Mud volcanoes which eject hot mud by the force of eruption of steam, which occur in volcanic districts, are of volcanic origin. They are caused by the passage of hot water and steam through beds of volcanic rock such as tufa, or hardened volcanic mud and other volcanic products. The hot water or steam raises the temperature of the mud through which it passes to the boiling point. As Dana remarks, the mud varies in consistency from very liquid muddy water to a thick mass like boiling soap, or in some cases like masses of mud or paint, and, in still other cases, to material like soft mortar, the consistency of the mud varying with the dryness of the season. There are three regions where mud volcanoes are especially common. One of the best known is in the Yellowstone National Park, four miles north of Yellowstone Lake, and six miles from Crater Hill. Some of these mud volcanoes have circular craters about ten feet in depth around which they have built mounds, the rims of which are several feet above the general level. There are well-known regions of mud volcanoes in different parts of Iceland. Here, according to Lyell, they occur in many of the valleys where sulphur vapor and steam bursts from fissures in the ground with a loud hissing noise. In these regions there are pools of boiling water filled with a bluish black clay-like paste, that is kept violently boiling. Huge bubbles, fifteen feet or more in diameter, rise from the surface of the boiling mass. The volcanoes pile up the mud around the sides of their craters or basins. Another part of the world where mud volcanoes are especially numerous is on the western shores of the Caspian Sea at a place called Baku. These are of the gaseous type and are attended by flames that blaze up to great heights often for several hours. These flames are due to the presence of natural gas and petroleum vapor that pass out through the water. Large quantities of mud are thrown out from the craters of these mud volcanoes. There are also many mud volcanoes in a district in India about 120 miles northwest of Cutch near the mouth of the Indus. In this region the cone built up around the crater is sometimes as high as 400 feet. The following description of mud volcanoes on Java is quoted from Daubeny's book on volcanoes. "It would appear likewise from Dr. Horsfield's description, that Java exhibits phenomena of a similar kind to those noticed in Sicily and at the foot of the Apennines, and there known under the name of 'Salses.' In the calcareous district (which I suspect to belong to the same class of formations as the blue clay and tertiary limestone of Sicily) occur a number of hot springs, containing in solution a large quantity of calcareous earth, which incrusts the surface of the ground near it. Of these, some are much mixed with petroleum, and others highly saline.