Inasmuch as no earthquakes are felt in connection with such outflows as have been described, it is probable that the hot lava fuses a passageway for itself into some open channel underneath the flanks of the mountain. Such a course is well illustrated by the outflow of Kilauea in 1840, when, it will be remembered, occurred the great down-plunge of the crater that yielded the pit below the black ledge. At this time the lava first made its appearance upon the flanks of the mountain at the bottom of a small pit or inbreak crater which opened five miles southeast of the main crater of Kilauea (Fig. 104). Within this new crater the lava rose, and small ejections soon followed from fissures formed in its neighborhood. Some time after, the lava sank in the first new crater, only to reappear successively at other small openings (Fig. 104, _B_, _C_, _m_, _n_) and finally to issue in volume at a point eleven miles from the shore and flow thereafter _upon the surface_ of the mountain until it had reached the sea. Only the slightest earth tremors were felt, and as no rumblings were heard, it is evident that the lava fused its way along a buried channel largely open at the time (see below, p. 112). In a majority of the eruptions of Mokuaweoweo, when the outflowing lavas have become visible, the molten rock has apparently fused its way out to the surface of the mountain at points from 1000 to 3000 feet below the bottom of the crater, and this discharge has corresponded in time to the lowering of the lava surface within the crater. There are, however, three instances upon record in which the lava issued from definite rents which were formed upon the mountain flanks at comparatively low levels. In contrast to the formation of fused outlets, these ruptures of a portion of the mountain’s flank were always accompanied by vigorous local earthquakes of short duration. In one instance (the eruption of 1851) such a rent appeared under the same conditions but at an elevation of 12,500 feet, or near the level of the lava in the crater. =The outflow of the lava floods.=—In order to properly comprehend these and many otherwise puzzling phenomena connected with volcanoes, it is necessary to keep ever in mind the quite remarkable heat-insulating property of congealed lava. So soon as a thin crust has formed upon the surface of molten rock, the heat of the underlying fluid mass is given off with extreme slowness, so that lava streams no longer connected with their internal lava reservoirs may remain molten for decades. [Illustration: FIG. 105.—Lava of Matavanu upon the Island of Savaii flowing down to the sea during the eruption of 1906. The course may be followed by the jets of steam escaping from the surface down to the great steam cloud which rises where the fluid lava discharges into the sea (after H. I. Jensen).]