File:The Quarterly journal of the Geological Society of London (12893585345).jpg

356
BR. C. CALLAWAY ON THE ORIGIN OF
.Aug. 1897,
with this hade. On the western side the altered diorite is very
irregular, projecting into several promontories

but here also the
grey gneiss is foliated parallel to the margin of the diorite, the
laminae following exactly the projections and recesses of the latter.
This foliation is accentuated by very thin dark lines.
Pig. 3. — Gneiss of secondary injection, Llangaffo.
Diorite
passing into
Schist
Grey Gneiss
.The grey gneiss or sheared felsite is injected with decomposition-products from
the adjacent diorite. The black lines in the grey gneiss represent planes
of injection, but only a few are shown..
The explanation of this gneiss is similar to that which I have given l
of an infiltration-gneiss at Malvern, felsite being substituted for
granite. The infiltration-products are substantially the same as at
Malvern, consisting mainly of chlorite, iron-oxide, and epidote

and
out of the chlorite and iron-oxide, with the addition of materials
from the felsite, black mica has in like manner been sometimes
constructed. The following is a description of a slice of this gneiss.
It shows a breadth of § inch cut across the foliation, and is taken at
the junction of the grey gneiss with the altered diorite.
1. A seam of minute transparent granules, generally elongated in
the direction of foliation, many of them being lenticular in
shape. It is rarely possible to determine which of these
are quartz and which felspar, bat scattered among them are
several comparatively large lenticles of quartz. Some minute
crystals of epidote, elongated with the foliation, are present.
2. A dark seam, chiefly composed of interfelted chlorite, pale green
in ordinary light, almost extinguishing under crossed nicols.
It occasionally passes into greenish, or even brownish, mica.
There is a little epidote, and lenticles of clear quartz or felspar
lie amid the meshwork.
3. A minute interfoliation of several of the felsitic seams with the
dark green material.
4. A comparatively thick folium of the chloritic rock.
Yeins of calcite cross the slide transversely to the foliation.
This specimen may be taken as typical of the structure at the
junction of the grey and green rocks. It does not contain much
mica, but in some of the junction-slides there is a fair proportion
of that mineral, the white variety being predominant.
Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 496 & vol. xlix. (1893) p.