ANGLESEY AND CAERNARVONSHIRE, 195 
majority of the lines extend to the edges of the quartz fragments, 
but others appear sealed up. The latter may have originally been 
lines of fracture which have been closed up by a secondary deposi- 
tion of quartz. The vesicles of vapours and liquids would naturally 
be arranged in rows if along fissures, but not necessarily so if 
they were original inclusions. These lines of vesicles therefore 
may be the result of infiltration. A rock of so old a date as the 
Dimetian has suffered many great changes since it was first formed, and 
it may have even been several times under the influence of heated 
waters and vapours. The false schistosity exhibited by the more 
massive portions of the Dimetian may be chiefly due to this crushed 
character, and to the abundance of the green mineral along the 
joints, sometimes present in such proportions as to give the rock a 
pseudo-brecciated appearance. The secret why this old granitoid 
rock is useless for building- and other purposes, may be found in its 
crushed character; but I am inclined to think that it is also partially 
due to its being, as I have lately maintained, a metamorphic rock, 
and not, as | formerly supposed, an igneous rock of Pre-Cambrian age. 
It varies so much in character at different horizons here, as at St. 
David’s, Rhos Hirwain, and Twt Hill, that it seems more natural to 
regard it as originally a deposited rock, like the older gneisses of 
Scotland and Canada, than as an igneous rock. Itis, however, now 
so completely crystalline that its origin even, as in the case of the 
massive gneisses, can only be a matter of conjecture. It seems that 
for the present, therefore, we cannot do better than be satisfied with 
the critical opinion of the late Mr. Tawney, who devoted much time 
to the examination of the Dimetian rocks, when he says, in a paper 
published last February in the Geological Magazine (p. 67), “‘ The 
constitution of the rock, however, is against its being igneous, and its 
great variability within short distances also.” 
About the centre of the Dimetian axis there are some green 
schistose rocks, which I believe are much newer than the Dimetian, 
and either folded or faulted inat this point. In the railway-cutting 
it appears as if, with the exception of the rocks above mentioned, we 
had an ascending section towards Ty-Croes Station, as explained in 
my former paper. The Dimetian at the east end of the cutting is 
lighter in colour than that towards the west, and it contains a 
certain proportion of a silvery mica. Beyond this are the compact 
-quartzose rocks which we classed as belonging to the hiilleflinta 
series, and these are followed by the quartz-schists and the chloritic 
schistose rocks so common in Anglesey. Dr. Callaway, in his paper, 
placed the granitoid rocks above the schists and compact quartzose 
rocks; but in this I feel confident he is in error, and that my 
original view as to the order of succession here is in the main correct. 
Fragments of almost every variety of the rocks in this section, 
besides those of the Dimetian already referred to (see Prof. Bonney’s 
note 8, p. 203), occur in the conglomerates ; therefore the evidence 
that all the metamorphic schists in Anglesey are of Pre-Cambrian 
age is abundantly clear. Fragments of the various schists have 
been frequently mentioned by other authors as occurring in con- 
glomerates in Anglesey; and Prof. Hughes states in an paper, 
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