CORAL FORMATIONS OF OAHU. 251 
they probably lie near or about the vent from which they were thrown 
out, now so obliterated that it cannot be distinctly traced. 
At the same pass several dikes intersect the ridge. There are eleven 
within a space of fifty yards, and some of them stand out like artificial 
walls. In other instances the rock of the dike is removed, leaving a 
deep channel in the declivities. There is a narrow fissure through 
the mountains at the gap; and the formation of this fissure may have 
been the origin of the gap, which was subsequently enlarged by de- 
gradation. ‘The mountain rises to a height of a thousand feet either 
side of the pass. 
There is a region of small cones and comparatively recent appear- 
ances of eruption, near the southwest angle of the island, at the base 
of the mountains. ‘The craters are three in number, and half a mile 
to a mile apart. ‘They consist mostly of black compact lava or basaltic 
rock, and are surrounded by immense quantities of loose blocks, from 
one to twenty cubic feet in size, resembling the regions near Diamond 
Hill and on Kaneohe Point. ‘There are remains of a crater in each 
hill, and in two of them the rock stands up in irregular columnar forms. 
The largest of the three may be three hundred feet high; and the 
lowest is but little raised above the plain. 
4. CORAL FORMATIONS OF OAHU. 
In the remarks on Molokai, we have alluded to some instances of 
coral rock found at a considerable height above the sea. About 
Oahu the elevation of the once submerged reef has not been carried 
so far; but its extent is much greater, the evidence of elevation is be- 
yond doubt, and the circumstances connected with it are peculiarly 
interesting. 
Coral reef rock.—The elevated-reef forms a plain at the foot of the 
mountain declivities, from five to twenty-five feet above the level of 
the sea. It occurs along the whole southern shores, a distance of thirty 
miles, forming also the large flats in the Pearl River lagoon. On the 
northeastern side, it continues, with few interruptions, from Makapuu 
to Kahuku Point, the north cape; on the northwestern, it is met with 
for several miles about Waialua; on the southwestern, along by 
Waianae and the southwestern cape.* 
* On the map of Oahu the surface covered by it is indicated by a yellow colour. 
