452 
THE TERTIARY VOLCANOES 
BOOK VIII 
sunk down, the drainage lias been arrested, and some 20 miles of the 
course of the Bann are now under a sheet of shallow water. This subsidence 
was not brought about by faults. It seems rather to have resulted from a 
general sinking of the ground. The movement was probably comparatively 
rapid, otherwise the river-course would hardly have survived so well. 
3. These inferences, based upon purely geological considerations, have an 
interesting bearing upon the allusions to the origin of Lough Neagh con- 
tained in some ancient historical documents. Various legends have from 
an early period been handed down as to the first appearance of this sheet of 
water. These myths, though differing in details, agree in describing such a 
sudden or rapid accumulation of water as destroyed human life, in a district 
which had previously been inhabited by man. The earliest records indicate 
that the alleged catastrophe took place in the first century of the Christian 
era. 1 It appears to me notimprobable that the tradition, thus preserved in these 
legends, may have had its basis in the actual disturbance which, on geological 
grounds, can be shown to have determined the existence of Lough Neagh. 
Though the event may go back far beyond the first century, there can be no 
doubt that, in a geological sense, it was one of the most recent topographical 
changes which the British Isles have undergone. 
Thus the Antrim basalt-plateau, in addition to the high interest of its 
volcanic history, has the additional claim to our attention that it has 
preserved, more fully and clearly than any other of the plateaux, the 
evidence for the latest subterranean movements that followed the long series 
of volcanic eruptions during Tertiary time. It contains the record of a 
post-Glacial subsidence that gave birth to the largest lake in Britain. 
ii. DISLOCATIONS 
Though I have not observed any features among the Tertiary basalt- 
plateaux of the British Isles that can be compared to the remarkable rifts 
and subsidences of Iceland, it can be shown that these piles of volcanic 
material have undoubtedly been fractured, and that portions of them have 
subsided along the lines of dislocation. 
Careful examination of the basalt-escarpments of the Inner Hebrides 
discloses the existence of numerous faults which, though generally of small 
displacement, nevertheless completely break the continuity of all the rocks 
in a precipice of 700 or 1000 feet in height. Not infrequently such 
dislocations give rise to clefts in the cliffs. Some good illustrations of this 
feature may be noticed on the north side of the island of Carina, where the 
precipice has been fissured by a series of dislocations, having a hade towards 
1 For versions of tire legends, see Dr. Todd’s “Irish Version of the Historia Britonum ot 
Nennius,” Roy. Hist, and Arcltaol. Assoc. Ireland; Dr. Reeves’ “Ecclesiastical Antiquities of 
Down,” etc., p. 37(1 ; Mr. J. O'Beirne Crowe’s “Ancient Lake Legends of Ireland,” No. 1 in 
Journ. Hoy. IHst. and Archwol. Assoc. Ireland, vol. i. (1870-71), p. 94 ; Giraldus Cambrensis, 
vol. v. cap. ix. p. 91 — “de lacu inagno miram origineni hahente.” Moore’s well-known lines 
embody the popular belief that round towers and other buildings were submerged by the inun- 
dation. 
