352 W. T. BRIGHAM ON THE VOLCANIC PHENOMENA 



The channels between the different islands are deep and quite irregular, so far as sound- 

 ings have been made. These have been limited to the principal harbors and shores where 

 shoals were suspected. Generally where rock cliffs meet the sea, the water is deep and 

 the shore destitute of coral reefs. Where a sand-beach or low lava streams are found, the 

 fringing reef extends some distance from the shore, but it is generally quite level to its 

 outer edge where it meets deep water at once. The limits of these reefs may be readily 

 seen by the marked change in the color of the water. In none of the channels are there 

 known to be detached coral reefs ; there seem to be no sunken rocks at any considerable 

 distance from the shore which might indicate the existence of some submerged or abortive 

 volcanic peak. It is well known, however, that to the north-west of the Hawaiian Group, a 

 long range of shoals extends many hundred miles, but beyond the mere fact of its existence, 

 we know nothing. It would be most interesting to institute a series of soundings to the 

 south-east of Hawaii, and it is hoped that the Hawaiian Government will undertake a 

 complete hydrographic survey of the " Seven Seas." 



OAHU. 



(See Map Plate XII.) 



General Description. — 0£hu is ninety miles distant from Kauai, in a S. E. direction, the 

 channel between being called Ieiewaho. Its shape is somewhat irregular, the greatest 

 length being thirty-five miles, the breadth twenty-one, and the area six hundred square 

 miles. Unlike Kauai, Oahu has no central mountain, but on the N. E. and S. W. coasts, two 

 high ridges extend in a nearly parallel direction for many miles, the plain between being 

 the base of the N. E. or Konahuanui range. 



The windward side of the island is well watered and fertile, and even the leeward side 

 possesses several streams besides many subterranean springs. As on all the other islands of 

 the group, except perhaps Kauai, the larger portion of the surplus rainfall passes to the sea 

 by subterranean channels, often appearing as springs near the line of low tide. The lava 

 of the mountains is more porous and less decomposed than on Kauai. Dykes are not so 

 common, nor so large, nor have the valleys such high and closely approaching walls, with 

 the single exception of the valley of Kaliiwaa. 



The shores of (Mhu are mostly fringed with coral reef which is often half a mile or more 

 in breadth, and is composed of the cemented coral fragments, shells, sand and growing spe- 

 cies of zoophytes. The ancient reefs are elevated thirty, forty, or even a hundred feet in 

 various places, and several of the valleys have by this barrier been changed from lagoons to 

 solid ground. Volcanic action is perhaps more evident than on Kauai, although many ages 

 must have elapsed since any outbreak. Vegetation extends to the summits of the moun- 

 tains, and during the rainy season even the dry plains and tufa cones support a crop of grass. 



South-west Range. — The mountains on the S. W. were perhaps the earliest formed, and from 

 the overlying strata of the eastern range, it is evident that they first ceased to give out lava 

 streams. They are much broken into peaks and ravines, the slopes are steep and difficult 

 to ascend, the summits are often marshy, and on one of them a lake of some size is said to 

 exist. This range is usually called the Kaala from its highest peak, which Dr. Gairdner 

 ascertained to be three thousand eight hundred and fifty feet high (3850). There are two 



