Even if we confine our attention to the larger volcanoes, which merit the name of 'mountains,' and such of these as we have reason to believe to be in a still active condition, our difficulties will be diminished, but not by any means removed. Volcanoes, as we have seen, may sink into a dormant condition that may endure for hundreds or even thousands of years, and then burst forth into a state of renewed activity; and it is quite impossible, in many cases, to distinguish between the conditions of dormancy and extinction. Concerning certain small areas in Southern Europe, Western Asia, and Northern Africa, historical records, more or less reliable, extend back over periods of several thousands of years; but with regard to the greater part of the rest of the world we have no information beyond a few hundred years, and there are considerable areas which have been known only for far shorter periods, while some are as yet quite unexplored. In districts almost wholly uninhabited, or roamed over by nomadic tribes, legend and tradition constitute our only guides--and very unsafe ones they are--in the attempt to determine what volcanoes have recently been in a condition of activity. [Sidenote: NUMBER OF ACTIVE VOLCANOES.] We shall, however, probably be within the limits of truth in stating that the number of great habitual volcanic vents upon the globe, which we have reason to believe are still in an active condition, is somewhere between 300 and 350. Most of these active volcanic vents are marked by more or less considerable mountains, composed of the materials ejected from them. If we include the mountains which exhibit the external conical form, the crateral hollows, and other features of volcanoes, but concerning the activity of which we have no record or tradition, the number will fall little, if anything, short of 1,000. The mountains composed of volcanic materials, but which have lost through denudation the external form of volcanoes, are still more numerous. The smaller temporary openings which are usually subordinate to the habitual vents, that have been active during the periods covered by history and tradition, must be numbered by thousands and tens of thousands. The still feebler manifestations of the volcanic forces--such as are exhibited in 'stufas,' or steam-jets, geysers, or intermittent hot springs, thermal and mineral waters, fumaroles, emitting various gases, salses or spouting saline and muddy springs, and mud volcanoes--may be reckoned by millions. It is not improbable that these less powerful manifestations of the volcanic forces, to a great extent make up in number what they want in individual energy; and the relief which they afford to the imprisoned activities within the earth's crust may be scarcely less than that which results from the occasional outbursts at the 300 or 350 great habitual volcanic vents.