1 879.] Prof. Hartt on the Brazilian Sandstone Reefs. 355 



siliceous grains, with broken or perfect shells and finer calcareous 

 particles, derived from shells, corals, stony sea-weeds, etc. 



The action of the tides is not limited to the surface of the 

 beach, but this, from its porous character, absorbs a certain 

 amount of the water. At low tide the beach just below high 

 tide level is wet but not soaked with water ; going downwards, 

 however, we find the beach becoming gradually wetter and wetter 

 until it is completely saturated, producing little rills which run 

 down the surface. With the rise of the tide the level of com- 

 plete saturation also rises, and when the tide is high the upper 

 part of the beach, for some distance above high tide mark, is com- 

 pletely charged with water, thrown upon it by the waves. A 

 constant movement of water is thus produced in the interior of a 

 beach, but on account of the friction against the grains of sand, 

 this movement can only extend over a comparatively slight 

 width, at least in the upper portions of the beach, which are 

 under the water for only a few hours each day. 



Now sea-water, in many parts of the world, and especially 

 within the tropics, is very highly impregnated with bi-carbonate 

 of lirne,»and this solution, from evaporation or other causes may 

 deposit its lime in the form of a carbonate, which acts as a cement. 

 Where evaporation goes on over a beach wet with sea-water of 

 this character, the surface materials may become consolidated, as 

 occurs at the Abrolhos islands, on the coast of Brazil, and else- 

 where. Not only, however, may the upper sands be soldered to- 

 gether, but the hardening may even go on below the level of the sea. 



On the Brazilian coast, the surfaces of the beaches are seldom 

 hardened during the ebbing of the tide ; but under the hot tropi- 

 cal sun and strong prevailing winds, there must be, in the upper 

 part of the beaches, a concentration of the calcareous solution, 

 which, on sinking downwards to the level of complete saturation, 

 tends to deposit its carbonate of lime as a cement, uniting the 

 grains of sand. The water from rains, percolating through the 

 beaches, may also bring lime, arising from the dissolving of 

 shells and corals in the upper layers, and lagoons, which frequently 

 lie back of the beach ridge, may contribute to the same result. 

 It is evident, however, that the work is mostly done by the sea- 

 water, and that this agent, under certain conditions, has the power 

 of solidifying sea-beaches to a variable distance inland, and to a 

 depth varying from about high tide level to a few feet below low 

 tide level. 



