252 HAWATIAN ISLANDS. 
The elevated coral reef is, therefore, not a doubtful heap of corals, 
but extensive tables of calcareous rock, in many places a mile in width. 
The whole island is fringed by it to the same extent as now by a 
growing reef. It has been mentioned that a ledge of reef rock occurs 
also on the south bank of the salt lake basin, which is now shut off 
from the sea by a barrier forty or fifty feet in height. 
The reef rock is a white compact rock, quite various in structure, 
as we have described when speaking of coral reefs and islands. Some 
portions contain the corals imbedded as they grew; but a larger part 
is a solid homogeneous rock, with only here and there a coral or shell 
distinguishable ; it gives a clinking sound under the hammer, and 
breaks with a splintery fracture, throwing off, at the same time, sharp 
fragments. Other varieties consist of aggregated shells, or shells im- 
bedded in a compact calcareous base, and are generally as firm in tex- 
ture as any secondary limestone. A still more remarkable variety 
resembles chalk, having its colour, its earthy fracture, and soft, homo- 
geneous texture, and being an equally good writing material, as has 
been mentioned in the account of coral reefs. 
The growing reefs of the island have the same structure as here 
described for the elevated reef. But, while Astreas appear to have 
been abundant during former times, at present they are compara- 
tively rare, and the corals belong now, toa very large extent, to the 
genera Porites and Pocillopora, especially the former. The building 
material of Honolulu comes from the harbour reef, and consists of a 
closely branched Porites with intermingled fragments of shells and 
smaller fragments of corals and coral sand; and very frequently the 
Porites is 7m place, with the interstices filled more or less completely 
with shells and coral fragments. 
The elevated reef rock appears to form one, two, or three layers, five 
to ten feet thick. In some places, it is full thirty feet above the sea; 
but the average height is from ten to eighteen feet. At Honolulu, it 
has been frequently penetrated to some depth in making wells; but 
it has not been bored through. In excavations of this kind, it is usual 
to find first, two to five feet of soil; then two to three feet of volcanic 
sand ; and below this, the coral reef rock, the depth of which has not 
been ascertained. At Waialua, on the northwest side of the island, 
the limestone stands twenty to twenty-five feet above the sea in some 
parts of the plain, varying from this height to ten feet. There is here 
an isolated block, which stands on the lower flat of reef rock, ele- 
vated upon a kind of rude pedestal, to which it is cemented by large 
