CHAP. XLIX 
SUBSIDENCES OF THE PLATEAUX 
449 
area, marks it out in strong contrast to most of the larger British lakes. 
Its surface is only 48 feet above the level of the sea. 
The origin of Lough Neagh, the theme of various legends, has been 
seriously discussed by different writers, but most exhaustively by the late E. 
T. Hardman of the Geological Survey. 1 This author connected the forma- 
tion of the lake-basin with a series of large faults which are found inter- 
secting the rocks around the basin, and passing under the water in a general 
north-easterly direction. He showed that these faults have produced serious 
displacements of the strata, amounting sometimes to as much as 2000 feet, 
and he believed that it was by the concurrent effect of such dislocations that 
the depression of Lough Neagh had been caused. 
It is possible that these displacements may have contributed to at least 
the earlier stages in the history of the Antrim subsidence. They have 
undoubtedly taken place after the outpouring of the basalts, for these rocks 
are involved in their effects. But in the hollow of the Bann valley north 
of Lough Neagh the faults which have been detected in the basaltic plateau 
are few and trifling. The bold and bare escarpments, that so clearly display 
the relations of the rocks, reveal few traces of any important transverse 
dislocations. Nor has any proof of large longitudinal faults parallel with 
the axis of depression been obtained within the area of the Bann valley. 
The earliest evidence for the existence of a lake on the site of the 
present Lough Neagh has been supposed to be furnished by certain fine 
clays, sands, seams of lignite and clay-ironstone, which have been referred 
to the Pliocene period. These deposits have been regarded as indicating 
the accumulation of fine sediment with drift vegetation brought down into 
a quiet lake by streams entering from the south. . Their fresh-water origin 
was believed to be further corroborated by the occurrence of shells be- 
longing to the lacustrine or liuviatile genus, Unior 
The thickness of this series of strata, their position above sea-level, and 
their distribution are important parts of the evidence for the geological 
history of the locality. At one place the deposits are said to have been bored 
through to a depth of 294 feet, and Mr. Hardman believed them to be not 
less than 500 feet deep. The same observer found that they certainly reach 
a height of 120 feet above the sea, and he was of opinion that in some 
places their height was not less than 140 feet. The deposition of strata to 
the depth of 300 feet below a level of 120 feet above the sea would, of 
course, entirely fill up Lough Neagh, and spread over a large tract of low 
ground around it. The pottery-clays and lignites, however, appear to be 
confined to the southern half of the lake, from which they rise gently 
into the low country around. 
The distribution of these deposits and their extraordinary variations in 
1 “On the Age and Formation of Lough' Neagh,” Journ. Roy. Gaol. Soc. Ireland, vol. iv. 
(1875-76), p. 170 ; also Explanation of Sheet 35 of the Geol. Surv. Ireland (1877), p. 72. 
s These shells were regarded as forms of Unio by the late VV. H. Baily ; but Dr. Henry 
Woodward assigned them to Mytilus. See Prof. Hull’s Physical Geology and Geography of Ireland, 
2nd edit. p. 101. The shells have boen more recently dug out by Mr. Clement Reid, who has 
found them to be the common Mytilus edulis. 
VOL. II 2g 
