24 
PRE-CAMBRIAN VOLCANOES 
BOOK H 
convenient adjective to distinguish them. As a provisional term for tliem 
I have pi’oposed the term “ Dalradian,” from Dalriada, the name of the old 
Celtic kingdom of the north of Ireland and south-west of Scotland.^ 
The special feature for which this Dalradian series is cited in the present 
volume is the evidence it furnishes of powerful and extensive volcanic 
action. In a series of rocks so greatly dislocated, crumpled and meta- 
morphosed, we cannot look for the usual clear proofs of contemporaneous 
eruptions. Nevertheless all over the Scottish Highlands, from the far coast 
of Aberdeenshire to the Mull of Caiityre, and across the west of Ireland 
from the headlands of Donegal into Galway, there occur.s abundant 
evidence of the existence of rocks which, though now forming an integral 
part of the scliists, can be paralleled with masses of undoubtedly volcanic 
origin. 
Intercalated in the vast pile of altered sediments lie numerous sheets 
of epidiorite and hornblende-schist, which wore erupted as molten materials, 
not improbably as varieties of diabase-lava. Most of these sheets are 
douljtless intrusive “ sills,” for they can be observed to break across from one 
liorizon to another. But some of them may possibly be contemporaneous 
lava-streams. A sheet may sometimes be followed for many miles, occupying 
a, Mi<.*a*selii.s^ ; h h, Sills. 
the same stratigraphical platform. Thus a band of sills may be traced from 
the coast of Banffshire to near Ben Ledi, a distance of more than 100 
miles. Among the hornbleudic sills of this band some occur on a number 
of horizons between the group of Ben Voirlich grits and the Ben-y-Glo 
(j^uartzite. One of the most marked of these is a sheet, sometimes 200 feet 
thick, which underlies the Loch Tay Limestone. Anotlier interesting group 
in the same great band lias been mapped by the Geological Survey ou the 
hills between Loch Tay and Amulree, some of them being traceable for several 
miles among the mica-.schists with which they alternate (Fig. 37). 
In Argyllshire also, between Loch Tarbert and Loch Awe, and along the 
eastern coasts of the islands of Islay and Jura, an abundant .series of sheets 
of epidiorite, ampliibolite and hornblende-schist runs with the prevalent 
strike of the schists, grits and limestones of that region. Similar rocks 
reappear in a like position in Donegal, where, as in Scotland, the frecpiency 
of the occurrence of these eruptive rocks on the horizons of the limestones 
is worthy of remark. The persistence, number and aggregate thickness of 
the sills in this great band mark it out as the most extensive series of in- 
trusive sheets in the British Isles. 
Presidential Address to Geoloijical Society, 1891, p. 39. 
