&. 
ON CAVES. 79 
itself would soon destroy ajointed rock. As each storm-wave 
rolls in, it deals a tremendous blow on the fissured mass: 
Every thin packing of clay between contiguous blocks is soon 
washed out, and the fissures themselves enlarged. ‘hen there 
comes into play another action. The space behind the block 
is filled with water; the thumping wave falls on the narrow 
opening on one side of it, not on the whole at once; the 
force is multiplied ten or a hundred times by the hydro- 
static paradox, and the block is hammered out. Even in 
a river this operation is seen going on. Along the valley 
above Sedewick’s old home in Dent, thin beds of car- 
boniferous limestone with shaley partings form the bed of 
the stream. The shale perishes, and the great slabs, 5 feet to 
10 feet across and nearly 1 foot thick, lie side by side on the 
bed of the stream. Then in one of the floods so frequent in 
that district the fissured limestone is filled, and the surplus 
water rushes in a torrent over the usually almost dry channel. 
A slab is lifted by the hydrostatic paradox, turned over by 
the torrent, perhaps swept down, or often left a record of the 
lifting force which got it out of its bed, but in doing so 
destroyed the machinery by which it lifted it. So sea-cliffs 
are more apt to be scooped out into caves and crannies 
where the rock is jointed or crushed. Any- soft, readily- 
decomposed dyke traversing the harder rock is also more 
easily removed. 
But that is not the only process by which these sea-caves 
are formed. On the coast of Pembrokeshire, near St. Davids, 
- there is a hole among the crags near high-water mark where, 
at a certain state of the tide, with each recoiling wave there is 
a loud sucking noise as the air is being forcibly drawn in 
through small, wet, weed-covered fissures to take the place of 
the receding water. It is known as Llesugn from the sound. 
Were it not for the cracks communicating with the air above we 
should not be reminded of this force being exercised by every 
wave in the cave below. Any loose material would he drawn 
back with the wave, and perhaps carried out of the cave 
altogether. Many. of us are familiar with the phenomena 
known as “ Blow Holes,” or “ Puffing Holes.” The incoming 
wave fills the tapering cave, and, just as the bore coming up a 
tidal river rises higher and runs more fiercely when the con- 
verging banks force it to pursue its way through a more 
contracted channel, so when the wave rushes into a narrowing, 
funnel-shaped cave, with a small aperture communicating with 
the surface, the water is forced up through the opening, and 
often a spout of spray is carried high into the air. Al] these 
phenomena tell us of the enormous force exerted by the waves 
