Second, Mr. Green finds sympathy in the action between the two volcanoes not merely when the eruptions are synchronous but when increased activity has been observed in the supposed quiescent column either shortly before or after the main outburst. Lava does not exhibit as clearly as water the tendency to pre serve a level, because it is viscid' even at high temperatures. In all cases of the Mauna Loa eruptions at high altitudes the lavas in Kilauea have also been high, though not to the point of discharge.
In February, 1852, the lavas ran in a large stream on the north east side of Mokuaweoweo. A month later Kilauea exhibited increased action. October 3o, 1865, a brilliant light was dis played in Mokuaweoweo and continued for four months. No streams were known to have flowed from it, and no visitors climbed to the summit. Just after the disappearance of the light new lakes of fire and cones appeared from Halemaumau to the northwest, north and northeast, as well as towards Keanakakoi. The surface was flooded for a distance of two miles and a breadth sometimes of half a mile, preventing access by the usual route to the great lake for months. Now the chronicler says no sympathy was exhibited by Kilauea with the summit fire ; but Mr. Green calls attention to the fact that the moment action ceased at Mo kuaweoweo it began in Kilauea. "The lava columns in the two mountains have been rising steadily together. The cessation of apparent action in the summit crater of Mauna Loa may merely mean that the immense mass of lava discharged over the bottom of the crater of Mokuaweoweo had finally cooled and sealed up the opening. But the gradual rise of the lava in Kilauea keeps on," and culminates a year and a half later in the double erup tion of 1868.
The igneous manifestations on Mouna Loa from 1872-7 cul minated in an earthquake and submarine eruption the last of February. The activity was soon transferred to Kilauea, as manifested at Keanakakoi in August, and almost continuously about Halemaumau till the collapse of April, 1879; after which it was quiet for a couple of months.
Both the volcanoes were active in May, 188o, in South Mokua weoweo and Halemaumau. The lava had been steadily rising in both columns and continued ready for a discharge till November, when relief was afforded by the flow from Puka Uahi, lasting for nine months and reaching to the margin of the village of Hilo.
The next action was at Kilauea in 1886, there being lakes of lava varying slightly in detail but gradually increasing from 188o till the minor collapse in March, 1886. Mokuaweoweo seemed almost extinct till the time of the double eruption in January, 1887. The scene of activity was again exhibited in Kilauea alone, culminating in 1891 and 1894. After that came the two displays of 1896 and 1899 upon Mauna Loa and the contemporaneous in activity of Kilauea. The latter awoke again briefly in 1902 and Mauna Loa in 1903, but there was not much stirring in either pit till the last recorded double eruption in 1907.
Third. The intervals between the periods of joint action are somewhat suggestive of an approximate twenty year cycle, the figures being in order from 1832, 17, 6, 13, 19, 20. Possibly the 1855 episode was a supplement, as the intervals of 6 and 13 add up to 19.
Fourth. The old notion that volcanoes are safety valves is not so far astray after all. It is pressure from below that gives rise to eruptions. If relief is not afforded by discharges at high levels, the tension brings on earthquakes low down, and after great chasms have been opened the lava flows out and then quietude is restored.
It is conceivable that the retaining walls might be strong enough to withstand the pressure down to or below the sea level. If so the probability would favor the coming of a violent explosion like those from Vesuvius or Tarawera ; unless the discharges in the ocean as in 1844, 1877, 1884, etc., afforded the proper relief.