674 DR, C, CALLAWAY ON THE [Nov. 1902, 
normally coincides with the direction of shearing in the diorite, 
and we may fairly infer that the injection has followed the pre- 
existing shear-planes. 
Some of the best sections of the banded gneiss occur in the old 
quarry north of the site of Y Werthyr. The beds here lie in a gentle 
anticline, and the granite-veins bear a larger proportion to the diorite 
than in any other Anglesey locality known to me. They attain 
a maximum thickness of 6 inches, but most of them are much 
thinner. In one variety of the gneiss, there are six veins, with 
interfoliated micaceous films, in the thickness of 1 inch. Under 
the microscope this rock (577) is seen to consist mainly of felspar 
and quartz, the latter predominating. Twinned and untwinned 
felspars are in about equal proportion. The slide is crossed by two 
interrupted films of intermixed biotite and chlorite, with a minute 
proportion of white mica. Discontinuous cracks coincide with the 
films, and may be the partial survivals of the original shear-planes 
along which the granite was forced. Both the quartz and the felspar: 
are moulded to the flakes of mica and chlorite. The felspar of the 
original diorite would seem to have been incorporated with the 
haplite. 
Another specimen of gneiss from this quarry exhibits signs of 
crushing subsequent to the final consolidation. The thin slice (575) 
is traversed by four subparallel cracks (shear-planes), which pass 
with an undulating course from side to side. One of these is filled 
almost from end to end with chlorite. The others are occupied 
partly by chlorite, and partly by opaque brown matter and biotite. 
The opaque matter, presumably iron-oxide, is confined to the cracks ; 
but the mica often extends for some distance on each side of them. 
The rock in the bands between the cracks is mostly felspar and 
quartz. The seams of mica and chlorite have apparently yielded 
readily to pressure, and hence are traversed by the shear-planes. 
Another variety of the banded gneiss is nearly all granite, the in- 
tervening films of mica being so thin as to be just perceptible to the 
naked eye. Superficially this rock suggests the gneiss of secondary 
injection seen south of the Wych at Malvern.’ In that example, 
the granite is sheared, and infiltration-products, sometimes changed 
into black mica, lie between the lenticles of haplite. In this case, 
it is the diorite which has been sheared, and the granite has been 
injected along the planes of fracture, so that the injection is 
primary. 
The behaviour of the granite-veins is well seen in this quarry in 
strike-sections, some of them running for several yards with uniform 
thickness, others thinning out more rapidly. Here and there in the 
quarry the veins are fewer and less regular, but the general mass 
of diorite has been so densely charged with the haplite that no 
hornblende could be detected in it. 
I do not think that any original biotite occurs in the Craig-yr- 
Allor district. This mineral makes its appearance so uniformly in 
* Quart. Journ, Geol. Soc. vol. xlv (1889) pp. 496-98. 
