100 J. D. Dana- — Geological History of Oahu. 



The fact that the volcano of East Oahu was in full action 

 long after the extension of the western cone, is shown (as I first 

 observed in 1840 and again in 1887) by the encroachment of 

 the eastern lava-streams over its base, and the burial in part of 

 j , the valleys. The accom- 



panying sketch is a view, 

 ^t^'^'^'^P-'^^'^^^y):^^ looking westward from the 

 \i^^^%^.. : ^S/ k^Y^'i^'tV • plain made by the encroach- 



MW\^'h : ^V '4^|?i)&' % " ; ing lavas ' snowin g now tne 

 ; ^<mn,::3sMi7 : lavas dammed up the al- 



ready made valleys of West 

 Oahu, and forced the drain- 

 age waters to take a north 

 or south direction, nearly 



parallel with the base of the mountain, in order to reach the 



sea. The courses of these streams are shown on the map. 



The depth of burial by the East Oahu lavas was j>robably some 



hundreds of feet. 



2. Evidence of recent change of level. 



1. Elevation. — Evidence of recent upward change of level is 

 afforded by the elevated coral reef along the sea-border. The 

 dotted line on the map (Plate 4) has already been pointed to as 

 approximately the inner limit of the raised reef; the small 

 dotted areas about Kahuku Point, the prominent north cape of 

 the island, and in Laie, the district next southeastward, besides 

 others west of Waimanalo, are the positions of hills or bluffs 

 made of the reef rock and consolidated drift sands. The rock 

 is in some parts a beautiful white, fine-grained building stone ; 

 but generally it has sudden transitions in texture and firmness, 

 and much of it is a consolidated mass of broken corals, or else 

 of standing corals made compact or nearly so with coral sand. 

 Along southern or southwestern Oahu the height of the reef is 

 fifteen to thirty feet ; and I estimated the amount of elevation 

 indicated by it in 1840 at 30 to 40 feet. 



At the Kahuku bluffs, which I visited anew in 1887 (see 

 figure 2), the solid coral reef rock extends up in some places 

 to a height by estimate of fifty to sixty feet above tide level ; 

 and this is surmounted by drift-sand rock, made of beach coral 

 sands that were drifted into hills on the coast when the reef- 

 rock was submerged, adding twenty feet or more to the height. 

 There are large caverns in the bluffs, which are mostly within 

 the upper layer of the coral reef -rock and have the drift-sand 

 rock as the roof. In the sketch, a faint horizontal line may 

 be seen passing by the top of the cavern ; it separates the beds 

 of different origin. The coral reef-rock consists mostly of ce- 

 mented masses and branches of corals of the kinds common in 



