GEOLOGY AND MINERALOGY. 
71 
down into the sea, are broken into fragments and washed away. 
Many instances of this undermining may be seen around the coast; 
at Bankses there is a very good example, where a thick layer 
of lava is deeply undercut. It is, however, not so quick a process 
as may at first sight appear, and such falls around the coast of St. 
Helena rarely occur, though, when they do, a good slice of rock comes 
away. The northern coast has in this manner been cut into by 
the sea making its way inland for a distance of 1233 yards, or 
nearly three-quarters of a mile, leaving a perpendicular face to the 
cliff of 600 or 1000 feet. This is what we find on the leeward side, 
where the sea is generally calm, excepting occasionally when dis- 
turbed by the rollers ; but the denuding action of the Atlantic waves, 
which sweep with great force before the south-east trades against the 
windward side, has been much greater. There, at Holdfast Tom 
near Prosperous Bay, Stone Top, and Old Joan Point, the 
Island has been worn away until perpendicular cliffs of full 2000 
feet have been formed. 
An effect of this kind is visible also at Ascension, which, being 
a more recent formation, has been less acted on by the rollers along 
its northern coast, but has been considerably denuded on that side 
which is exposed to the prevailing winds. Again, at Madeira it is 
seen that those parts of the coast which are exposed to the broad 
sweep of the Atlantic are the most worn away. 
By prolonging the direction of the strata, and making due 
allowance for any displacement, the original coast line can be ascer- 
tained with tolerable correctness, and this we find at Holdfast 
Tom was once 3300 yards, or nearly two miles, further seaward than 
it is at present. If then the wearing away, in a horizontal direc- 
tion, of the coast at this exposed point, was at the rate of three 
inches in a year, it would require a period of 39,600 years to elapse 
since the action of the sea commenced to produce this change ; or, 
taking the less exposed side of the Island, as at Ladder Hill, an 
encroachment by the sea of one inch horizontally per year would 
suffice to give 44,388 years. This estimate of the encroachment by 
the sea on those hard iron-bound basaltic cliffs, is, to one who has 
for many years carefully watched its progress, rather a minimum 
than a maximum rate. Year after year passes without any per- 
ceptible change in the line of cliff; the undercut strata at Bankses, 
already mentioned, has through a number of years remained appa- 
