The shape and steepness of a volcano depend on the nature of the materials ejected. The finer the volcanic ash, the steeper and more conical is the mountain. The building up of a volcano may be fairly illustrated by the little cone of sand formed in an hourglass as the sand-grains fall. These settle down at a certain slope, or angle, at which they can remain, instead of falling down to the bottom, as they do directly this slope is exceeded. Some volcanoes are built up almost entirely of volcanic ash and its embedded blocks. Vesuvius, Teneriffe, Jorullo, in Mexico, and Cotopaxi, in the Andes, are examples of steep volcanic cones built up in this way. Others, less steep and more irregular in shape, are chiefly formed of successive lava-flows. Little minor cones are frequently formed on the side of a volcano; and these during an eruption give rise to small outbursts of their own. They are easily accounted for by the dykes which are mentioned just now; for when molten rock forces its way through fissures, it sometimes finds an outlet at the surface, and being full of steam, as soda-water is full of gas, it gives rise to an eruption. The great opening in the centre of a volcano, with its molten lava, is like a very big dyke that has reached the surface and so succeeded in producing an eruption. The opening of a soda-water bottle not infrequently illustrates a volcanic eruption; for when the pent-up carbonic acid cannot escape fast enough, it forces out some of the water, even when the bottle is held upright. Every volcano has been built up on a platform of ordinary stratified rocks; and at some period _after_ these had been laid down in water and raised up into dry land, molten rock found its way through them, and so the volcano was built up by successive eruptions during many years. It is probable that earthquake shocks, preceding the first eruption, cracked up these strata, and so made a way for the lava to come up. The main point we wish to emphasize is that _volcanoes are never formed by upheaval_. In this way they differ from all other mountains. They have not been made by the heaving up of strata, but have been gradually piled up, something like rubbish heaps that accumulate in the Thames barges as the dustmen empty their carts into them, only in the case of volcanoes the "rubbish" comes from below. It is not necessary to suppose that the reservoir down below, from which the molten rock is supplied, exists at any very great depth below the original land surface on which the volcano grows up.