CHARACTER OF ERUPTION 51 



way through limited oritices, ami have llowed down the mountain lor 

 weeks or months. 



The other kind of eruption was manifested in 18G8 and 1887, near the 

 south end of the island. It commenced with a light at the sunnnit, hut 

 discharged on a long line of vent from 10,000 to 12,000 feet below the fused 

 pool in Mokuaweoweo. The outburst was preceded by violent earth- 

 quakes, and continued to flow only two or three days, the amount of the 

 discharge being approximately equal to that sent out at the higher level- 

 The eruption of 1899 clearly belongs to the first categor3\ 



Areas of Weakness 



Two areas of weakness are indicated : the first at the southwest base 

 of the mountain, whei'e the violent discharges have come to the surface ; 

 the second is a considerable tract on the northeast slope, which carries 

 the sources of the flows of 1823, 1843, 1852, 1855, 1880, and 1899. They 

 bunched together from 9,000 to 11,000 feet above the sea and to the 

 northeast from Mokuaweoweo. Figure 1 '^ exhibits roughly their i)osi- 

 tions and comparative importance. Probabl}^ a dozen terminal f cones 

 are scattered over this area, and they are plainly visible 20 miles away. 

 The hill Puu ula ula is the most prominent elevation in this tract, and 

 it is one of the signal stations of the Hawaiian Trigonometrical Survey. 

 It was incorrectly located on the early maps, and the propinquit}^ of the 

 terminal cones Avas not observed till the effort was made to fix the pre- 

 cise position of the latest flow with reference to the others. 



The Mauna Loa Dome 



Mauna Loa is an elongated dome, 74 by 53 miles in the two diameters, 

 as measured at the sealevel, and 13,650 feet in altitude. Sometimes it 

 has been spoken of as extending downward more than 16,000 feet far- 

 ther to the level of the submarine plain on which the whole archipelago 

 is based. That would present a cone 30,000 feet in altitude, and we can 

 imagine it to contain a tube through which lava has welled up from this 

 enormous depth. Perhaps the recent discovery of the Tertiary age of 

 the foundations of Oahu may suggest that the base of the cone rests on 

 sedimentary material — a low island — thus avoiding the hypothesis of an 

 ejection built up from the sea bottom by means of volcanic debris. The 



*Tlie map is compiled from data afforded by Professor W. D. Alexander, chief of the government 

 survey. 



+ This term has Vjeen commonly applied to the small craters built up from the several orifices 

 of the different fiows. 



