KIMBERLITE FROM THE UNITED STATES 67
so
frequent, or so large generally, though one, which shows a moderately
good cleavage and is apparently olivine, attains to quite a quarter of
an inch. It has a distinctly rounded outline. Mica also is more
plentiful in this specimen, occurring generally in small rounded flakes
; but these in one or two cases are about one-fifth of an inch in
diameter. It is distinctly brown in colour. In microscopic structure
the rock is like the last, but without any garnet or spots suggestive
of filled-up cavities; there are also one or two semi-opaque grains,
minutely granular in structure, which are possibly fragments of some
rather compact and decomposed rock.1
The
above hand specimens from both Syracuse and Elliott Co. are perhaps a
little more like a porphyritic igneous rock than those which I have
seen from Kimberley, owing to the absence of distinct rock fragments
and the more uniform size of the included minerals; and the same may be
said of their microscopic structure. They are very like serpentine, but
the points to which attention has been called produce a
difference—marked, though not easily expressed in words—from the
normal specimens of that rock. This also is very perceptible in looking
at the thin slices with the unaided eye or with a common pocket lens,
when they are held up to the light. The rather rounded outline of the
enclosures, their unequal size and distribution, give them a
fragmental, rather than a normal porphyritic, aspect. I believe that
had I been asked to guess what the rocks were, from the evidence of the
hand specimens and from this mode only of studying the slices (i.e. without actually using a microscope), I should have answered ' probably a non-scoriaceous unstratified tuff.'
1 See Plate II. for a figure of a portion of this specimen.
