PRE-CAMBRJAN VOLCANOES 
BOOK n 
and fractures are numerous, and much of the surface of the ground is 
obscured hy the spread of Fahiiozoic formations and superheial deposits. 
These Anglesey schists are so obviously au altered sedimentary series 
that it is not surprising that they should liave been regarded as metamor- 
phosed Cambrian strata. All that can he positively affirmed regarding 
their age is that they are not only older tlian the lowest fossiliferous rocks 
around them — that is, than Arenig or even Tremadoc strata — but that 
they liad already acquired tlieir present metamorphic character before these 
strata were laid down unconformably upon them. There is no actual proof 
that tliey include no altered Cambrian rocks. But when we consider their 
tlistinctly crystalline structure, and the absence of sucli a structure from any 
portion of tlie Cambrian areas of the mainland ; wlien, moreover, we reflect 
that the metamorphism which has affected them is of the regional type, 
and can hardly have been restricted to merely the limited area of Angle.sey : 
Fk:. 38. — SkeUili of cru.' 5 lie<l basic igneous rock iuiiong tlie schists, E. side ot 1 ortli-tyvsjii-mawi, 
E. .side of ttolyliead Straits. 
we must agree with those oltsei’vers who, in spite ot the iihseuce ot positive 
proof of their true geological horizon, have regained these rocks as of much 
higher anti(iuity than the Ciimbrian strata of the neighbourhood. No one 
familiar with the Dalradian locks of Scothind and Ireland can tad to he 
struck with the close resemblance which these youngei- Anglesey schists beiir 
t(i them, down even into the minutest details. Betrograpfliicidly they are 
precisely the counterparts of the (juartzites and schists of I eithshire and 
Ponegal, and a further connection may be established of a palaeontological 
kind. The upper part of the Holyhead quai'tzite w'as found by Mr’. B. N. 
Beach and myself in the autumn of the year 1890 to be at one qflace 
crowded with annelid-pipes, and 1 subsequently found the same to be the 
case with some of the flaggy quartzites near the bonth btack. 
For the purpose of the inquiry which forms the theme of this w’ork, 
the feature (jf greatest interest about these younger schists of Anglesey is 
the association of igneous rocks with them. They inclirde hands of dark 
l,.asic material, the less crushed parts of which resemble the diabases of 
