KIMBERLITE FROM THE UNITED STATES 63
tine
takes place rapidly in the cross fractures approximately parallel to
the base, but very slowly along the numerous minute fissures in the
prism zone. Cleavage parallel to the brachypinacoid is barely
discernible. Magnetite is excluded in the usual manner, and in some
cases dolomite appears as an ultimate product of alteration. The
en-statite has an irregular (corrosion) border. The biotite is dark
coloured, is strongly dichroic, and is sometimes surrounded by a
secondary border of biotite (differing in optical properties) and of
magnetite. The pyrope occurs in spherical and ellipsoidal grains,
varying from 1 to 12 mm. in diameter, of a clear, deep-red colour, its
specific gravity being 3-673. It is often surrounded by a border, the
outer band of which is commonly rendered opaque by a dark powder, the
inner one being a greyish or reddish-brown colour, generally fibrous in
structure (the substance named kelyphite by Schrauf, but shown by
Lasaulx to be a mixture of several minerals, chiefly of the pyroxene
and amphibole groups). Of the other minerals, the octahedrite occurs in
yellowish clouded grains, ranging in size from -004 to -0G mm. in diameter. The dolomite is irregularly distributed. Of the serpentine there are two forms.
This
peridotite cannot be seen in contact with the adjacent sandstone and
shale ; but within a short distance from it both appear to be
indurated, and the latter in some cases to be converted into a kind of
spilosite. The fragments of shale included in the peridotite are
always surrounded by a border of colourless mica, its scales being
intricately intermingled, and have undergone other mineral changes,
generally on a minute scale. On a review of the whole evidence, Mr.
Diller concludes that ' the peridotite is a truly eruptive rock, which
has been forced up through the carboniferous strata.'
At
the end of the paper Mr. Diller gives a series of analyses of the
adjacent sedimentary rocks, of the peridotite, and of the minerals
contained in it.
