200 
THE TERTIARY VOLCANOES 
BOOK VIII 
It then strikes inland, and making a wide curve in which it reaches a height 
of more than 1300 feet, comes to the sea again at Garron Point. From that 
headland the cliffs of basalt form a belt of picturesque ground southwards 
beyond Belfast, interrupted only by valleys that convey the drainage of the 
interior of the plateau to the North Channel. Above the valley of the 
Lagan the crest of the plateau rises to a height of more than 1500 
feet. 
Throughout most of its extent the basalt-escarpment rests on the white 
limestone or Chalk of Antrim, beneath which lie soft Lias shales and 
Triassic marls. Here and there, where the substratum of Chalk is thin, 
the action of underground water on the crumbling shales and marls below 
it has given rise to landslips. The slopes beneath the base of the basalt 
are strewn with slipped masses of that rock, almost all the way from 
Cushendall to Larne, some of the detached portions being so large as to be 
readily taken for parts of the unmoved rock. On the west side also, a 
group of huge landslips cumbers the declivities beneath the mural front of 
Benevenagh. 
I have found some difficulty in the attempt to ascertain what was the 
probable form of surface over which the volcanic rocks of this plateau began 
to be poured out. The Chalk sinks below the sea-level on the north coast, 
but, in the outlier of Slieve Gallion, three miles beyond the western base of 
the escarpment, it rises to a height of 1500 feet above the sea. On the 
east side also, it shows remarkable differences of level. Thus, below the 
White Head at the mouth of Belfast Lough, it passes under the sea-level, 
but only 16 miles to the south, where it crops out from under the basalt, 
its surface is about 1000 feet above that level. If these variations in height 
existed at the time of the outpouring of the basalt, the surface of the 
ground over which the eruptions took place was so irregular that some 
hundreds of feet of lava must have accumulated before the higher chalk 
bills were buried under the volcanic discharges. But it seems to me that 
much of this inequality in the height of the upper surface of the Chalk is 
to be attributed to unequal movements since the volcanic period, which 
involved the basalt in their effects, as well as the platform of Chalk below 
it. Had the present undulations of that platform been older than the 
volcanic discharges, it is obvious that upper portions of the basalt-series 
would have overlapped lower, and would have come to rest directly on the 
Chalk. But this arrangement, so far as I am aware, never occurs, except 
on a trifling scale. Wherever the Chalk appears, it is covered by sheets of 
the lower and not of the upper of the two groups into which the Antrim 
basalts are divisible.. We have actual proof of considerable terrestrial dis- 
turbance, subsequent to the date ol the formation of the volcanic plateau. 
Thus, near Ballycastle, a fault lets down the basalt and its Chalk platform 
against the crystalline schists of that district. On the east side of the 
fault, the Chalk is found far up the slope, circling round the base, of the 
beautiful cone ol Knocklayd — an outlier of the basalt which reaches a 
height oi 1695 feet (Fig. 263). The amount of vertical displacement of 
