The spiracles represent rather uncommon phenomena at Kil auea. Steam rises readily in water, less easily in lava because of the greater degree of resistance to the upward movement. The projectile force must depend upon the viscidity of the lava. Be cause of the liquidity of basaltic lavas, the jets in Kilauea rise only a few yards ; the drops fall back still in the melted condition and the jets dance in very lively manlier. When somewhat stiffened the lava mud falls around the vent and cones result, having aper tures in the top through which the steam rushes noisily. A still later episode is where the explosive force throws the pulverized fragments high into the air as lapilli, ashes or dust. Various ob servers have watched this rising bubble in Stromboli, with its bursting and cinder projections.
The lunar spiracles are of larger dimensions than those seen in Kilauea or Hualalai, some of the latter being a thousand feet high. The pinnacles are also more conspicuous upon the Moon than upon the Earth. The same is true of caves and tunnels re sulting from the hardening of the surface of lava flows, allowing the liquid to run till the supply is exhausted. These caves were
found better developed in the Azores than in Hawaii, where lakes of fresh water and solid cones also appear.
Faults or cracks are common in all volcanic regions, especially mentioned in the palis south of Kilauea and in the fractures pro duced in the eruption of i868. Upon the Moon these cracks are known as rills, and are very numerous ; and may be also related to the canals upon Mars.
Keanakakoi (Plate 45) is also cited as an illustration of Lunar craters with smooth floors.
It is not practicable to set forth farther the similarities between the Lunar and Hawaiian volcanic features. They are to be ex plained primarily as phenomena characteristic of volcanoes in whatever world they occur ; and secondarily as belonging to Cal deras, of which the most celebrated examples are in Mokuaweo weo and Kilauea. See Lunar and Hawaiian Physical features compared, Memoirs of the American Academy, Boston, Vol. XIII ; and Appalachia, Vol. XI, No. 4.