Wolo 5S. | PLUTONIC COMPLEX OF CENTRAL ANGLESEY. 663 
the previous year, I had communicated to the British Association * 
an outline of the igneous theory of origin; and in 1897 I read to 
this Society an account of the ‘ Origin of some of the Gueisses of 
Anglesey ’* from evidence obtained in the eastern district. 
I have to express my great obligations to Prof. Bonney for his 
opinion on some of the thin slices of rocks used in this paper ; 
to Prof. Theodore Groom for working out certain microscopic 
details ; and to Mr. Philip Holland for an elaborate chemical analysis 
of the felsite from which the grey gneiss is derived. 
Il. Tur Materials WHICH coMPosE THE PLUTONIC CoMPLEX. 
In the eastern district, the materials employed in the gneiss- 
making process are mainly felsite and diorite. These varieties 
occur also in Central Anglesey ; but associated with them is the 
well-known binary granite (haplite) originally called ‘ Dimetian’ 
by the late Dr. Hicks. A fourth variety, which occupies an important 
place in this paper, is the quartz-felsite claimed by that geologist as 
‘Arvonian. It forms a part of the same magina as the granite, 
and must be carefully distinguished from the quartzless felsite, into 
which it is sometimes intruded in dykes and veins. 
(1) The Diorite and its Modifications. 
The diorite is well known.* It undergoes numerous modifi- 
cations, of which the following are the most important :— 
(a) Hornblende-Gneiss.—This rock forms a large proportion 
of the gneissic dome. It will be more fitly noticed farther on 
(p. 670, ete.). 
(6) Decomposed diorite and chlorite-gneiss.—At the 
southern end of the dome, especially on the Holyhead Road, the 
diorite is much decomposed. In the old quarry on the south side 
of the road, near the ;% milestone, it passes into a soft greenish 
rock, with hardly any granite-veins, and with only a few indistinct 
traces of foliation. Under the microscope, the latter (579) * 
can hardly be distinguished from a grit. It iscomposed of angular 
fragments in a greenish matrix. The fragments are mainly felspar, 
plagioclase predominating. Hornblende comes next in quantity: 
it has broken up along the cleavage-planes, and some of it is 
rather drawn out. Some minute, highly refracting granules, which 
polarize in brilliant colours, are apparently epidote. Iron-oxides in 
crystalline forms are scattered through the slide. The groundmass, 
which is not abundant, is mainly chlorite. This rock is clearly a 
diorite, which has suffered from crushing and decomposition. 
1 Rep. Brit. Assoc. 1887 (Manchester) p. 706. 
* Quart. Journ. Geol. Soc. vol. liii (1897) p. 349. 
* T. G. Bonney, Geol. Mag. 1880, p. 126; J. F. Blake, Rep. Brit. Assoc. 
1888 (Bath) pp. 403-406 & Quart. Journ. Geol. Soc. vol. xliv (1888) pp. 479-99. 
* The numbers between parentheses throughout this paper refer to the slides 
in my cabinet. | 
