Vol. 58.] PLUTONIC COMPLEX OF CENTRAL ANGLESEY. 675 
proportion to the abundance of granite-veins, that the relation 
between them of cause and effect may fairly be inferred. 
Compressive forces seem to have affected the district repeatedly. 
Before the diorite had consolidated, an imperfect fluxion-structure 
was produced. After solidification, the diorite was sheared, and 
granite was injected along the planes of discontinuity. Subsequently 
to the secondary consolidation, shearing took place along the seams 
of the softer minerals. 
(c) The dark gneiss originally a xenolith of diorite.— 
The mass of diorite the modifications of which gave rise to the 
gneisses of the Craig-yr-Allor area is seen in plan to be entirely 
surrounded by granite, and as it is almost everywhere penetrated by 
eranite-veins it must be underlain by the granite, probably at no 
great depth. It must once have been of much greater vertical 
thickness, for it has been exposed to intermittent denudation ever 
since the early part of the Ordovician’ Period. It was therefore 
at one time a huge block of diorite, immersed in granite. It may, of 
course, have been continuous under the granite with other masses of 
diorite. Before the granitic intrusion, the diorite must have acquired 
its schistose structure. Whether the domical arrangement of the 
schistosity was produced in one period by expansion under constraint, 
or resulted from two linear forces acting simultaneously or succes- 
sively, I have not sufficient material to determine. 
This insulation of a diorite-mass is no new phenomenon. Isolated 
blocks and masses of the diorite appear in the felsite 7 miles to 
the south-east, in the Llangaffo district.2 As early as 1885, I 
showed * that the granite of Northern Donegal contains xenoliths of 
hornblendic, micaceous, and quartzose schists. Two years later I 
described * how granite invades the diorite of County Galway, forcing 
its way along the joints, and carrying away with it isolated blocks 
of the diorite. In this way pseudo-conglomerates were formed 
in Western Galway, and a gneissic rock near Galway town. 
Recently, Prof. G. A. J. Cole* has recognized similar phenomena 
in Eastern Tyrone and Southern Donegal, and he considers that 
the relations described as existing between the granite and various 
xenoliths ‘ will probably be found to prevail throughout the whole 
of North-west Ireland ’ (op. czt. p. 467). 
Several other writers ° have noted the occurrence in the British 
area of xenoliths in plutonic magmas; but I am not aware of any 
other example of an insulated mass on so large a scale. 
' Conglomerates of at least Llandeilo age contain fragments of diorite in 
Anglesey, as well as pebbles of the haplite, T. G. Bonney, Quart. Journ. Geol. 
Soe, vol. xl (1884) pp. 485, 586. See also H. Hicks, cbéd. pp. 187-96. 
2 Quart. Journ. Geol. Soe. vol. liii (1897) p. 355. 
3 Thid, voi. xli (1885) pp. 222-24, 227. 
* Ibid, vol. xliii (1887) pp. 518-24. 
5 Trans. Roy. Irish Acad. vol. xxxi (1900) pp. 431 e¢ segq. 
° J. A. Phillips, Quart. Journ. Geol. Soc. vol. xxxvi (1880) p.1; E. Hill 
& T. G. Bonney, ibid. vol. xlviii (1892) p. 127; A. Harker, zbéd. vol. lii (1896) 
p- 320; J. Parkinson, ibid. vol. lv (1899) p. 480 & vol. lvi (1900) p. 320. 
oA 
