There are also springs which are inflammable without being hot. This generally arises from a quantity of inflammable gas, or oily matter, which floats on the surface of the water; as in the instance of a brook in the vicinity of _Bergerac_, in the south of France, the surface of which may be set on fire by a lighted straw. Others, being mixed with bitumen, which often floats on the surface, will easily take fire, as at _Baku_, and other places in Persia. The waters of some springs and lakes have a petrifying, and others an incrusting quality. The former is impregnated with extremely fine silicious particles, which penetrate the pores of the substances immersed in them, and change their nature. This property is possessed by Lough Neagh. The Danube and the Pregel have also the same quality, but in a less degree. The waters which possess the incrusting property operate in a more rapid and manifest manner, by depositing the earthy particles they hold in solution, on the surfaces of bodies submitted to their action. This effect is produced by both hot and cold springs, particularly by the former. The matter deposited is usually calcareous, but in the instance of the Great Geyser it is silicious. Waters holding salt in solution, or muriated waters, as they are commonly called, are perhaps the most common of all; but they are rarely found in a state of purity. Among the Uralian and Carpathian mountains, they are frequent, and in general in the zone comprised between the parallels fifty and thirty north latitude. More to the north they are rarely found; farther toward the south crystallized salt is abundant in certain regions, as in the great desert of Africa; but we find only a few salt springs there. CHAPTER VIII.――CAVERNS. THE most celebrated cave in the United States, is that in Rockingham county, Virginia, known by the name of _Madison’s Cave_. It is in the heart of a mountain, about two hundred feet high, which is so steep on one side, that a person standing on the top, might easily throw a pebble into the river which flows round the base; the opposite side of it is, however, very easy of ascent, and on this side the path leading to the cavern runs, excepting for the last twenty yards, when it suddenly turns along the steep part of the mountain, which is extremely rugged, and covered with immense rocks and trees from top to bottom. The mouth of the cavern, on this steep side, about two thirds of the way up, is guarded by a huge pendant stone, which seems ready to fall every instant; it is impossible to stoop under it and not reflect with a degree of awe, that, were it to drop, nothing could save you from perishing within the dreary walls of that mansion to which it affords an entrance. The description which follows, is from the Travels of Mr. Weld.