DR. BUNDJIRO KOTO ON SOME JAPANESE ROCKS. 445 
needles and magnetite grains. This ground-mass presents another 
peculiar feature; the interstices between the microliths are filled 
up with a colourless anisotropic substance, which has a striking 
resemblance to tridymite, and I consider this to be a crystalline 
form of silica. The chemical analysis gives the following result :-— 
S102 ee ee 69°10 
VAIO NGL A Uo et 16°32 
He ON res kane 3°70 
EO: ont pre 1°37 
CaQhacre mise. 5°10 
Mis Oar Moeiae es Hed 
IN ARO TRG Eto « 2°91 
KOF eee a. 1:06 
ag J Specific gravity 
10:68) To oMaGs 
The large amount of silica may be ascribed to the presence of 
tridymite. Another locality of the hornblende-bearing augite- 
andesite is Funabara. It is similar to that of the Izu-San. Here 
the alteration of the horblende into augite grains can be more 
favourably observed than in the Izu-San rock. 
The augite-andesite from Yawatano belongs to the dark grey 
porphyritic type. It has a brown glass-basis with trichites and 
augite, the latter with liquid-lacune. Apatite is not rare and 
shows strongly developed prismatic faces. A clinopinacoidal section 
contains a large number of oval glass enclosures (0:016 mm.) with 
fixed bubbles, which, when observed from the orthopinacoidal face, 
appear as black rods running parallel to the axis C. 
The rock from Kitayama is a dark grey porphyritic andesite, and 
contains augite with granular augite borders of a deep brown colour. 
This singular structure is not the result of a simple adherence of the 
augite grains around the larger individuals, but that of an actual 
decomposition caused by the caustic action of the once semi-fluid 
magma*. The reason (so, at least, 1 suppose) for this assumption 
is that the granular wall behaves optically just like the inner main 
mass, or what is the same thing, the wall is optically the continua- 
tion of the inner augite mass. On the contrary, if the external wall 
were the result of the simple aggregation of irregularly arranged 
augite grains, then it it is most likely that the individual grains 
would behave in a manner optically independent of each other, 
which in this instance is not the case. No doubt this action took 
place after the crystallization of the augites, but before the solidifi- 
cation of the ground-mass. Apatite and the glass-basis are wanting. 
In the Amagi-San rock the augite is changed into a dark green 
fibrous substance with pleochroic undecomposed augite kernels in 
the centre. Different stages of the decomposition can be easily 
traced; it commences from the periphery and from the cracks; the 
* Vide page 437. 
Q.J.G.8. No. 159. 24 
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