ON VOLCANOES AND EARTHQUAKES. 367 



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 leucites of 

 Vesuvius are almost peculiar to that mountain, as are 

 many of the very compound lavas in which it abounds. 

 Augit, arid 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. 



