Note 18 — LETTER IV.

This must depend greatly upon the quality of the lava's; some have been in a more perfect state of vitrification than others, and are consequently less liable to the impressions of time. I have often observed on Mount Vesuvius, when I have been close to the mouth from whence a lava was disgorging itself, that the quality of it varied greatly from time to time: I have seen it as fluid and coherent as glass when in fusion: and I have seen it farinacious, the particles separating as they forced their way out, just like meal coming from under the grindstones. A stream of lava of this sort, being less compact, and continuing more earthy particles, would certainly be much sooner fit for vegetation, than one composed of the more perfect vitrified matter.