Vol. 58. ] PLUTONIC COMPLEX OF CENTRAL ANGLESEY. 673 
seems to follow no law, for while they increase in number towards the 
margin in the area south-east of Llechcynfarwy and in the section. 
east of Gwyndy, I did not observe them at all in the schist south- 
west of Boleyn, although the junction with the granite is within 
50 yards, and they are not numerous in the quarries south of the 
Holyhead Road. They vary in thickness between a foot or more 
and a line or less. They occasionally cut across the schistosity 
of the diorite, either directly or 
Fig. 4.—Granite-vein in foliated obliquely; but generally they 
diorite, Craig-yr-Allor. coincide with it, as if it had 
provided the planes of least. resist- 
ance. Fig. 4 shows a combination 
of the two. 
Where the veins are few in 
number, contact-effects may be 
very slight. Sometimes, however, 
fusion is produced at the contact, 
with aggregation of fresh horn- 
blende, as’ inv fig. 3 1(p. G71): 
Or the fusion may have pro- 
ceeded for some distance into 
the diorite. In this intermediate 
zone, the granite and diorite are 
‘intermixed, hornblende (and perhaps black mica) in aggregates 
being immersed in a granitic groundmass. Large phenocrysts of 
hornblende also occur away from the diorite, scattered in a granitic 
magma. Some of these are corroded at the margin, where they 
are in contact with quartz. How far the felspars of the two 
magmas are intermixed I have not ascertained. 
Abundance of granite-veins in the diorite is usually (indeed, so 
far as I saw, invariably) accompanied by the generation of black 
mica, and the number of veins appears to be in direct ratio to the 
quantity of the mica produced. Where the veining is very close 
all the hornblende has disappeared. When the veins lie in the 
planes of shearing 1n the diorite, they are parallel one to the other, 
and the result is a banded gneiss, seams of quartz and felspar (the 
haplite) alternating with seams of biotite and felspar (the modified 
diorite). 
I have not succeeded in obtaining microscopic specimens showing 
the details of the production of this banded gneiss. This defect 
appears to be due to the degree of fusion which the granite has 
caused in the diorite. Sharp lines of contact between the diorite 
and the invading granite I have not been able to observe, so that 
it is impossible to determine exactly in my slides how much of a 
specimen is diorite and how much granite. We have therefore to 
rely mainly upon field-evidence. We know that the diorite has 
been often cut by parallel and closely approximated planes of 
division, and we know that an invading granite is much more likely 
to penetrate these than to flow in the direction of a fluxion-structure 
in a solid rock. We find in the field that the granite-banding 
Q.J.G.8. No. 232. 3A 
