912
T. G. BONNET ON THE SERPENTINE AND
projecting, as happens with the gabbro of the Cuchullin Hills, Skye.
I have had slides cut from one of the dark bluish unweathered
specimens, and from one of the exterior white and green. The
former shows the rock to be an olivine gabbro. There is plagio-
clase felspar, generally in rather short irregularly oblong crystals,
showing bright colours and twinning. Some of these exhibit a pecu-
liar strongly marked cleavage (or minute twinning, with one set of
crystals dominating), which gives them a general resemblance to the
structure of diallage. The felspar is in places rather decomposed.
There is a fair amount of diallage, and a few crystals of common
augite. These with ordinary light are as nearly as possible colour-
less, and are in good preservation. The olivine is rather rough in
texture and much cracked ; the cracks are marked out by a deposit
of granular opaque mineral, probably magnetite, which in some
cases appears to penetrate the intermediate spaces (which are
commonly fairly translucent), rendering them almost opaque ; now
and then it assumes a browner tinge, as from haematite or limonite,
and the grains are slightly stained with brown or green. Most of
the olivine grains have a finely granulated aspect at the edges, and
are sometimes bordered by a finely fibrous mineral, probably ser-
pen tinous and a secondary product ; the grains, however, show
very little trace of conversion into serpentine. Except the minute
granules described above, there is very little magnetite or other
iron- oxide visible. The above appearance would lead us to conclude
that the olivine is a rather ferruginous variety. The other slide (fig. 7),
Fig. 7. — Diallage partly altered into Hornblende, from outer part
of the great Gabbro mass at Coverack.
A. Decomposing felspar. B. Diallage. C. Hornblende.
cut from a somewhat weathered mass, which in appearance closely
resembled that of the veins, exhibits plagioclase felspar beginning
to pass into the saussuritic mineral, and diallage, more or less con-
verted into minute hornblende, but no olivine or serpentine that can
be recognized — the slide on the whole being remarkably like one