OF THE HAWAIIAN ISLANDS. 345 



places in this district the elevated coral-reef forms the roadway, and the coral-sand has often 

 blocked up the mouths of the small streams which are compelled to seek another outlet 

 through the porous reef beneath the surface, and the traveller may pass over the breach 

 after a storm unconscious of the existence of a watercourse until his horse commences to 

 sink in a quicksand. 



Very curious masses of black basalt occur imbedded in ochreous earth in the hills near 

 the shore. They often exhibit a columnar structure, are always smooth on the surface, 

 as if long exposed to the action of water, and are seldom found in masses of more than 

 two tons weight. 



The southern boundary of Koolau is formed by Mauna Kalalea, a curved ridge meeting 

 Mauna Pueo nearly at right angles. Its peaks are sharp, thin, and needle-like, about a 

 thousand feet in height, often columnar. Near Anahola, the ridge ends rather abruptly, and 

 is so thin that a round hole has been formed through the wall high up the slope, through 

 which the sky may be seen, from a base line of more than a mile at an average distance 

 of half a mile. The clip of this ridge is quite regular seaward. From Anahola southward 

 the mountain slopes are more gentle, not so much broken by ravines, and the streams of 

 water are small. Seven miles beyond is the Wailua River, the second in size on Kauai, and 

 navigable for canoes two miles and a half from its mouth, which is sometimes nearly closed 

 by a changeable sand-bar which reduces its breadth from one hundred and fifty to twenty- 

 five feet. Its depth does not exceed fifteen feet. 



Two miles and a half from the sea on the western branch is a fall one hundred and sixty 

 feet high, which, after a rain, presents a most beautiful spectacle as the river dashes over the 

 rocks, finally leaping in a broad sheet of foam into a dark basin walled on either side by 

 cliffs so steep and rocky that even the ferns and mosses which revel in the spray cannot 

 cover the naked rock. These walls continue nearly to the sea, and often present a columnar 

 structure. One of the layers of basalt on this river is convex in the manner of a lava bub- 

 ble, leaving a cave underneath. The fractures in this rock seem to follow nearly the course 

 of radii drawn from the centre of the sphere of curvature. 1 The cave underneath the fall 

 seems to be of similar structure, but can only be examined during very low water. 



Several dykes intersect the high walls of the gorge about a mile from the sea, where the 

 river changes its course. They are from three to six feet wide, and seem to have the direc- 

 tion of radii from the centre of the Island. On the eastern branch of the Wailua is a fall 

 of exceeding beauty. 



Both above and below this the river passes through a deep canon with nearly inaccessible 

 walls. The whole region about these falls abounds in small cones, some of which have 

 crateriform cavities at the top, several are broken down, and many of them are wooded to 

 the summit. Probably this region was once a malpays, quite like that described by Humboldt 

 as surrounding Jorullo in Mexico. 2 



The Wailua intersects a mountain ridge about half a mile from the sea, which presents 

 many curious features. The northern part is about six hundred feet high, extending two 

 miles, and is called Mauna Nounou, while the southern ridge, Mauna Kapu, is about two 

 thirds this height and extends double the distance. Both of these ridges are nearly parallel 

 with the coast line, are uneven, much degraded, and face the interior of the island with an 

 abrupt wall. The stratification is not very apparent from the decomposed state of the lava, 



i Geology of the U. S. Expl. Exped.,p. 269. a Vuesdes Cordilleres, pi. xliii. p. 239. 



MEMOIRS BOST. SOC. NAT. HIST. Vol. I. ft. 3. 88 



