92 J. D. Dana — Geological History of Oaliu. 



tress here and there lengthening out into a ridge ; and only far- 

 ther northwest, beyond Knaloa, are there the longer valleys or 

 gorges and ridges and the mountain architecture characteristic 

 of deeply worn windward slopes. 



The only broad valley of the leeward or south and southwest- 

 ward slope that is continued upward with gradual ascent to the 

 very edge of the precipice is that of Nuuanu, behind the city of 

 Honolulu. It is the valley to the left in fig. 2 on Plate 4. 

 Six miles up it ends in the " pali" or precipice, and overlooks 

 the northeastern sea-border plains and hills. The height of the 

 " pali " is only 1207 feet above the sea ; but on either side are 

 the highest peaks of the range, Konahuanui 3105 feet in 

 height, and Lanihuli, 2775 feet. 



Great denudation on the leeward side of an island is an ex- 

 ception to the usual rule. It is a consequence, on Oahu, of 

 the sharp-crested 20-mile precipice. The trade winds become 

 chilled on striking the summit of the precipice and ready, 

 therefore, to drop their moisture ; but as they are moving on, 

 they get beyond the summit before much of the moisture 

 falls, and so the leeward slopes receive the water. In the 

 upper part of the JSTuuanu valley, within two miles of the pali, 

 132 inches of rain fall a year, and nearly 100 inches less than 

 this at Honolulu, although brief sprinklings occur almost daily 

 over the city. Konahuanui and Lanihuli, as seen from Hono- 

 lulu are generally under clouds, but from Kaneohe they are 

 usually uncovered. 



A nearly similar condition exists in West Maui, owing to 

 the thinness of the rocky walls at the head of its' great valleys. 

 Yery broad valleys are consequently made on the leeward side, 

 as in Oahu ; but these valleys soon end below in a slender 

 gulch, which may be, for the most of the year a " dry run ;" 

 the excessive dryness and heat of the lower plains evaporating 

 powerfully and supplying no water. 



2. Orographic condition of East Oahu. — From the facts 

 mentioned, it appears to be plain that the chief structural dif- 

 ference between East Oahu and East Maui is that the latter is 

 a whole volcanic mountain, and the former a piece of one. 

 By some means the Oahu mountain-cone or dome has lost, as 

 I concluded in 1840, a large piece from its mass — all that once 

 existed northeast of the 20-mile precipice. The size of the 

 lost piece it is not easy to determine. The lava streams of the 

 leeward slopes, which dip away from the precipice mostly at 

 an angle of 3° to 5° (as seen in the intersecting valleys), must 

 have come from some point or points beyond it to the north- 

 eastward. 



Following the leeward slopes around westward and north- 

 ward we find all pointing upward toward the higher part of 



