOME JAPANESE ROCKS. 
the macropinacoid is also developed, making an angle of from 86° to 
87° with that parallel to the brachypinacoid*. 
It is difficult to get mathematically accurate microscopic sections, 
and therefore optical researches are possible only in a few cases. 
In the basal section parallel to P, the maximum extinction of light 
takes place between 15° and 20° (in the sense of Schuster) with the 
edge P/M ; but the exact point could not be fixed with any degree 
of certainty owing to the so-called undulatory extinction. The 
angular values are not constant, varying from 13° to 35°. In the 
brachypinacoidal sections parallel to M, the direction of maximum 
extinction makes an angle of from 5° to 6° with the edge P/M. 
These measurements indicate that the felspar is labradorite, 
according to the observations made by Max Schuster 7, while the 
following chemical analysis confirms this view. 
The porphyritic felspar, by virtue of its difference in specific gravity 
from the other constituents, may be mechanically separated by 
Thoulet’s solution (a mixture of potassium and mercuric iodide {). 
By repeated cautious treatment of coarsely pulverized samples of 
the rock from Ihama, Izu, I was able to obtain felspar grains, the 
purity of which was evident under the microscope. 
The alkalies were determined by the new method of Prof. W. 
Knop §. 
The analysis gives the following result :— 
UO tre tigate: 55°97 
AOE eras 27°60 
Fe Ovni canon 1°68 
CaQe oe acer hiess 
Vt O crsret are 0°66 
Na Orr inves: 3°83 
dal 0 Mlerey ree rida 0:08 
LOL-70 
Subtracting the quantities of Fe,O,, MgO, and K,O from the sum, 
and calculating the remainder up to 100, the following numbers 
are obtained :— 
* According to Stelzner, theangle made by two systems of lamellz, one parallel 
to the macropinacoid o Po, the other parallel to the brachypinacoid oo Po, 
amounts to 86° 40! in labradorite. By comparing the angular measurements 
of the two felspars, our felspar is found to have a remarkable coincidence as 
regards optical behaviour with that of Stelzner. Berg- und Hiittenm. Zeit. xxix. 
. 150. 
i T “ Ueber die Orientirung der Plagioklase,” in Tschermak’s ‘ Mineralogische 
und petrographische Mittheilungen,’ iii. p. 186. 
t “ Ueber die Verwendbarkeit einer Kaliumquecksilberjodlosung bei mineralo- 
gischen und petrographischen Untersuchungen,” N. Jahrb. f. Mineralogie, &c. 
i. Beilage Band. 
§ W. Knop, “Neue Methode fir quantitative Trennung des Natrons und 
Kalis,” Bericht der math-phys. Classe der k. stichs. Gesellschaft der Wissen- 
schaften, 1882. 
