TRANSACTIONS OF THE SECTIONS. 69 
their easterly limits; but it is tolerably certain that the sheets of lava did not 
reach the shores of Galloway or those of the Isle of Man. Basaltic dykes, how- 
ever, as is well known, traverse the north of England and the south of Scotland ; 
but if referable, as Professor Geikie concludes, to the Miocene period, they cannot 
be included in the volcanic region as here described and understood. 
Thus the volcanic plateau of Antrim, like the Campagna of Naples, is washed 
on one side by the sea, and its limits become indefinable in consequence; but to 
the south, the west, and to some extent to the north, the limits of the region are 
marked out by mountains of considerable elevation. Within this region craters 
poured forth lavas or other volcanic products, which extended in great sheets 
until they were intercepted by the uprising of these natural barriers, 
The floor of the area thus partially circumscribed was formed of various 
materials, as the accidents of denudation admitted. Over the central portions it 
was chiefly Cretaceous limestone (or Chalk), but to the southward it was New 
Red Sandstone and Lower Silurian, and to the north, Chalk, Lias, Carboniferous, 
and Lower Silurian beds in different directions. The whole region composed of 
rocks thus distributed was probably converted into dry land towards the close of the 
Eocene period—when, at various points, highly silicated felspathic lavas burst 
forth, consolidating into sheets of trachyte porphyry, rhyolite, and more rarely 
pitchstone, such as are found at Brown Dod Hill and Tardree, near Antrim, and 
west of Hillsborough. These trachytic lavas were therefore the oldest of the 
voleanic eruptions of the north of Ireland, and seem to have been represented by 
the newer granitoid rocks recently described by Zirkel, Geikie, and Judd in the 
Island of Mull on the one hand, and by the trachytes of Mont Dore in Central 
France on the other, They have been described in this district by Berger and 
Bryce; but it is only recently that their relations to the other lavas have been 
clearly determined ; and as such rocks are exceedingly rare in the British Isles, 
I trust the Members of the Association will take this opportunity of paying a visit 
to the quarries near Antrim, where they are fully opened to view. In composition, 
both at Hillsborough and at Antrim, they present a felspathic base, enclosing 
erystals of sanidine (or glassy felspar) and grains of quartz. At Brown Dod Hill 
they are disposed in sheets, showing lines of viscous flow and dipping beneath the 
overlying beds of basalt. 
As I have already stated, the outpouring of these trachytic lavas may, with 
eyery probability, be referred back to the later Eocene period. At any rate, a 
considerable interval probably elapsed before the eruption of the next series of 
lavas of Miocene age, which are essentially augitic, and may be comprehended 
under the heads of basalt and dolerite with their amygdaloidal varieties. Sheets 
of these lavas were formed, from various vents, over the uneven surface of the 
older rocks, and to a far greater extent, both as to area and thickness, than in the 
case of the preceding eruptions of trachyte*. These beds, which are often vesi- 
cular, attain in some places a thickness of 600 feet, and are surmounted by decom- 
posed lava and volcanic ashes, which mark the close of the second period of 
eruption. 
he sheets of augitic lava which were poured forth during this stage are remark- 
able for their vesicular character and the numerous thin bands of red ochre 
(bole or laterite) which separate the different lava-flows, and which have been 
recognized by Sir C. Lyell as probably ancient soils formed by the decomposition 
of the beds of lava, similar to those in Madeira and the Canary Islands, resulting 
from streams of subaerial origin. Microscopic examination bears out this view; 
for a thin slice of one of the more compact beds of bole from the north coast 
showed that the felspar-prisms retained their form, while the augite and magnetite 
ingredients had passed into the state of an’ ochreous paste. 
The vesicular and amygdaloidal character of these older beds of lava shows the 
probability that they have been poured forth under no greater pressure than that 
of the atmosphere, and together with the evidence derived from the bands of ochre 
leads to the conclusion that they have been erupted over land-surfaces. Some of 
* Tn this respect they resemble the corresponding rocks in Central France, where, as: 
Mr. Scrope has shown, the trachytes have a more restricted range than the basalts 
(* Volcanoes of Central France’), 
