It is practically certain that many of the eruptions producing this dust occurred within historic times. There must, therefore, have been many times in these parts of our country when the dense ash clouds hiding the sun turned the day into night and destroyed the forests and other vegetation by showers of red hot ashes. There were produced, too, the same great dread, and possibly loss of life as common during historical eruptions. It is pleasing, however, to think that while these great catastrophes brought suffering and dread to the people who then lived on the earth, they were, nevertheless, but the forerunners of those fruitful fields that at a much later age were to bless the people who afterwards lived on them. CHAPTER XXI MUD VOLCANOES AND HOT SPRINGS Mud volcanoes are the more or less conical hillocks from which, under certain conditions, mud is thrown out through the crust of the earth. Geikie defines mud volcanoes as follows: "Conical hills formed by the accumulation of fine and usually saline (salty) mud, which, with various gases, is continuously or intermittently given out from the orifice or crater in the centre. They occur in groups, each hillock being sometimes less than a yard in height, but ranging up to elevations of 100 feet or more. Like true volcanoes, they have their periods of repose, when either no discharge takes place at all, or mud oozes out tranquilly from the crater, and their periods of activity, when large volumes of gas, and sometimes columns of flame, rush out with considerable violence and explosion, and throw up mud and stones to a height of several hundred feet." There are two kinds of mud volcanoes: those in which the mud is thrown out by the action of different kinds of gases, and those in which the mud is thrown out by the action of steam. Mud volcanoes may or not be volcanic phenomena. Those which occur in the neighborhood of volcanoes whether active, dormant, or extinct, are probably of volcanic origin. There are others, however, which occur in regions far removed from volcanoes. These are probably due not to volcanoes, but to chemical action and the eruptions are caused by the action of gases. The gases producing these eruptions are either carbonic acid gas (the gas that is given off from soda water); carburetted hydrogen (the gas that is sometimes seen escaping from the bottom of marshy ground); sulphuretted hydrogen (a gas that is given off from rotten or decomposing eggs, and possessing the characteristic odor of decayed eggs) and nitrogen gas derived from the atmosphere. In mud volcanoes of the gaseous type the mud is generally cold, and the water salty. In this latter case the mud volcanoes are also called _salses_. Daubeny has pointed out that the mud volcanoes of this class that occur in the neighborhood of Sicily are due to the slow burning or oxidation of beds of sulphur.