BELOW THE TIDEMARK 271 



selective force in the run of the tidal current along the shore. 

 Along the south coast each tidal wave from the Atlantic 

 drifts the shingle eastward, dropping the small stones before 

 the large. The most perfect exhibition of this process is on 

 the great twelve-mile bank of shingle which forms the Chesil 

 Beach. Instead of the nooks and capes of an ordinary 

 coastline, the Chesil Beach forms an even curve where the 

 process of sifting the stones can be carried out without the 

 smallest interruption from cross currents and sheltering 

 rocks. All the way from Bridport to the Isle of Portland 

 the size of the pebbles increases regularly ; and behind the 

 great barrier of stones, in a still lagoon, lie hidden the bays 

 and headlands of the original coastline. 



Sand is the finest dust of the sea's floor ; it is the residue 

 of the hardest and most insoluble rocks, and of broken sea- 

 shells. Sea-sands vary in colour in accordance with their 

 substance. Pure white sands are formed either of white 

 quartz or of sea-shells ; other quartzes give varying shades of 

 yellow, while the grit from rocks stained with iron gives 

 certain sands a tinge of red. Brown sands are usually due 

 to an infusion of mud ; but the waste of granite rocks forms 

 sands of a cool speckled brown which is pleasantly novel to 

 the eye accustomed to yellow sands. In certain spots on the 

 coast the sea seems to have the faculty of disgorging sand 

 from its depths, almost inexhaustibly, while the winds pile it 

 up into drifts which endanger the land. The moving sand- 

 hills on both coasts of the Bristol Channel have at various 

 periods eaten up farms, churches, and villages. At times the 

 drift of the sand changes, and the sea lays bare again what 

 it covered ; but the only sure way of checking such inroads 

 of sand is by planting the dunes with marram grass, and after- 

 wards with pines and other sand-loving vegetation. Below the 

 tide-mark the peril corresponding to the drifting sand-hill 



