52 ('. IT. HITCHCOCK — GEOLOGY OF OAHU 



The most westerly exposure of these ashes is at an old ceirietery between 

 the Insane asylum and the Bishop museum. Obviously the Nuuanu 

 valley may have been filled with this deposit, which has nearly all been 

 removed by llnviatile erosion, leaving this remnant of one or two acres 

 in extent. This may be 10 feet thick, as shown by excavations, with caves 

 and pillars of a similar material made to cohere by concretionary attrac- 

 tion. Here may be seen the pebbles overlying the ashes. They have 

 been seen also on the north side of Punchbowl. Hence there are three 

 localities of stones thrown out from Punchbowl subsequently to the dis- 

 charge of the ashes. It is to be noted that the ashes at the crest of 

 Punchbowl near the Mag-staff and those below Tantalus and over Round- 

 top contain numerous nodules. These are not present in the deposit in 

 the lower grounds about the city. Perhaps their greater weight explains 

 why they are limited to locations near their point of departure. 



My conclusion in regard to the origin of this coarse black ash is that 

 it probably originated in at least three craters — Tantalus, the pond 

 east of Kakea, and Punchbowl. The other shore craters, Diamond 

 head and the Kokos, have poured out freely a similar but finer 

 grained material, and Makalapa may have been the source of the con- 

 solidated ash plant beds near Halawa. I have also found the same sub- 

 stance intercalated in a limestone on the shore of Fords island. Refer- 

 ence has been made elsewhere to red ashes discharged from the Laeloa 

 craters and from secondary openings on Koolau. 



Rocks of the basaltic Areas 



There is little variety in the rocks of the older igneous areas on Oahu. 

 All of them are basalts, consisting of the common mixture of augite and 

 plagioclase, usually labradorite, with a slight sprinkling of magnetite. 

 Chrysolite is very common. The basalt may be compact, fine grained, 

 coarser grained, vesicular, scoriaceous, and it occurs also in every stage 

 of decay. In general these rocks correspond with those on Hawaii de- 

 scribed petrographically by E. S. Dana.* I will mention a few general 

 facts about the occurrence of these varieties at specific localities. 



At Kaena point the basalt has been cut by a large dike, 11 feet wide 

 in the railroad cut. Of this the outer 3 feet on each side are slaty, be- 

 cause of joints parallel to the sides, and the inner 5 feet, with a few hor- 

 izontal joints, correspond better with our usual notions of the appear- 

 ance of basalt. Southeast from this first exposure the dike becomes 

 larger — 13 feet of the slaty mass on one side, with a solid core of 8 feet 

 thickness. It may be followed readily for a quarter of a mile along the 



* Characteristics of Volcanoes, pp. 318-354. 



