* 
[3 
1879.) 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 lime, 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 toa 
depth varying from about high tide level to a few feet below low 
tide level. oe BS es - 
