Lava-streams vary greatly in size; in some cases the lava, escaping from craters, comes to rest before reaching the base of the slopes of the volcano; in other cases a lava-flow not only reaches the plains below, but extends for many miles over the surrounding country. Hence lava-streams are important geological agents. Let us look at some famous instances. The most stupendous flow on record was that which took place from Skaptar Jökull in Iceland, in the year 1783. In this case a number of streams issued from the volcano, flooding the country far and wide, filling up river gorges which were in some cases six hundred feet deep and two hundred and fifty feet broad, and advancing into the alluvial plains in lakes of molten rock twelve to fifteen miles wide and one hundred feet deep. Two currents of lava which flowed in nearly opposite directions spread out with varying thickness according to the nature of the ground for forty and fifty miles respectively. Had this great eruption taken place in the south of England, all the country from the neighbourhood of London to that of Gloucester might have been covered by a flood of basalt of considerable thickness. Sometimes, when the lava can only escape at a point low down on the mountain, a fountain of molten rock will spout high into the air. This has happened on Vesuvius and Etna. But in an eruption of Mauna Loa, in the Sandwich Islands, an unbroken fountain of lava, from two hundred to seven hundred feet high and one thousand feet broad, burst out at the base of the mountain; and again in April, 1888, the same thing happened on a still grander scale. In this case four fiery fountains continued to play for several weeks, sometimes throwing the glowing lava to a height of one thousand feet in the air. Surely there can be no more wonderful or awful sight than this in the world. The volcanoes of Hawaii, the principal island in the Sandwich Islands, often send forth lava-streams covering an area of over one hundred square miles to a depth of one hundred feet or more; but they are discharged quite quietly, like water welling out of a spring. Repeated flows of this kind, however, have in the course of ages built up a great flat cone six miles high from the floor of the ocean, to form this lofty island, which is larger than Surrey; and it is calculated that the great volcanic mountain must contain enough material to cover the whole of the United States with a layer of rock fifty feet deep.