st mountain crests of North Wales, such as those
of Cader Idris, Aran Mowddwy, Arenig and Moel Wyn. A similar series is
also represented in Ireland, ranging from Wicklow to Waterford, forming
a double group of felstones, porphyries, breccias, and ash-beds, with
dykes of basalt and dolerite. The same series again appears amidst the
Lower Silurian beds of Co. Louth, near Drogheda.

_Metamorphic Series presumably of Lower Silurian Age._--If, as seems
highly probable, the great metamorphic series of Donegal and Derry are
the representatives in time of the Lower Silurian series, some of the
great sheets of felspathic and hornblendic trap which they contain are
referable to this epoch. These rocks have undergone a change in
structure along with the sedimentary strata of which they were
originally formed, so that the sheets of (presumably) augitic lava have
been converted into hornblende-rock and schist. Similar masses occur in
North Mayo, south of Belderg Harbour.

_Cambrian Period._--In the Pass of Llanberis, along the banks of Llyn
Padarn, masses of quartz-porphyry, felsite and agglomerate, or breccia,
indicate volcanic action during this stage. These rocks underlie beds of
conglomerate, slate and grit of the Lower Cambrian epoch, and, as Mr.
Blake has shown, are clearly of volcanic origin, and pass upwards into
the sedimentary strata of the period. A similar group, first recognised
by Professor Sedgwick, stretches southwards from Bangor along the
southern shore of the Menai Straits. Again, we find the volcanic
eruptions of this epoch at St. David's, consisting of diabasic and
felsitic lava, with beds of ash; and in the centre of England, amongst
the grits and slates of Charnwood Forest presumably of Cambrian age,
various felstones, porphyries, and volcanic breccias are found.

Thus it will be seen that every epoch, from the earliest stage of the
Cambrian to the Permian, in the British Isles, gives evidence of the
existence of volcanic action; from which we may infer that the
originating cause, whatever it may be, has been in operation throughout
all past geological time represented by living forms. The question of
the condition of our globe in Archaean times, and earlier, is one which
only can be discussed on theoretic ground, and is beyond the scope of
this work.

[1] The reader is referred to Sir A. Geikie's Presidential Address to
the Geological Society (1891) for the latest view of this subject.

[Illustration: VO