44 CORAL FORMATIONS. 
there are also beach formations derived from the reefs. The tides 
and the attending currents carry to the shores more or less coral sand 
with shells and other reef-relics, and these sometimes form large 
deposits. The material is mostly like common sand in fineness, but 
often somewhat coarser, or even like a bank of pebbles. When the 
barrier is distant, only the sand and smaller pebbles are met with; 
but if the reef is quite narrow, there may be larger fragments and 
masses of coral rock. 
These deposits become cemented by being alternately moistened 
and dried, through the action of the recurring tides, and the wash of 
the sea on the shores. ‘The waters take up some carbonate of lime 
which is deposited and hardens among the particles on the evapora- 
tion of the moisture at the retreat of the tides. In some places the 
grains are loosely coherent, and seem to be united only by the few 
points in contact; and with a little care, the calcareous coating which 
caused the union may be distinctly traced out. In other cases, the 
sand has been changed to a solid rock, the interstices having been 
filled till a compact mass was formed. Generally, even the most solid 
varieties show evidence of a sand origin, and in this they differ from 
the reef-rock. The pebbly beds produce a pudding-stone of coral. 
In all instances observed, these calcareous sandrocks or conglome- 
rates, form a number of parallel layers along the coast, which dip 
regularly at an angle of five to eight degrees towards the water. The 
layers are from a few inches to a foot in thickness. They appear as 
if they had been tilted by some force below, and are seen to outcrop 
successively, on receding from the water. 'Tutuila and Upolu in the 
Navigator Group, and Oahu in the Hawaiian, afforded us many ex- 
amples of these beach formations. They seldom rise more than a few 
inches above high tide. At certain localities they appear to have been 
washed away after they were formed; and occasionally large masses 
or slabs have been uplifted by the sea, and thrown back on the 
beach. 
The same kind of deposits sometimes includes detritus from the 
hills. Black basaltic pebbles are thus cemented by the white calca- 
reous material, producing a rock of very singular appearance. Near 
Diamond Hill on Oahu, is a good locality for observing the steps in 
its formation. Many of the pebbles of the beach are covered with a 
thin incrustation of carbonate of lime, appearing as if they had been 
dipped in milk, and others are actually cemented, yet so weakly that 
the fingers easily break them apart. 
