faults, which descend to an indefinite depth, far below any point which it is possible for us to reach. Generally, they serve only for the direct descent of waters which, having been swallowed into them, find a little farther down an outlet from which they issue still cold. But it also happens, and the fact should be kept clearly in mind, that a fault offers a return route to water which has been heated at great depths. This is the case at Bourbonne-les-Bains. In the Alps, according to M. Lory, the same fault feeds the thermal springs of Monestier, Briançon, Brides, and Salines near Montiers. The fault which cuts and terminates the chain of the Alps near Vienna in Austria is the emissary of a considerable number of springs. Most of them are cold; but some hot springs, as those of Baden and Vöslau, are ranged along a distance of eleven kilometres.
The return branch of these natural siphons is often filled up and obstructed by incrustations which the water has formerly made, so as to form metalliferous veins. If the obstruction is not complete, or if it has been opened by raining excavations, these veins may be still serving for a way of ascent. A passage pierced at Plombières some thirty years ago in the granitic flank of the valley for the regulation of the warm waters, cut several veins of quartz and fluor-spar, along the sides of which springs gushed out forcibly. A similar incident took place at Lamalon, in Hérault, where it became necessary to stop the working of the veins of copper and lead, in order not to compromise the existence of the thermal establishment, the source of which was only a few dozen metres away. In the Comstock lode torrents of water having a temperature of 158° Fahr. gave out such heat that the workmen had to use ice to cool their shafts; and after twenty years of most profitable operation the mines became the object of great expenditures to obviate this difficulty.
It is for the most part in the neighborhood of extinct volcanoes and rocks of a volcanic nature that faults produce thermal spoutings. While such springs are usually wanting in the greater portion of the French central granitic plateau, they abound in those regions of the same plateau which are traversed by volcanic rocks.
There is no reason, therefore, to be surprised at finding the region of active volcanoes itself rich in emanations of this kind. Puzzuoli, Baiæ, and the baths of Nero, are situated near the solfataza of Puzzuoli and the ancient craters of Agnano and Lake Avernus. On the island of Ischia, as at Guadeloupe, the hot waters gush copiously from the flanks of the volcanoes.
We also observe near volcanoes boiling water violently projected into the air by torrents of vapor. The noise they make, which is like that of a steam-boiler, has caused the name of "Steamboat Springs" to be given to a group of this kind in the State of Nevada. Such springs are in close analogy with others in which the water is forced up, in the shape of a tall column, by intermittent eruptions. The latter have
