1 6 SEA AND LAND 



readily to the battering action of the stones which the waves 

 hurl against them ; or in other cases, the rock may be riven 

 by dykes and veins, that is, by fissures which have been filled 

 with la\a, or materials deposited by the action of water. 

 These deposits may be, indeed most often are, softer than 

 the stone in which they are laid, and may thus afford weak- 

 nesses which are searched out by the sea and developed 

 into rifts and caverns. 



As soon as any weak spot on the face of the cliff has 

 been worked back a little way so that the hard bits of stone 

 may gather in it, every wave sends these fragments with 

 energy sufficient to wear the place yet farther back into 

 the land. The effect of the boundary walls is to keep the 

 rolling stones in a position to do effective work, and as they 

 are tossed about by the waves, new bits find their wa)' into 

 the pocket as fast as old ones are worn out. In this way, 

 these cutting tools are much better supplied in these recesses 

 than alone the general face of the cliff, and thus the waves 

 do more effective work here than elsewhere. As the sea 

 cuts only for a short distance up on the face of the steep, 

 the excavation, if the rock be tolerably firm, often has at first 

 the form of a cavern with a wide portal. As the chamber 

 widens, this opening commonly becomes unable to support 

 its roof, which falls into ruins and is ground up by the waves. 

 The greater part of the permanent caverns which are formed 

 in this general manner are excavated in trap-dykes. These 

 sometimes extend back from the sea-face to a distance of 

 one or two hundred feet or more. Most commonly the floor 

 of the chamber rises pretty rapidly as we penetrate from the 

 light of day. In fact, a considerable inclination toward the 

 water is necessary to keep the mining machinery by which 



