72 r REPORT—1874. 
from that assigned by Dr. Zirkel as the normal structure of basalt, in which the 
base is “a glass,” and the other minerals (the augite, felspar, and olivine) are 
individually crystallized out*. This, indeed, is the case with the Carboniferous 
melaphyres of the south of Irelandt, and probably with all the rocks in which 
augite is deficient; but the basalts of Antrim contain augite so largely in excess 
of the felspar that it has, in nearly every case, formed the base of the rock f. 
The basalt itself is often so rich in iron as to become an impure iron-ore. This 
is owing to the presence of the metal in the form of minute grains of titaniferous 
iron-ore, which is the principal cause of the black appearance of the rock, and also 
as one of the components of the augite. 
From the above general review of the volcanic history of Tertiary times in the 
north of Ireland it will be evident that it presents us with three distinct periods, 
similar to those which Mr. Judd has recognized in the succession of events in the 
Island of Mull :— 
The earliest, possibly extending as far back as the later Eocene period, charac- 
terized by the trachytic lavas. 
The middle, referable to the Miocene period, characterized by vesicular augitic 
lavas, tuffs, and plant-beds. 
The latest, referable to a still later stage of the Miocene period, characterized by 
more solid sheets of basalt and numerous vertical dykes. 
These three stages were probably separated from each other by long intervals of 
repose and the cessation of volcanic action. The succeeding Pliocene period seems to 
have been characterized by considerable terrestrial movements, resulting in the 
production of fractures in the earth’s crust, and (as my colleague, Mr. Hardman, 
supposes) in the formation of that large depression which was filled with waters 
having a greater area than the Lough Neagh of the present day. Some of the faults 
which traverse the upper sheets of basalt, and are therefore of later date, have ver- 
tical dislocation amounting to 500 or 600 feet, as, for instance, that which runs along 
the valley under Shane’s Hill near Larne. Such great fractures must necessarily have 
been accompanied by denudation, and it is probable that many of the present 
physical features had their origin at this (Pliocene) period. The extent to which 
the original plateau of volcanic rocks has been broken up and carried away within 
such comparatively recent times is vaster than is generally supposed. As there is 
evidence that the sheets of lava to the north of Belfast Lough were originally con- 
nected with those of Scrabo Hill to the south, we must suppose that this arm of 
the sea and the valley of the Lagan have been excavated since the Miocene period ; 
while on the north-west the high elevation to which the escarpment of the basalt 
reaches, leads to the supposition that the basaltic sheets spread over the ground 
now occupied by Lough Foyle. Both along the west and along the eastern sea- 
bord the sheets of lava are abruptly truncated, and must have extended far 
beyond their present bounds; while many deep valleys, such as those of Glenarm, 
Cushendall, and Red Bay, have been excavated. 
But the most remarkable result of the denudation, as bearing upon the subject 
before us, is the complete obliteration of the volcanic cones which we may well 
suppose studded the plateau. Some of these cones, at least, were contemporaneous 
with those now standing upon the granitic plateau of Central France, and which 
are but little altered in elevation since the fires which once burst forth from them 
became extinct. But since then the north of Ireland has been subjected to 
vicissitudes from which Central France has been exempted. The surface of the 
country has been overspread by the great ice-sheet of the earliest stage of the 
Glacial period, which appears to have stretched across from the Argyleshire High-- 
lands, if we are to judge by the direction of the glacial strie at Fair Head §. i 
a “a 4 ge iiber d. mikrosk, Zusammensetzung und Structur der Basaltgesteine’ 
70). 
t E. Hall, ‘On the Microscopic Structure of the Limerick Carboniferous Melaphyres,” 
Journ. Roy. Geol. Soc. Ireland, vol. iii. p. 112 (with plates). 
{ Mr. Allport, F.G.8., states (Geol. Mag. 1873) that he has found the augite indiyi- 
dually crystallized out in a specimen from near the Causeway. Such a case, however, 
must be exceptional; but the rule as stated above certainly holds good. ‘ 
§ A view also held by Mr, James Geikie and Mr, Campbell of Islay, 
