Hawaii.--The Sandwich Islands in the North Pacific are the best known examples of volcanoes formed by the welling up of lava without explosive action. They are all volcanic, and all exhibit similar features, though only one of them (Hawaii) now has active vents. Recent soundings have disclosed the fact that these islands are the summits of a gigantic submarine mountain chain, the loftier peaks

Fig. 7. Map of the island of Hawaii, showing some of the recent lava streams. The dotted lines are contours of 2,000 feet interval.

of which are not far off 30,000 feet above their sub-oceanic bases. The depth of the water within thirty or forty miles of Hawaii varies from 14,000 to 16,000 feet, and the Hawaian volcanoes rise to nearly 14,000 feet above the

sea.

Hawaii comprises four volcanic centres, one of which is

apparently extinct (Mauna Kea); but three (Mauna Loa, Hualalai, and Kilauea) are still active. As already stated (p. 24), the most notable characteristics of their eruptions are (1) the comparatively quiet uprise of the lava and the consequent absence of fragmentary ejecta; (2) the great liquidity of the lavas. To these causes the low, broad outline of the mountains is due (see fig. 3). The positions of these centres, and some of the more recent lava-flows, are shown in the plan, fig. 7.

Mauna Loa, the "Great Mountain," is 13,700 feet high, and its summit is a nearly level plateau, in the centre of which is a huge pit three miles long and a mile and threequarters broad in the middle. The floor of this pit is in two parts, the outer ends being about 600 feet below the rim, while the central section is about 180 feet lower, as if it had sunk down bodily in consequence of the removal of support below. Many eruptions have taken place from this mountain, but the lava has generally issued from some rent below the summit, and seldom from the crater. The lava seems to rise in a central pipe, and to exert such a pressure on its walls that they give way before it can reach the summit. Thus in 1852 there was first a small issue of lava near the summit, and then a larger outburst from a point about 4,000 feet below it.

In April, 1868, there was an outflow of lava from the southern border of Mauna Loa, at a point only 3,700 feet above the sea. This was preceded by earthquake shocks, which continued for two weeks. The lava was thrown up in a series of jets or fountains at intervals along a line of fissure, and the lava flowed with such rapidity that in a little over two hours the stream had reached the sea, a distance of 10 or 11 miles.

In February, 1877, there was an eruption of six hours duration near the summit of the mountain; lava-fountains were again formed, and a representation of them is given in fig. 8. It was followed by a submarine eruption in Kealakekua Bay. Other more extensive eruptions occurred in 1881 and 1887, producing lava-flows thirty miles in length.

Kilauea (pronounced Kilouea) has usually been described as a subsidiary vent of Mauna Loa, situate on the

Fig. 8. Eruption of Mauna Loa, Hawaii, on February 14, 1877. After a sketch by M. Ballieu, French Consul at Honolulu. Viewed from the north-west.

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west flank of that mountain, but Captain Dutton has recently given good reasons for dissenting from this view, and for regarding it as a distinct volcano, having no more connection with Mauna Loa than with either of the other two volcanic centres. He points out that the ground falls slightly from Kilauea towards Mauna Loa, as well as in every other direction, though the lavas from both centres have naturally tended to obliterate the intervening depression. Again, Kilauea is about as far from the summit of

Mauna Loa as that is from the other two mountains. Finally, he found that the real centre or apex of the Kilauea dome lay some 4 or 5 miles east of the great pit which bears that name, and that numerous old fissures radiate from this focus. Kilauea, therefore, is an independent mountain, smaller than, but similar to, Mauna Loa, and its present focus or crater lies a little west of its apex, and at a level of 4,200 feet above the sea.

Kilauea is a great pit, from 2 to 3 miles wide, and about 9 miles in circumference. It is filled with liquid lava, and

1 U.S. Geol. Survey, 4th Ann. Report, p. 120.

after an eruption it is often 1,000 feet deep; the ordinary action in this pit consists of small ebullitions, producing jets of lava which play like fountains to a height of 30 or 40 feet. Sometimes there are only two or three active pools, but in times of greater activity the pools overflow, and the pit fills to within 400 or 500 feet of the rim. An eruption may then be expected; an addition of 400 feet to the column of lava within the crater causes a corresponding increase of pressure, and eventually one or more fractures are produced below the crater, through which the lava makes its way to the surface. During the eruption of 1840 the lava first rose to a great height in Kilauea, and then broke out at several points, one below the other, finally issuing in a continuous stream at a place about 10 miles from the sea, into which it flowed at Nanawale.

Volcanic Islands and Submarine Volcanoes.— A volcanic island may have originated in two ways, it may be either the highest peak of a sunken tract of land, and therefore an island, because the sea has overflowed the ground on which it stands; or, it may have been built up from the sea-bottom, repeated eruptions increasing the height of the volcano till it lifts its head above the waves and becomes an island.

An interesting example of the latter kind of volcanic island was formed in the Mediterranean about 30 miles off the south-west coast of Sicily during the summer of 1831, at a spot where previous soundings had ascertained the depth of the sea to be 600 feet. Its formation was preceded by a violent spouting up of steam and water, and the sea around was covered with floating cinders and shoals of dead fish. At length a small island gradually appeared, having a crater on its summit which ejected steam, ashes, and scoriæ. This crater attained an elevation of nearly 200 feet, with a circumference of about 3 miles, having a circular basin full of boiling water of a dingy red colour.

The island received various names, but is best known by the English one of "Graham's Island," and the French one of "l'Isle Julia." It continued in activity for three weeks and then gradually disappeared. In 1833, two years after its destruction, a dangerous reef remained 11 feet under water, in the centre of which was a black volcanic rock