34 C. H. HITCHCOCK — GEOLOGY OF OAHU 



Largely composed of the upper earths of the section, constituting the sub- 

 stratum of the soils found to be very suitable for the growth of the sugar 

 cane. At a deep railway cut one-fourth of a mile west from Aiea station 

 is a thick mass of earth, capped by 8 or 10 feet of coarse pebbles and 

 cobbles, cemented together so as to constitute a conglomerate, all of whose 

 constituents are rounded. These stones increase in size in passing across 

 a stream near the business center of the Honolulu plantation. Starting 

 at the sealevel, at Aiea station, the following is an approximate section 

 up to the top of the plateau, about 60 feet : At the base, 4 feet of greenish 

 clay and pebbly earth; 1 foot fine volcanic ash, consolidated ; 4 feet of 

 tuff; 1 foot of clayey ash ; pebbles and clay, 4 feet ; tuff and ash, 8 feet. 

 Back of this cliff" is an indefinite amount of drab and gray earths, with 

 layers of silica. On the summit of the plateau I found marine shells 

 and corals, some of which are like those used for food by the natives, so 

 that this is not a clear case of a submarine deposit, though it probably 

 is, as some of the organisms are not edible. On the branch railroad lead- 

 ing from Halawa up to the sugar plantation is an interesting cut through 

 earth capped by a fine grained volcanic ash, 3 feet thick, well filled with 

 leaves of dicotyledonous plants. The ash was apparently blown from 

 Makalapa and consolidated. Along the seashore the lower pebbly ash 

 of the Aiea section has been folded and slightly faulted. It is covered 

 by an earth or old soil, which can be traced eastwards directly beneath 

 the tufa of Makalapa, which comes as far west as Halawa stream. 



It would seem that this Pearl River series is a combination of marine 

 deposits, reefs, decayed rock, secondary volcanic products, ashes, and 

 solid basalt. The natural conclusion is that volcanic ejections were in- 

 tercalated with beds of marine origin, illustrated further by the finding 

 of a fine black ash intercalated in the limestone of Fords island, several 

 miles away from the nearest volcanic vent. At present it is not possi- 

 ble to separate them. Passing southerly toward the mouth of the river, 

 the limestones grow thicker and merge into the limestones proved to 

 extend deep into the earth by the artesian bore-holes. The series is of 

 Tertiary age. 



Rocks at the Pali 



s 



A trip to the Pali gap from Honolulu reveals many points of interest, 

 and the many excavations beyond the gap for a carriage road bring to 

 light phenomena that would not otherwise have been noted. The rocks 

 revealed are the older Koolau basalts, intermediate dikes, and recent 

 Hows of ashes and lapilli. 



Starting from the city, the first exposures are of the older basalts — 

 vesicular, chrysolitic, agglomeratic — covered by reddish earth, and we 



