ON VOLCANOES AND EARTIIOiUA KES. ,‘567 volcanic eruptions, and if they resemble in every point that family, while they differ essentially from the present volcanic rocks, it might be concluded that such volcanoes, under some other condition possibly, were anterior to the present state of the globe and were permanent sources of subterranean heat- Thus also it might be inferred, that the present overlying rocks occupied the places of antient volcanoes, though dif fering perhaps in some essential circumstances from the existing examples. It is not here necessary to describe minutely the various characters and composition of the lavas: all that may be required on this subject, will be found in the Classification of rocks. In a general sense, they present a much stronger resemblance to the traps than to any others, as well in their aspect as in their con stituent minerals; though it is in this last circumstance that the most important differences will be found. Yet even these differences, when most striking, are rather of a local than a general nature: the leucitcs of Vesuvius are almost peculiar to that mountain, as are many of the very compound lavas in which it abounds. Augit, and compact felspar, or a mineral analogous to this last, form the leading ingredients of lavas, as they do of many of the trap rocks. With respect to gene ral character, the lavas are simple and compact, or porous, or amygdaloidal, or porphyritic, or of a gra nitic character ; and two or more of these features are sometimes united in one example. Where the occa sional minerals are numerous, they sometimes form the great bulk of the compound, as is the case with Leucite. Obsidian is a glass; and pumice varies, only as it happens to consist of this glass inflated by air or steam, or of other lavas rendered so thoroughly cavernous by inflation as to float on water.