Vol. 62.] IxV THE SOUTH-WEST OF CAPE COLONY, 77 



There is a further and more important difference in the Knysna 

 dune-area — namely, the base upon which the sand rests is below 

 sea-level. It seems at first sight strange that so loose a material as 

 the sand-rock should remain consolidated when it is sunk beneath 

 the water, but we have had ample proof that it does. Near Danger 

 Point, the bluffs of wind-deposited sand dip steeply seawards, and 

 pass beneath the tide-level ; and at Cape Infanta there is a great cave 

 excavated in the sand-rock, with an old beach forming the roof. 

 Dr. J. C. Branner mentions that, at Fernando de Noronha, wind- 

 bedded sand occurs below tide-level. 1 Off the George coast there 

 are several ledges, separated from each other by steps of a couple of 

 feet or so, made of consolidated sand-rock, the uppermost of which 

 is rarely covered by the sea, so that the spray evaporates in the pools 

 and yields salt ; but the lower ledges are covered and uncovered 

 at each ebb and flow of the tide, and the terrific surge of the 

 breakers beats upon them incessantly. The surface of the ledges is 

 exceedingly rough, and has been formed by the rock cracking along- 

 intersecting joints ; these become filled in with lime and form walls 

 round the enclosed blocks. When exposed to the waves the softer 

 interiors become eaten away, leaving the walls outstanding, and 

 where two or more cracks intersect a little pinnacle is produced. 

 Sometimes the cracks are very scanty and the included blocks 

 correspondingly large, so that wide, shallow pools are formed, with 

 low enclosing ridges: these are densely crowded with calcareous 

 algae, worms, and other lime-secreting organisms, and thus the 

 surface is further strengthened by a thick deposit of calcareous 

 matter. 



The question arises : if the sand-rock can be so hardened, might 

 these ledges not be the ordinary shore-sand hardened in situ, by 

 a process similar to that which, as Dr. Branner (in the memoir just 

 mentioned) maintains, built up the stone-reefs of Brazil ? At the 

 western end of the George sand-belt the limestone-ledges do not 

 appear above tide-level ; but they exist below it, as shown by the- 

 numerous fragments that are thrown up along the shore, yet there 

 is no tendency for the beach-sand to form similar ledges. As 

 certain proof exists in the caves of Cape Infanta that the sand-rock 

 has at one time gone beneath sea-level, and as all sections along the 

 coast where the ledges occur are favourable to the view that the 

 sand-rock does pass under the sea, we can accept with confidence 

 the statement that these limestone-ledges are formed of wind- 

 bedded sand hardened by sea-water. 



There is a very simple explanation of this hardening. When the 

 tide recedes, it leaves behind it in the pools and hollows of the 

 ledges a certain amount of sea-water, which contains carbonate 

 of lime in solution, in virtue of the carbonic acid which it holds; 

 when the water is exposed in shallow sheets to the heat of the 

 sun, the gas is driven off and the lime deposited, the process thus 

 necessitating, in the first place, a more or less solid ground for the 



1 Bull. Mus. Comp. Zool. Harvard, vol. xliv (1904) p. 220. 



