GEOLOGICAL SOCIETY OF DUBLIN. 
65 
As the subsidence took place, the breakers pushed before them the 
masses that they had riven from the cliffs, breaking many into frag¬ 
ments, and rounding them as they progressed with the advancing 
boundary of the sea. In this manner was effected the transit of boul¬ 
ders, continually decreasing in size over the face of the submerging 
country; and thus, also, were the blocks driven up the hills while these 
went down progressively below the water line. But while the breaker 
action on the surface directly exposed to it would tend to drive the drift 
towards the dry land, or upwards, the currents would produce a con¬ 
trary effect, and, running deeply, and with great force, among the 
submerged hills, they would carry the smaller drift down their sides, 
and bear it along to a distance. On the sides of the hills not fronting 
the force of the waves, these would also drive the drift downwards, but 
only to the extent limited by the trifling depth that can be reached by 
breaker-action; and by this separation of the effects of breakers and 
currents, the course of the latter during a period of submergence might 
be inferred from the drift that has come from distant and higher levels, 
if not subsequently disturbed, and the direction of the prevailing winds 
may be conjectured from the great boulders about the summits of hills. 
Accordingly, we have grounds for concluding that at the time of the 
“boulder-drift” currents the predominant winds were not far from due 
west. The tendency of the large blocks to gather round the summits of 
the hills shows that the latter existed at that period, and it may be, that 
few considerable changes in the relative levels of the district have 
occurred since. During the re-emerging of the land, the forward mo¬ 
tion of boulders under breaker action would be continuously down hill on 
the sides not opposite to the force, but on the side opposed to it the 
great boulders of a former period would be little changed in position. 
They could not now be rolled any further up hill, as the sea was re¬ 
treating ; nor would they be carried down the slope, which would be a 
motion of advance against the force, and to the extent of the excess of 
length that the ordinates towards the base of the hill would have over 
those towards its summit in a curve of its vertical profile. Thus the 
great boulders of the submerging period have, in certain situations, been 
left as its memorials; in other places the drift of that time was subse¬ 
quently swept away, and portions of it now represent the escar drift, in 
which we find materials, as before stated, mixed with the prevailing 
limestone gravel, that lie at various points between S. E. and H. E. 
from their sources, according to the distance that they were carried away 
from their first resting-place where they had been deposited as boulder 
drift. In consequence of this displacement it is difficult to define the 
course of that drift within many degrees of its true direction, but we can 
scarcely err in saying that it came from some point between the north 
and west. At the same time it is possible—though the probabilities do 
not seem favourable to the supposition—that the portions of the “boul¬ 
der drift,” which have come from higher levels may have been first 
removed during the emergence of the land early in the period of the 
escar drift, and that subsequently a change in the direction of the cur- 
