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DR. BUNDJIRO KOTO ON SOME JAPANESE ROCKS. 431 
31. Srupres on some Japanese Rocks. By Dr. Bunpsrro Koro. 
(Read April 2, 1884.) 
(Communicated by F. Rutley, Hsq., F.G.S.) 
Tur materials for this paper were kindly sent to me by Mr. T. Wada 
and Mr. Kato, Rector of the Tokio University, and were obtained 
- partly from the petrographical collection of the Tokio University 
and partly from that of the Geological Survey of Japan. During the 
last few years those districts, in which our rock-specimens were 
collected, have been examined by O. Kuntze, EK. Naumann, J. J. Rein, 
and other European geologists, but as yet no full descriptions of 
their researches have been published. Many specimens helonging 
to the Geological Survey were collected in the provinces Izu and 
Kai by Wada, and those from Kojuke during a two months’ journey 
by myself. The rock-specimens belong mostly to grey porphyritic 
pyroxene-andesites, basalts, diabases, granites, diorites, and por- 
phyrites. 
The microscopic investigations have been prosecuted in the 
Mineralogical Institute of Leipzig under the direction of Geheimerath 
Prof. F. Zirkel, and the chemical analyses were made in the Labo- 
ratory of Agricultural Chemistry under Prof. W. Knop, to both of 
whom I here take the opportunity of expressing my hearty thanks, 
PyYROXENE-ANDESITES. 
The andesitic rocks were for the most part collected in the 
neighbourhood of Tokio and in the province of Izu, the latter a 
mountainous volcanic district in which the well-known Atami geyser 
and many other mineral springs of less note are situated. Most of 
the specimens belong to the coarse porphyritic type. They are grey 
or dark-brown in colour, somewhat porous and with a trachytic 
appearance. With the aid ofa simple lens, or even with the naked 
eye, the component minerals may be clearly distinguished from one 
another. ‘These rocks are not at all like the typical glassy augite- 
andesites from Santorin, Hungary, Java, and Australia, which are 
of a pitch-black colour, and have a resinous lustre and conchoidal 
fracture, but rather resemble typical hornblende-andesites. The 
eround-mass of these specimens is not, as is usually the case, a glassy 
paste containing felted microliths, but is mostly holocrystalline. 
Sometimes a few of the specimens show a glassy interstitial sub- 
stance in small quantity. The rocks from the Kozuke province, 
however, approach in structure to the typical augite-andesites. 
Porphyritic Constituents. 
Plagioclase occurs in crystals ranging up to three millim. They 
are usually twinned on the albite type. 
Besides the albite type, another system of twinning parallel to 
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