On Coral Reefs and Islands. 369 
In all instances observed, these calcareous sand-rocks or con- 
glomerates form a number of parallel layers along the coast, which 
dip regularly at an angle of five to eight degrees towards the wa- 
ter. ‘The layers are from a fewinches to a foot in thickness. 
They appear as if they had been tilted by some force from below, 
and are seen to outcrop successively, on receding from the water. 
Tutuila and Upolu in the Navigator Group, and Oahu in the Ha- 
; walian, afforded us many examples of these beach formations. 
| They seldom rise more than a few inches above high tide. A 
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. 
i Deposits of the same kind sometimes included 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 for observing the 
Steps in its formation. Many of the pebbles of the beach are cov- 
ered 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. 
The lime in solution in waters washing over these coral shores, 
is also at times deposited in the cavities or seams of the basaltic 
tocks; the cavities of the lava or basalt become filled with white 
calcareous kernels, and the cellular lava is changed into an amyg- 
: daloid. In large cavities or caverns, it often forms stalactites or 
if Stalagmitic incrustations. eee 
if Drift sand-rock.—Still another kind of beach formation is 
| going on in some regions through the agency of the winds in 
i connection with the sea. It occurs only on the windward side 
of islands when the reefs are narrow, and proceeds from the drift 
sa 
The drifts resemble ordinary sand-drifts, and are often quite 
|. €Xtensive. On Oahu, they occur at intervals around the eastern 
: Shores, from the northern cape, to Diamond Point which forms 
; the south cape of the island,—the part exposed to the trades ; 
and they are in some places twenty to forty feet in height. 
are most remarkable on the north cape, a prominent point ex- 
posed to the winds that blow occasionally from the westward, as 
Well as to the regular trades. They also occur on Kauai, another 
of the Hawaiian Islands. But at Upolu, (Samoa,) where. the pro 
»  tecting reefs are broad, I met with no instance worthy of ention. 
- These sand-banks, through the agency of infiltratirig waters, 
fresh or salt, become cemented into a sand-rock, more,or Jess fyi- 
Mr. Darwin as observed on the shores of Ascension, 
are given ePoceng calcareous incrustatfons on 
49. They were observed by the writer upon ira, 
as well as among the basal islands of 
te 
