Page:A Treatise on Geology, volume 2.djvu/234

 220 question we must survey volcanic phenomena in a variety of other aspects.

The Solfatara, near Puzzuoli, is in shape like other volcanic cones, with craters at the summit; it is formed of a trachytic rock, naturally hard and dark-coloured, but in proportion as it is exposed to the vapours given off from the "fumaroles" in the crater (the steam contains sulphuretted hydrogen and a minute proportion of muriatic acid), its texture and colour undergo a remarkable alteration. It passes through various stages of decomposition, and finally appears a white siliceous powder. Saline compounds effloresce on the surface of the rock (muriates of ammonia, &c., sulphates of alumine, lime, soda, magnesia, iron, &c.), and sulphur (not sublimed alone, but derived from sulphuretted hydrogen) lines the walls of its cavities. The ground is hollow (probably in fissures) below, and a stream of trachyte has formerly flowed from it, and formed the promontory called the Monte Olibano.

Craters in which the volcanic fires are utterly extinct are sometimes filled by water, as the Lago Agnano, and the more celebrated Lake Avernus, where no longer rise the sulphureous fumes which once procured it the formidable character of a gate of hell. (Æneid, vi.) They may be compared with some of the craters of the extinct Rhenish volcanos, as the Laacher See, near Andernach, which is about 2 miles in circumference, the Meerfeld, and other circular lakes or "maars" of the Eifel district. Sulphureous exhalations, which resemble those of the Solfatara, and lakes in craters like those of the Eifel, occur in Hungary and Transylvania; and the central districts of France show us, in addition, a variety of facts, to complete the view of the condition of countries where, though volcanic action, as commonly understood, is entirely extinct, the effects of subterranean