130 REPORT—1858. 
from a Wexford newspaper :—‘‘ The day was misty and dark, wind S.S.W. to S. 
Thunder heard at noon; wind lulled, and fog became dense. At Kilmore, ten miles 
south of Wexford, and directly opposite the Saltee Islands, about noon, a number of 
short, loud, smothered reports like cannon were heard. The tide had flowed consider- 
ably at the time, and the fishing-boats at the pier were all afloat, when, within the 
space of two or three minutes, the water suddenly receded from the pier, and people 
walked dry-shod where a little before there had been five to six feet of water. 
After a few minutes, again the tide began as suddenly to return; and, after re- 
suming its level, continued to rise to high water in the usual way. There was no 
extraordinary commotion, only an increased surf. The sky cleared after thunder 
and showers.” 
The question, however, here chieflyin point is, whence come these waves? what is their 
origin? The direction of translation, on entering the wide Bay of Ballyteague, here was 
almost exactly from the south-west, and if transmitted from a considerable distance, 
the origin of disturbance must have been beneath the deep waters of the Atlantic 
Ocean, and it is scarcely probable that an earthquake blow sufficiently powerful to 
have originated waves so large after so long a transmission, should have occurred and 
not have been generally felt in the South of Ireland, where the hard and elastic cha- 
racters of all the formations are so favourable to the distant transmission of impulses. 
It is equally difficult to assume, as here operative, a condition which upon coasts 
of shoal water and encumbered with banks and bars, may unquestionably originate 
great sea-waves, and which very probably is actually the cause of those of not un- 
frequent occurrence upon the east and south-east coasts of England. 
Almost all great submarine banks are constantly subjected, at the same time, to 
aggregation by deposition, and to partial degradation, by the sweeping away of 
material along their bases and flanks, by tidal action, either constant or at certain 
periods of tide. Deposition takes place by vertical, or more or less inclined preci- 
pitation of suspended matter; this form of degradation, by horizontal removal. 
The conjoint effect is very frequently to increase the steepness of the angle of slope 
of the degrading flank of the bank, matter being constantly added on top and re- 
moved from lower down, and with most energy at a level intermediate between the 
surface-water and bottom. 
A time arises, therefore, at which the angle of slope of the bank is increased be- 
yond the limits of repose of the material, whether mud, sand or gravel, or any mix- 
ture of these ; and then a great under-water slippage takes place, and a mass often 
of enormous magnitude at once slides from the top and flank of the bank down into 
deep water, and spreads and levels itself out upon the bottom, to be in its turn swept 
away and replaced by fresh materials and to give rise to another slippage. Thus, in 
figs. 9 & 10, if s,s represent the surface of the sea, b, b (fig. 9) the sea-bottom in 
Fig. 9. 
transverse section through the flank of the bank in a plane at right angles to the 
stream of abrasion; then, at the point where the equilibrium of repose of the mass 
is lost, the mass 7, ” slips and is suddenly transported from its original position to 
n,m. The effect upon the surface of the sea, is at the same moment to originate a 
positive and a negative wave, w and v, whose crests shall more or less approximate 
to the general line of the flank of the bank ; and these will be immediately succeeded 
by two solitary waves of translation, a greater, w (fig. 10), and a less, v, whose mo- 
tions of translation will be opposite. 
See ee 
