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KIMBEIILITE FROM THE UNITED STATES 59
compact
base containing numerous lighter spots, with a sharp crystal outline,
thus producing a porphyritic structure. Some singular blood-red patches
are scattered through this rock. The first type, under the microscope,
appears as a medium-grained aggregate of brown mica, of octahedral
crystals of two minerals—one yellow and transparent, the other black—of
green or colourless serpentine, and of carbonates. Through this matrix
occasional larger crystal forms are scattered. In spite of nearly all
the substance of the rock being secondary, these preserve the shape of
crystals. Among the minerals present Professor Williams identifies
olivine or enstatite ; the latter being completely, the former mostly,
replaced by serpentine. The mica is the peculiar brown biotite, well
known to be characteristic of eruptive peridotites, without a definite
crystal form, and often with a bleached peripheral zone. Of the
octahedral minerals, the opaque are usually chromite, but a few may be
magnetite, the transparent are proved to be perovskite.
The
second type of this rock, under the microscope, is identical with the
former, except as regards structure. The ground-mass is finer grained,
thus throwing the characteristic crystal forms of the olivine and
enstatite into sharp relief; the octahedral crystals are much smaller
than in the other rock and are confined wholly to the ground-mass ; the
brown mica is less abundant. ' The persistence of structure in this
rock, in spite of the profoundest chemical changes, is remarkable. In
one porphyritic specimen, the ground-mass is almost all carbonate, the
mica has become quite colourless, and the olivine crystals are changed
to a perfectly isotropic, colourless substance, enclosing the sharpest
possible rhombohedra of dolomite ; in fact none of the original
components remain, except the chromites, and yet the structure is just
as sharp and characteristic as in a specimen,' of which Professor
'Williams gives a figure.
The rock, when Professor Williams wrote his earlier paper, was no longer visible, being concealed beneath
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