14 Physko-Geog^nostk Sketch of the Island of Oahu. 

 such regularity from tlie central rides of the two chains, are an- 

 cient lava streams. This is proved not only from the nature of 

 their component rocks, but also from the form, being narrow 

 and more elevated on their upper part, broader and less elevated 

 as they descend into the plains. One of the finest views of this 

 arrangement is obtained in the streams descending from the 

 north-east side of Mouna Raala, as seen at a distance of five to 

 six miles from the sea-shore at Wailua. The depth of the ravines 

 which separate these lava streams is so great, and their bound- 

 ing walls, the sides of the lava streams, so steep, that the only 

 way of reaching the central summits is by following one of these 

 lateral ridges. Even when more open, as on the south side of 

 the chain of Ronahuanui, they always terminate in a cul de sac 

 or circus, bounded by tremendous precipices reaching from the 

 bottom of the valley to the ridge of the lava stream, here, near 

 its summit, almost elevated to the crest of the central chain, or 

 even, as at Ronahuanui, on the side of the valley of Annanu, to 

 the very highest summits themselves. It is probable that this 

 last arrangement holds also on the north-east side of Ruala, but 

 the valley is here more elevated at its upper termination than in 

 those which surround the base of Ronahuanui, and consequent- 

 ly the bounding precipices, though reaching to the highest sum- 

 mits, are not so lofty. These valleys are directed, as might be 

 expected from the above mode of origin, at right angles to the 

 central ridge, and their walls are straight, without any salient or 

 re-entrant angles. 



An interesting question suggests itself. How have the lime- 

 stone cliffs in the vicinity of Kahuku attained their present po- 

 sition .? for their summits are elevated upwards of fifty feet 

 above the highest level reached now by the sea. They must 

 have been under water at the time of their formation, for coral 

 never increases above the surface, the tenants which formed it 

 then dying. Successive elevations of a moderate amount would 

 account very well for the terraced form assumed by the succes- 

 sive reefs rising above each other in regular succession. That 

 the outer reefs become gradually deeper in proceeding out to 

 sea, is evident from the successive increase in the magnitude of 

 the breakers in proceeding outwards, indicating a greater depth 

 of roll, and the outermost is invariably the largest and most 

 magnificent. 



