492 6. H. HITCHCOCK MOHOKEA CALDERA 



Mr Emerson's final conclusion is that we mnst seek for the source of 

 the ash in the district where it abounds. Considering the shape of our 

 supposed caldera, he thinks the ashes must have proceeded from some 

 part of it. This was the "source of the stupendous explosions or series 

 of explosions which has rescued Kau from being a waste of unproductive 

 rock and transformed it to so large an extent into a land of pastures and 

 plantations." 



I have already treated of this question in the paper already cited, 

 looking to Mokuaweoweo as the probable source of this and other local- 

 ities of ash on Hawaii. What is conceived to be the same duplex deposit 

 is recognized at Puakala on the south flank of Mauna Kea, at Hilo, all 

 through Olaa, as well as in Kau and Kona. I have the past year discov- 

 ered the same deposit on the north side of Mokuaweoweo a dozen miles 

 west of Humuula sheep station, so that now the great crater has been 

 proved to be encircled by this light, fine grained material. The absence 

 of it about Kilauea, Puu o Keokeo, and on the north slope of Mauna 

 Loa is occasioned by its removal by the later historic discharges of lava. 

 It would not be found near the central vent because the heated air would 

 carry the particles many thousand feet in the air, whence they would de- 

 scend miles away from their place of origin. The fact that the Mohokea 

 caldera is covered by the ashes is evidence that they came from a distant 

 vent. Had the eruption been in the midst of the depression, we should 

 look for them in an encircling belt, if not upon the southwest side almost 

 exclusively, where they were deflected by the trade winds. 



Order of Events in the History of Mohokea 



Several events can be clearly discriminated in the history of the Moho- 

 kea caldera. 



1. The formation of the cone of Mauna Loa. This is really composite, 

 but may be treated as a unity for convenience. Basalt came from below 

 and flowed over the edge of the primeval crater till the whole dome, 

 75 by 53 miles in the two diameters and 13,650 feet altitude, had been 

 formed, composed of millions of layers gradually superimposed upon one 

 another. The altitude must have been even greater, so as to allow for 

 the falling in of the surface to develop the caldera of Mokuaweoweo. 



2. After the material ceased to flow over the surface, two styles of 

 eruption commenced or continued to be manifested — those high up, 

 allowing streams of molten lava to flow away quietly, and those starting 

 from comparatively low levels, discharging with violence. The base of 

 the cone was filled by these ruptures of the basaltic sheets and the dis- 



