STRUCTURE OF CORAL REEFS. 53 
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 consolidated into a solid limestone 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 pro- 
duce a pudding stone of coral. 
In most localities the rock is an odlite or odlitic limestone. 
The grains become coated by the agglutinating carbonate of 
lime, and each enlarges thus into a minute sphere—a spherical 
concretion ; and the aggregation of these concretions makes 
the odlite. The grains are usually much smaller than the roe 
of most fishes, a resemblance which is alluded to in the name, 
from the Greek wor, egg. 
These beach deposits consist of regular layers, commonly 
from a few inches to a foot in thickness, and are generally con- 
solidated up to a line a little above high-tide mark. In all in- 
stances observed, the layers dip at an angle of six to eight de- 
grees down the beach. ‘This dip is nothing but the slope of the 
beach itself, and arises from the circumstance that the sands 
are deposited by the incoming waves, or tides, on such a slop- 
ing surface. Tutuila and Upolu, in the Navigator Group, and 
Oahu in the Hawaiian, afford many examples of these beach 
formations. At certain localities the beach sand-rock has been 
washed away after it was formed ; and occasionally large mass- 
es or slabs have been uplifted by the sea and thrown high up 
on the beach. 
Deposits of the same kind sometimes include detritus from 
the hills. Black basaltic pebbles are thus cemented by the 
white calcareous material, producing a rock of very singular 
appearance. Near Diamond Hill, on Oahu, is a good locality 
