CORAL REEF.
Oahu is mostly encircled by a fringing coral reef. At low tide one can walk a long distance on this reef in various direc tions, off the city of Honolulu, near Koko Head, and in Kaneohe Bay. The polyps living on and enlarging the reef are of the genera Porites, Pocillopora, Astrea, Meandria and Fungia, to gether with Millepora, echinoderms, mollusks, serpulae, gorgoniae, nullipores with sea weeds, etc. The life is much better de veloped at Kaneohe Bay than at Honolulu, because the trade winds impinge directly against the shore, bringing food in great abund ance to the animals, while the harbor is on the lee side of the islands and subsistence is less easily obtained. Where the fresh water streams of Nuuanu and Kalihi valleys and Pearl river enter the sea, channels are produced, because the animals can not flour ish in fresh water. The Nuuanu channel is utilized for shipping, and the Pearl River outlet bids fair to form the entrance to the finest harbor in the Pacific Ocean when the bar at the mouth has been removed.
The great extent of the low apparently drowned land about Pearl River and inland from Waikiki gives the impression of submergence ; and on the northeast side of the island Kaneohe and Kahana Bays may be quoted as tending to the same con clusion. This is a controverted point between the advocates of the Darwin and Murray theories of the origin of coral reefs. Doubtless the land is somewhat lower now than it was formerly whichever theory is adopted.
The loose character of the ordinary reef rock is shown in the large blocks used for stone walls and buildings. A better quality is exhibited in the walls of the Kawaeahaa church, and the very best is a compact variety made by the washing of limestone f rag ments into fissures and cavities, which have been cemented by its own substance in solution. The sea water has worn the reef into very irregular shapes, not easy to walk on.
The plain of Honolulu rests on coral limestone, beginning easterly near Moiliili church and Paakea, and it has been covered by the basaltic flow of Kaimuki. It crops out in many places within the settled districts, as on the banks of the Nuuanu River near the Palama chapel and seaward from the trolley at Kapalama. A very large excavation in it shows an abundance of corals and shells. Boulders of basalt strew the surface of the unexcavated portion, and it may extend beneath the Kahemaheha Schools and Bishop museum, being fully twenty feet above the sea. The original floor of the crater of Aliapakai consists of coral, and it both overlies and is intercalated in the tuff that came from Makalapa, exposed along the railway in the southeast locks and the islands opposite. Most of the islands and points about Pearl River consist of this material, as at Ford's Island, Pearl City peinsula, Laulaunui, etc. About Ewa planta tion the limestone area is nine miles long and two and one-half wide. It skirts the shore and railroad the whole length of the
southwest shore of Oahu. At an abandoned quarry three miles north of Barber's Point (Laeloa) lighthouse the best quality of the sandstone is well developed, and was used in the erection of the Saint Andrew's English Cathedral. Alexander Agassiz speaks of this material as a "massive coral pavement sandstone." There are three varieties of material at this locality: At the base, the underlying rough reef loosely put together, a sandy lime stone, and above all, the compact pavement sandstone, capable of affording a good polish. The total thickness is about sixteen feet. This compact rock has been utilized also in the manufacture of quicklime. It is a good place in which to observe the manufacture of the sandstones, for shells and corals are strewn over the beach in all stages from the live animal to worn cobbles, pebbles, sand, and firm rock. Crystals of calcite are frequently seen in the con solidated rock.
Proceeding northerly, Professor Alexander reports a ledge of coral seventy-nine feet above the sea, at Katie, and seven hundred and thirty feet distant from the water south of Puna o Hulu, he mentions another ledge fifty-six feet above the sea and a quarter of a mile inland ; also on the south side of Lualualei, twenty feet high. At the south end of the ridge called Moiliili, the limestone reaches the height of eighty-one feet ; at other localities on this coast I have observed limited areas of the same substance more or less elevated.
The plain at Waialua shows many outcrops of the reef ; Ka huku, the extreme northern point of Oahu, is the most interesting locality. The Koolau highlands end in a bluff nearly two miles back from the extreme point, rising to a hundred feet or more from a flat plain. This bluff consists of coral rock up to sixty feet, capped by blown calcareous sand now firmly consolidated, which may extend inland to a height of two hundred and fifty feet. At various localities in the neighborhood I found corals and shells in the underlying limestone, but nothing in the sandstone above, save perhaps a shell brought by a hermit-crab. Professor Dana has given a very effective figure on page 302 of his "Character istics of Volcanoes," illustrating this plane between the two lime stones. Nowhere on the windward side of the island do the winds blow more vigorously than here, and hence the explanation of the great altitude attained by this blown consolidated sand. For five miles southeasterly, to even beyond Laie, the coral plain is quite extensive. Knobs of the consolidated sand with inclined strata rise to the height of thirty-five feet, and sometimes suggest an assemblage of lames. Several other localities of coral ma terials might be mentioned.