File:The Quarterly journal of the Geological Society of London (14073282461).jpg

ASSOCIATED KOCKS OF THE BREIDDEN HILLS.
541
Fig. 3. — Fehpar-cnistal in andesite at N. end of Moel-y-Golfa,
^00 ft. from summit^ showing fractarea of crystal (a) and inclitsion
of serjpentine (b).
Fig. 4. — -Edge of felspar crystal showing inclusions of base.
v\.
«v^^AA^^
2. Pyroxene. There are two forms of this mineral present :
(a) Rhombic pyroxene, in octagonal or elongated sections, indicating
a good development of pinacoidal faces truncated by those of the
prism. There is a good cleavage parallel to the best-developed
pinacoid, but no trace of any other cleavage. The crystals are ter-
minated by oblique faces, probably domes, which are generally only
developed at one end of the crystal. The structure is slightly
fibrous, and there are curious lamellae of darker green mineral gene-
rally parallel to the principal pinacoid. It is slightly dichroic,
passing from greenish yellow, when the cleavage is parallel to the
short axis of the nicol, to light straw-yellow when placed at right
angles. The maximum extinction takes place when the cleavage is
parallel to one of the vibration -planes of the crossed nicols, and the
polarization colours are not of a very high order. It occurs in
crystals of all sizes, retaining its shape do'w^n to the smallest prisms. It
is best observed in its unaltered state in the great crags running round
the E. and S.E. side of the hill, at a vertical distance of from 300 to
400 feet from the summit, but in an altered state it occurs in almost
all the lavas, being replaced by a green or brown serpentinous
pseudomorph, in the manner so admirably figured by Mr. W. Cross*.
The serpentine needles frequently have a radial arrangement, and
there is more alteration in the large than in the smaller crystals.

• Bulletin, United States Geol. Suit. I. 1885, pi. ii. fig. 8.