Pebbles and Sand. 9 



clays, and limestones, the waste of the softer strata has 

 been in many places calculated at about two yards a 

 year. Where the strata are harder, as on the west 

 coast in Devon, Cornwall, and Wales, the waste is often 

 so slow as to be generally ignored by ordinary observers. 

 But the form of the coast proves it. Hard rocks resist- 

 ing waste because of their hardness are apt to form 

 headlands, while softer or more friable strata, wasting 

 more rapidly, often occupy the recesses of coves and 

 bays. The removal of the fallen detritus by the restless 

 waters makes room for further slips of debris from 

 above, and thus it happens that all sea-cliffs are in a 

 state of constant recession, comparatively quick when 

 made of clay or other soft strata, and when the rocks 

 are harder, perhaps very slowly, but still sensibly to the 

 observant eye, so that in time, be they ever so hard, 

 they get worn more and more backwards. The material 

 derived from this waste when sea-cliffs are truly rocky, 

 generally forms, in the first instance, shingle at tbeir 

 bases, as, for example, with the pebbles of flint formed 

 by waste of the chalk which contains them. These, 

 being attacked by the waves, are rolled incessantly 

 backwards and forwards, as everyone who has walked 

 much by the sea must have noticed ; for, when a large 

 wave breaks upon the shore, it carries the shingle for- 

 ward, rolling the fragments one over the other, and in 

 the same way they recede with the retreating wave with 

 a rattling sound. As in the running water of torrents, 

 so this long-continued marine action has the effect of 

 grinding angular fragments into rounded pebbles ; and, 

 in the course of time, large quantities of loose gravel 

 have thus been formed. Such material when con- 

 solidated becomes a conglomerate. 



If, also, we examine with a lens the sand of the sea- 



