er various parts of the globe. The subject is far
too large to be treated otherwise than in a distinct and separate essay.
I will therefore content myself with a brief enumeration of the
formations of the British Isles in which contemporaneous volcanic action
has been recognised.[1]

There is little evidence of volcanic action throughout the long lapse of
time extending backwards from the Cretaceous to the Triassic epochs,
that is to say, throughout the Mesozoic or Secondary period, and it is
not till we reach the Palaeozoic strata that evidence of volcanic action
unmistakably presents itself.

_Permian Period._--In Ayrshire, and in the western parts of Devonshire,
beds of felspathic porphyry, felstone and ash are interstratified with
strata believed to be of Permian age. In Devonshire these have only
recently been recognised by Dr. Irving and the author as of Permian age,
the strata consisting of beds of breccia, lying at the base of the New
Red Sandstone. Those of Ayrshire have long been recognised as of the
same period; as they rest unconformably on the coal measures, and
consist of porphyrites, melaphyres, and tuffs of volcanic origin.

_Carboniferous Period._--Volcanic rocks occur amongst the coal-measures
of England and Scotland, while they are also found interbedded with the
Carboniferous Limestone series in Derbyshire, Scotland, and Co. Limerick
in Ireland. The rocks consist chiefly of basalt, dolerite, melaphyre and
felstone.

_Devonian Period._--Volcanic rocks of Devonian age occur in the South of
Scotland, consisting of felstone-porphyries and melaphyres; also at
Boyle, in Roscommon, and amongst the Glengariff beds near Killarney in
Ireland.

_Upper Silurian Period._--Volcanic rocks of this stage are only known in
Ireland, on the borders of Cos. Mayo and Galway, west of Lough Mask, and
at the extreme headland of the Dingle Promontory in Co. Kerry. They
consist of porphyrites, felstones and tuffs, or breccias,
contemporaneously erupted during the Wenlock and Ludlow stages. Around
the flanks of Muilrea, beds of purple quartz-felstone with tuff are
interstratified with the Upper Silurian grits and slates.

_Lower Silurian Period._--Volcanic action was developed on a grand scale
during the Arenig and Caradoc-Bala stages, both in Wales and the Lake
district, and in the Llandeilo stage in the South of Scotland. The
felspathic lavas, with their associated beds of tuff and breccia, rise
into some of the grande