398 COASTAL LIMESTONES 



the Bot Eiver mouth, at Cape Agulhas, Cape Barra- 

 couta and Cape Recife. These are calcareous sands 

 composed of a mixture of broken shells and fragments 

 of minerals, chiefly quartz. The strong winds and con- 

 stant supplies of fresh sand, as well as the facility with 

 which the dune sand is moved, account for the difficulty 

 of getting vegetation to gain and maintain a footing on 

 these sand areas, which are a source of danger to the 

 farms behind them. 



6. LIMESTONES OF THE COAST BELT. 



The calcareous sands of the coast belt pass into lime- 

 stone by the solution of carbonate of lime from parts of 

 the mass, and its deposition near the surface when the 

 water evaporates. In almost any part of the south-coast 

 dunes a thin hard crust can be found covering sand 

 which has been protected from the wind for some time ; 

 it may be less than a quarter of an inch thick, and is 

 easily broken. By the long-continued deposition of the 

 carbonate of lime the sand dunes are converted into 

 hard rock through a distance of many feet from the sur- 

 face, and where repeatedly wetted and dried, as happens 

 when the sea has encroached upon old dunes, the rock 

 becomes intensely hard and weathers with a peculiarly 

 jagged surface. This material sometimes makes a good 

 building stone (see chapter xiv.). 



False bedding is a very marked feature in many sand 

 dunes, being perhaps better developed in wind-borne 

 accumulations than in sediments deposited under water. 

 Magnificent examples of this structure can be seen in 

 several cliff sections through the hardened dunes on the 



