The rate at which a current of lava moves along depends upon its mass, upon its degree of fluidity, and upon the inclination of the ground. It has been stated that certain streams of lava have traversed more than 3,000 yards in an hour; but the rate at which they travel is usually much less, a man on foot being often able to outstrip them. These streams, also, vary greatly in dimensions. The most considerable stream of lava from Etna had, in some parts, a thickness of nearly 120 feet, with a breadth of a geographical mile and a half. The largest lava-stream which has been recorded issued from the Skaptár Jokul, in Iceland, in 1783. It formed two currents, whose extremities were twenty leagues apart, and which from time to time presented a breadth of from seven to fifteen miles and a thickness of 650 feet. A peculiar effect, and which only simulates volcanic activity, is observable in localities where _mud volcanoes_ exist. Volcanoes of this class are for the most part conical hills of low elevation, with a hollow or depression at the centre, from which they discharge the mud which is forced upwards by gas and steam. The temperature of the ejected matter is only slightly elevated. The mud, generally of a greyish colour, with the odour of petroleum, is subject to the same alternating movements which have been already ascribed to the fluid lava of volcanoes, properly so called. The gases which force out this liquid mud, mixed with salts, gypsum, naphtha, sulphur, sometimes even of ammonia, are usually carburetted hydrogen and carbonic acid. Everything leads to the conclusion that these compounds proceed, at least in great part, from the reaction produced between the various elements of the subsoil under the influence of infiltrating water between bituminous marls, complex carbonates, and probably carbonic acid, derived from acidulated springs. M. Fournet saw in Languedoc, near Roujan, traces of some of these formations; and not far from that neighbourhood is the bituminous spring of Gabian. [Illustration: IV.--Mud volcano at Turbaco, South America.] Mud volcanoes, or _salses_, exist in rather numerous localities. Several are found in the neighbourhood of Modena. There are some in Sicily, between Aragona and Girgenti. Pallas observed them in the Crimea--in the peninsula of Kertch, and in the Isle of Tamàn. Von Humboldt has described and figured a group of them in the province of Cartagena, in South America. Finally, they have been observed in the Island of Trinidad and in Hindostan. In 1797 an eruption of mud ejected from Tunguragua, in Quito, filled a valley 1,000 feet wide to a depth of 600 feet. On the opposite page is represented the mud volcano of Turbaco, in the province of Cartagena (PLATE IV.), which is described and figured by Von Humboldt in his “Voyage to the Equatorial Regions of America.”