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

38 GEOLOGICAL RESEARCHES IN
Beds of another rock occur, of brick-red and brown colors, and having an earthy
base, with small, brilliant crystals of glassy feldspar and grains of pellucid quartz,
and inclosing small fragments of other rocks.
This deposit is visible on the southern flank of the spur between Kalgan and
Siuenhwa, underlying the terrace loam in horizontal beds (Fig. 7).
At the base of the high hill north of Kalgan the tufa beds are seen to dip under
the porphyry at an angle of about 45~ (Fig. 8), and trending west they form a series
of detached hills. On the roads leading to Tutinza, Teutai, and Siwan, they are
traversed by a perfect network of dykes of the porphyry, which rock also caps the
summits of the hills, its vertical cliffs and outstanding dykes giving them a bold and
castellated appearance.
Although no analyses of these rocks have been made, there is, I think, little
doubt that we have here to do with a trachytic porphyry and its tufas.
Volcanic Formation of the Plateau.-The southern elevated edge of the Great
Plateau is formed, between the 112th and 115th meridians, of an immense lava bed.
How much further it extends beyond the limits given above, or how large its
breadth may be toward the north, is unknown; I have only tried to indicate on
the map the region which I observed it to occupy. Its breadth is, in places, not
less than forty miles, and this may be only a fraction of the real width.
The thickness of the formation is, necessarily, very variable as it fills the intex-utis xfi vhtai was once a mountainous country. At lanoor it seems to be not
less than fifteen hundred feet thick, and the same may be said of it in other localities visited, while we have seen it in places represented by only a thin sheet,
covering the metamorphic schists, where these rise to near the surface.
The rocks of this formation may be classed under two types-the one basaltic,
the other trachytic.
The basaltic rocks were observed more particularly near Hanoor and to the N.
E. of that place. Both compact and finely crystalline varieties occur. T.hey are
generally, especially the latter variety, poor in olivine and contain here and there
crystals of basaltic hornblende.
At many places in the neighborhood of Hanoor, fragments of a cellular variety
occur on the sides of the valleys, in a manner that would seem to indicate, that
there is a horizontal bed of it, marking the plane of contact between two flows of
lava.
The rocks of the other type are throughout crystalline, though often the texture
is very fine, and are generally porous. In color they vary from black to dark gray,
while some varieties, especially when weathered, are light gray. In some instances
hornblende, or augite, enter abundantly into the composition of the rock, but more
generally it seems to consist almost exclusively of white or yellow, triclinic feldspar with greasy lustre, partly in tabular crystals, partly massive. Scattered through
this mass are minute specks or grains of a dark to light green mineral, with glassy
lustre and conchoidal fracture, harder than the knife when fresh, soft and resinous
in lustre when altered. The feldspar is probably oligoklas. A characteristic
feature of the different varieties of this rock is the extreme rarity or total absence
of magnetic iron.