ON CAVES. 3 



itself would soon destroy a jointed 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. Then 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 Sedgwick'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 be 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. All these 

 phenomena tell us of the enormous force exerted by the waves 



