Ejected at the surface this water appears in fountains usually arranged in line over joints, or even in continuous sheets, and the sand collecting about the jets builds up lines of _sand_ or _mud cones_ sometimes described as “mud volcanoes” (Fig. 78). The amount of sand thus poured out is sometimes so great that blankets of quicksand are spread over large sections of the country. Most frequently, however, the sand is not built above the general level of the surface, but forms a series of _craterlets_ which are largely shaped as the water is sucked down at the time of the readjustment with which the play of such earthquake fountains is terminated (Fig. 79). Subsequent excavations made about such craterlets have shown them to have the form of a trumpet, and that in the sand which so largely fills them there are generally found scales of mica and such light bodies as would be picked out from the heterogeneous materials of the sand layers and carried upward in the rush of water to the surface (Fig. 80). =The earth’s zones of heavy earthquake.=—Since earthquakes give notice of a change of level of the ground, the special danger zones from this source are the growing mountain systems which are usually found near the borders of the sea. Such lines of mountains are to-day rising where for long periods in the past were the basins of deposition of former seas. They thus represent the zones upon the earth’s surface which are the most unstable—which in the recent period have undergone the greatest changes of level. [Illustration: FIG. 81.—Map of the island of Ischia to show how the shocks of recent earthquakes have been concentrated at the crossing point of two fractures (after Mercalli and Johnston-Lavis).] By far the most unstable belt upon the earth’s surface is the rim surrounding the Pacific Ocean, within which margin it has been estimated that about 54 per cent of the recorded shocks of earthquake have occurred. Next in importance for seismic instability is the zone which borders both the Mediterranean Sea and the Caribbean—the American Mediterranean—and is extended across central Asia through the Himalayas into Malaysia. Both zones approximate to great circles upon the earth’s surface and intersect each other at an angle of about 67°. It has been estimated that about 95 per cent of the recorded continental earthquakes have emanated from these belts. [Illustration: FIG. 82.—A line of earth fracture indicated in the plan of the relief, which may at any time become the seat of movement and resultant shock.]