Vol. 58.] PLUTONIC COMPLEX OF CENTRAL ANGLESEY. 671 
The gneiss is ‘simple,’ when it is merely the diorite or one of its 
modifications that has acquired a schistose structure; and ‘complex,’ 
when the simple variety is interbanded with granite-veins. The 
latter is an example of ‘ primary injection,’ * as distinguished from 
a gneiss of ‘secondary injection,’ in which the injected matter is a 
product of decomposition, as in the case of some of the gneisses of 
Eastern Anglesey.” 
The first stage in the process of gneiss-formation is the production 
of schistosity in the diorite. Much of the diorite is schistose ; 
but, even in hand- 
Fig. 3.—Granite-vein in diorite. specimens, the foliated 
(About + nat. size.) diorite may be some- 
times seen passing 
gradually into the 
massive rock. ‘The 
granite does not share 
in the foliation; for, 
whether the veins cut 
across the foliation 
or run with it, the 
granite remains mas- 
sive. Fig. 3 shows 
this very clearly. 
D= Diorite passing upward into hornblendie Jt represents a hand- 
neiss. © © 
G= aahis, containing epidote. At its upper Seah aeie tase side 
margin its constituents are interfelted with of which the diorite 
the hornblende of the diorite. is distinctly foliated, 
light - coloured lines 
of felspar alternating with the dark hornblende. A vein of the 
haplite cuts obliquely across the foliation, but is itself quite 
unfoliated. It has slightly fused the diorite at the junction, fresh 
erystals of hornblende forming a sort of fringe to the vein. 
Schistosity in the diorite produced by pressure was first noticed 
as occurring at Gaerwen, in a paper read by me before the British 
Association at Manchester in 1887 (p. 706 of Report). In the 
following year, the Rev. J. F. Blake presented his Report on 
Anglesey rocks, and described* the phenomena in some detail. He 
states that at Gaerwen the constituent minerals of the diorite 
have been more or less pulled out into lenticles, and that the 
rock is sometimes ‘crowded with mylonitic lines.” The term 
‘shearing’ is applied by him to the process which the diorite has 
undergone. With these descriptions I entirely agree. <A series of 
six slides in my collection shows the change from diorite to schist 
1 Prof. C. R. Van Hise indicates my ‘primary injection’ by the term ‘in- 
jection, and my ‘secondary injection’ by the word ‘ cementation, Princip. of 
N. Amer. Precambr. Geol. 16th Ann. Rep. U.S. Geol. Surv. 1894-95 (1896) 
pp. 666-68, 684-88. But surely there is ‘injection’ in both cases. 
2 Quart. Journ. Geol. Soc. vol. liii (1897) pp. 355-57. 
3 Rep. Brit. Assoc. 1888 (Bath) p. 405. 
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