Geological researches in China, Mongolia, and Japan, during the years 1862-1865.

CHINA, MONGOLIA, AND JAPAN. 83
The view in the distance is grand. On our left the shore of the beautiful Volcano bay forms a long, sweeping curve, parallel to which the mountains in the
background, covered with dense forests, appear in all the shades of green, blue, and
purple, as they stretch away on the far horizon. Far over the bay, rising as it
were from the sea, are several beautiful cones, long quiet, covered to the summits
with vegetation, while nearer, though seemingly among them, is the semi-active
Usu, a ruined cone whose yellow, sulphur-coated cliffs glisten even at this distance.
We descended into the crater by a talus of pumice, and crossing to the north
side came to the edge of a secondary crater, or pit, in the plain. This was
about 600 feet in diameter, with precipitous sides on which the stratification of the
mass of pumice that fills the bottom of the great crater is distinctly visible.
From the bottom and sides of this pit columns of steam were rising, incrusting
the walls with crystals of sulphur and salts. This inner crater must have been
formed after the falling in of the cone, and was, perhaps, the point of exit of the
ashes that fell after the breaking in of the peak.
On examining the long fissures that traverse the plain, their sides were found
incrusted with delicate crystals of sulphur and sulphate salts, while the pumice
walls were half turned to a bright red clay, impregnated with these crystals.
Putting my thermometer, which was graduated only to 800 C., into the steam, the
mercury instantly ran up to that point.
The recent covering of pumice conceals, in most places, the true structure of
the mountain, as it forms a deep mantle-over every slope not too steep to retain
it. This product is grayish-white, very irregular in its porous structure, and contains numerous crystals of felspar and grains of a translucent, greenish glass. It is
undergoing rapid disintegration. Bombs of black scoria were found containing
crystals of white felspar, and showing transition, in streaks, into pumice characterized by the same contents as that just described.
Blocks of a grayish trachytic lava, abounding in crystals of triclinic felspar and
grains of the greenish glass, mentioned above, occur in the crater, and seem to be
the rock of which the pumice and bombs are a variety.
The western side of the crater wall is the highest, and owes its better preservation to a broad dyke of rock consisting mainly of a dark paste with greenish-white
crystals of triclinic felspar, hornblende, and magnetic iron. The dyke has a tabular
structure, the plates being upright in the middle and horizontal on the sides, forming there a right angle with the cooling surface, as is the case with columnar structure. The rock traversed by this dyke was found very much disintegrated.
Without visiting the top of the northern wall we could clearly distinguish the
original outer mantle of the volcano, in the exposed edges of different colored strata,
bably the old cone remained. The greater part of at least the western and northern
walls appear to be of trachytic rock.
The general appearance of this mountain produced upon me the impression that
it had, before this, been a ruined cone, but was rebuilt by an eruption of pumice to
be again broken down and given over to the levelling solfatara-action.
Descending by the same route we returned to Skunope.