54 SEA AND LAND 



they present to the sea. Most commonly they are crescentic 

 in their outhne, hke the slenderest sickle of the new moon, 

 but they vary from this deeply re-entrant form through all 

 the stages in which we ma)' trace the filling of the lunar cup 

 until there is scarce a perceptible hollow left. When the 

 beach is newly formed it is always much curved ; if it has lost 

 this shape we can easily note the fact that it is b)' the pro- 

 gressive heaping up of the pebbles or sand in the central part 

 of the bay. If the supply of debris which is imported into 

 the beach is greater than the wave action which there takes 

 place can reduce to fine dust to be carried away by the 

 currents, then the basin fills and the shore line becomes 

 straighten If the waves at any time grind up more detritus 

 than is supplied to them, the shore is worn backward into the 

 land, and the beach may, if the process continue long enough, 

 utterly disappear. There is, however, as is the case in so 

 many natural forces, a beautiful |)rinciple of compensation 

 by which the several actions counterbalance or check each 

 other. Thus, when a beach is ill supplied with detritus, and 

 its curve becomes deeper, the waves which roll into it have 

 a longer journey over the shallow bottoni, are diminished in 

 energy, and are less effective in their work. On the other 

 hand, when the beach advances on the sea for any consider- 

 able distance the waves encounter less friction over the shal- 

 lows and o[>eratc with far more power. In this manner the 

 ill-fed beaches soon arrange their form so as to grind up less 

 rocky matter, and those which are gorged with pebbles con- 

 simie it more rapidly. 



There are many other noteworthy features in the pebbly 

 beaches, only a few of which can be considered in this essay, 

 for they are matters of detail, and, however interesting, have 



