But although the majority of volcanic craters are clearly formed by explosive action, there are some craters, like those of Kilauea in Hawaii, which probably owe their origin to quite a different set of causes. In this case the explosive action at the vent is but slight, and the crater, which is of very irregular form, appears to have originated in a fissure, which has been slowly enlarged by the liquid lavas encroaching upon and eating away its sides. Such craters as these, however, appear to be comparatively rare. Besides the great volcanic mountains composed of lava, scoriæ, tuff and ash, there are other structures which are formed around volcanic vents even when these do not eject molten rock-masses. The water which issues in these cases either as steam or in a more or less highly heated condition frequently carries materials in suspension or solution, and these sometimes accumulate in considerable quantities around the vent. [Sidenote: FORMATION OF MUD-VOLCANOES.] When fissures are formed in the midst of loose argillaceous materials, such as are frequently produced by the decomposition of volcanic rocks, the waters which issue through them are sometimes so charged with muddy matter that this accumulates to form cones having all the general characters of volcanic mountains, and which occasionally rise to the height of 250 feet. The gases and vapours which issue from these 'mud-volcanoes' are those which are known to be emitted from volcanic vents at which the action going on is not very intense. Daubeny and others have suggested that these mud-volcanoes may be the result of actions which have little or no analogy with those which take place at ordinary volcanic vents, and that the combustion of subterranean beds of sulphur and similar causes would be quite competent to their production. But inasmuch as these mud-volcanoes are almost always situated in regions in which the more powerful volcanic action has only recently died away, and the gases and vapours emitted by them are very similar in character to those which issue from volcanoes, there does not appear to be any good reason for doubting that they should be classed as truly volcanic phenomena. Mud-volcanoes are found in Northern Italy near Modena, in Sicily near Girgenti, on the shores of the Sea of Azof and the Caspian, in Central America, and in other parts of the globe. The gas frequently escapes from them with such violence that mud is thrown into the air to the height of several hundreds of feet. Sometimes this gas is inflammable, consisting of sulphuretted hydrogen, hydrogen, or some hydrocarbons, and these gases occasionally take fire, so that true flames issue from these mud-volcanoes. In other cases the mud-volcanoes appear to be formed by either hot or cold springs containing large quantities of suspended materials, and the liquid mud issues from the vent without any violent eruptive action. [Illustration: Fig. 79.--Sinter-cones surrounding the orifices of Geysers.